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Messages - AbruptSLR

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1
The first image shows atmospheric methane concentrations since 2005 from NOAA's South Pole Station measurements; and it appears clear to me that this plot shows an accelerating trendline.  Also, the second image shows that Antarctic atmospheric methane concentrations are currently well more than twice that of any value in the past 800,000 years.

2
The Paris Agreement calls for limiting the increase in the global average temperature to well below 2°C.; however, per the first attached image by the end of 2019 we have a most probable chance of being near 1.23C already, and per the second image following the SSP-Baseline scenario to say 2030-2035, most likely we will be near 2C by 2040.  However, such simple assessments do not fully represent our true probability of reaching Mid-Pliocene conditions around coastal Antarctica by 2040; which per Pollard, DeConto and Alley (2018) would initiate the collapse of key Antarctic marine glaciers.  Per the linked reference [Adger et al. (2018)], conventional/consensus risk assessments do not address well the deep uncertainty associate with potential cascading tipping points triggered by temporary perturbations such as:

a. The current cyclic slow-down of the MOC which could temporarily add about 0.4C (see the third image) to the GMSTA trendline shown the second image.

b. The current reduction in the frequency of La Nina events could increase the slope of the GMSTA trendline from 0.17C/decade to over 0.38C/decade (see the fourth image, which does not consider the impact of changes in ENSO on GMSTA in the coming two decades).

Such dynamic risk assessments call for proactive action per the Precautionary Principle.

Adger et al. (June 13, 2018), "Advances in risk assessment for climate change adaptation policy", Philos Trans A Math Phys Eng Sci., 376(2121): 20180106, doi: 10.1098/rsta.2018.0106

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5938640/

Abstract: "Climate change risk assessment involves formal analysis of the consequences, likelihoods and responses to the impacts of climate change and the options for addressing these under societal constraints. Conventional approaches to risk assessment are challenged by the significant temporal and spatial dynamics of climate change; by the amplification of risks through societal preferences and values; and through the interaction of multiple risk factors. This paper introduces the theme issue by reviewing the current practice and frontiers of climate change risk assessment, with specific emphasis on the development of adaptation policy that aims to manage those risks. These frontiers include integrated assessments, dealing with climate risks across borders and scales, addressing systemic risks, and innovative co-production methods to prioritize solutions to climate challenges with decision-makers. By reviewing recent developments in the use of large-scale risk assessment for adaptation policy-making, we suggest a forward-looking research agenda to meet ongoing strategic policy requirements in local, national and international contexts."

Extract: "In economic terms, climate change represents what Nick Stern refers to as the greatest market failure the world has ever seen. Ross Garnaut suggested that failing to adequately deal with the consequences of climate change ‘would haunt humanity till the end of time’.

Climate change creates cascading risks in physical systems, ecosystems, economy and society, often inter-related and creating the circumstances for irreversible and undesirable crossing of thresholds at multiple scales. To assess climate risks across domains, and in a manner meaningful to decision-makers, is therefore a major scientific challenge.

Risk assessment based on a reductive approach to risk was designed for familiar systems and well-defined issues; it has been shown to be less appropriate under conditions of uncertainty, ambiguity and ignorance, when reduction to a single risk metric or policy recommendation cannot be scientifically justified. … In times of global change, this approach is no longer adequate to capture future risks. Furthermore, in a decision-making context, inherent uncertainty associated with climate change has been used to show that a conventional ‘predict then act’ framing is paralysed by limits to prediction, whereas an ‘assess risk of policy’ framing can act as a better stimulus for action by showing where existing policy objectives may be threatened.

Hence risk assessment is not simply about better prediction of likelihood or consequence. Moreover, reducing uncertainties is only one means by which progress towards adaptation occurs."

3
I wonder what sort of effect (if any) the disappearance of summer sea ice in Antarctica would have on glacial melt?
wdmn,

Most of the Antarctic sea ice already disappears in the austral summer, but more global warming would certainly result in more sea ice loss, and eventually to loss of austral winter sea ice (see the attached image after 4 X CO2 forcing between 50oS and 60oS latitude). 

However, polar amplification is an important part of the aggregate ice-climate feedback mechanism, and as such the linked reference provides some background discussion of some of the many different feedback mechanisms that contribute to polar amplification.  That said Goosse et al (2018) errs significantly on the side of least drama by ignoring many of the dynamic polar feedback mechanisms discussed in this thread.  For example the attached image shows the nonlinearity of the ice-albedo feedback factor in both polar regions, but it entirely ignores the possible ice-albedo feedback contribution from the probable collapse of the WAIS due to ice-cliff failures and hydrofracturing, in the coming decades.

Goosse et al. (2018), "Quantifying climate feedbacks in polar regions", Nat Commun., 9: 1919, doi: 10.1038/s41467-018-04173-0

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5953926/

Abstract: "The concept of feedback is key in assessing whether a perturbation to a system is amplified or damped by mechanisms internal to the system. In polar regions, climate dynamics are controlled by both radiative and non-radiative interactions between the atmosphere, ocean, sea ice, ice sheets and land surfaces. Precisely quantifying polar feedbacks is required for a process-oriented evaluation of climate models, a clear understanding of the processes responsible for polar climate changes, and a reduction in uncertainty associated with model projections. This quantification can be performed using a simple and consistent approach that is valid for a wide range of feedbacks, offering the opportunity for more systematic feedback analyses and a better understanding of polar climate changes."

Extract: "In polar regions, the presence of different phases of water implies that many feedback parameters display a particularly strong dependence on the state of the system near the freezing point and are thus highly non-linear. For instance, phase changes play an important role in polar clouds leading to nonlinearities in the cloud feedback. Furthermore, feedbacks related to the cryosphere generally depend on the surface area covered by snow or ice. As temperatures rise, this area decreases and the feedback strength is reduced. This is illustrated in Fig. 2 for the surface albedo feedback in response to three consecutive doublings of CO2 in the Community Climate System Model version 3 (CCSM3). At many latitudes, the value of the feedback factor is smaller for the third doubling (8 × CO2–4 × CO2) than it is for the first (2 × CO2–CNTL). Between 50°S and 60°S the feedback approaches zero for the third doubling, since the Southern Ocean is already ice-free at these latitudes in the 4xCO2 climate, and no further melting can occur. On the other hand, the value of the feedback factor increases at northern high latitudes (75°N–90°N), as the sea ice edge retreats within the central Arctic at high warming."

4
The linked reference indicates that terrestrial carbon sinks degrade more quickly when starting increased radiative forcing from pre-industrial conditions than from glacial conditions.  Thus Earth System Models, that were calibrated based on paleo data, should be recalibrated to expect faster loss of terrestrial carbon sinks for the rest of this century:

Adloff, M., Reick, C. H., and Claussen, M.: Earth system model simulations show different feedback strengths of the terrestrial carbon cycle under glacial and interglacial conditions, Earth Syst. Dynam., 9, 413-425, https://doi.org/10.5194/esd-9-413-2018, 2018.

https://www.earth-syst-dynam.net/9/413/2018/

Abstract. In simulations with the MPI Earth System Model, we study the feedback between the terrestrial carbon cycle and atmospheric CO2 concentrations under ice age and interglacial conditions. We find different sensitivities of terrestrial carbon storage to rising CO2 concentrations in the two settings. This result is obtained by comparing the transient response of the terrestrial carbon cycle to a fast and strong atmospheric CO2 concentration increase (roughly 900 ppm) in Coupled Climate Carbon Cycle Model Intercomparison Project (C4MIP)-type simulations starting from climates representing the Last Glacial Maximum (LGM) and pre-industrial times (PI). In this set-up we disentangle terrestrial contributions to the feedback from the carbon-concentration effect, acting biogeochemically via enhanced photosynthetic productivity when CO2 concentrations increase, and the carbon–climate effect, which affects the carbon cycle via greenhouse warming. We find that the carbon-concentration effect is larger under LGM than PI conditions because photosynthetic productivity is more sensitive when starting from the lower, glacial CO2 concentration and CO2 fertilization saturates later. This leads to a larger productivity increase in the LGM experiment. Concerning the carbon–climate effect, it is the PI experiment in which land carbon responds more sensitively to the warming under rising CO2 because at the already initially higher temperatures, tropical plant productivity deteriorates more strongly and extratropical carbon is respired more effectively. Consequently, land carbon losses increase faster in the PI than in the LGM case. Separating the carbon–climate and carbon-concentration effects, we find that they are almost additive for our model set-up; i.e. their synergy is small in the global sum of carbon changes. Together, the two effects result in an overall strength of the terrestrial carbon cycle feedback that is almost twice as large in the LGM experiment as in the PI experiment. For PI, ocean and land contributions to the total feedback are of similar size, while in the LGM case the terrestrial feedback is dominant.

5
Here is another positive feedback mechanism that consensus climate scientist need to add to their Earth System Models:

Title: "New climate 'feedback loop' discovered in freshwater lakes"

http://www.bbc.com/news/science-environment-43990403

Extract: "Methane emissions from lakes in the northern hemisphere could almost double over the next 50 years because of a novel "feedback loop" say scientists.

Climate change is boosting the proportion of cattail plants growing in and around freshwater lakes they say.

But when debris from these reed beds falls in the water it triggers a major increase in the amount of methane produced."

See also:
http://www.nature.com/articles/s41467-018-04236-2

Extract: "Water-soluble phenolic compounds from plant litter have specifically been shown to bind to and inactivate extracellular enzymes and exert toxicity in methanogens. These compounds build-up in anaerobic soils and sediments because oxygen limitation restricts phenol oxidase activity and dark conditions prevent photodegradation. In this way, the buildup of phenolic compounds may act similar to a ‘latch’, suppressing CH4 production and holding in place large quantities of C in lake sediments that would otherwise be released as CH4."

6
The linked reference indicates that consensus climate science is currently underestimating the amount of methane that will be emitted as the permafrost continues to thaw:

Knoblauch et al (2018), "Methane production as key to the greenhouse gas budget of thawing permafrost", Nature Climate Change, doi:10.1038/s41558-018-0095-z

http://www.nature.com/articles/s41558-018-0095-z

Abstract: "Permafrost thaw liberates frozen organic carbon, which is decomposed into carbon dioxide (CO2) and methane (CH4). The release of these greenhouse gases (GHGs) forms a positive feedback to atmospheric CO2 and CH4 concentrations and accelerates climate change. Current studies report a minor importance of CH4 production in water-saturated (anoxic) permafrost soils and a stronger permafrost carbon–climate feedback from drained (oxic) soils. Here we show through seven-year laboratory incubations that equal amounts of CO2 and CH4 are formed in thawing permafrost under anoxic conditions after stable CH4-producing microbial communities have established. Less permafrost carbon was mineralized under anoxic conditions but more CO2–carbon equivalents (CO2– Ce) were formed than under oxic conditions when the higher global warming potential (GWP) of CH4 is taken into account. A model of organic carbon decomposition, calibrated with the observed decomposition data, predicts a higher loss of permafrost carbon under oxic conditions (113 ±  58 g CO2–C kgC−1 (kgC, kilograms of carbon)) by 2100, but a twice as high production of CO2–Ce (241 ±  138 g CO2–Ce kgC−1) under anoxic conditions. These findings challenge the view of a stronger permafrost carbon-climate feedback from drained soils and emphasize the importance of CH4 production in thawing permafrost on climate-relevant timescales."

7
The linked reference evaluates 19 previously proposed constraints on ECS, and it could not verify the applicability of 15 of these proposed constraints, but it did find that 4 of the 19 constraints "… all predict relatively high climate sensitivity."

 Peter M. Caldwell, Mark D. Zelinka, and Stephen A. Klein (2018), "Evaluating Emergent Constraints on Equilibrium Climate Sensitivity", Journal of Climate, https://doi.org/10.1175/JCLI-D-17-0631.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-17-0631.1?utm_content=buffer3197b&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

Abstract: "Emergent constraints are quantities which are observable from current measurements and have skill predicting future climate. This study explores 19 previously-proposed emergent constraints related to equilibrium climate sensitivity (ECS, the global-average equilibrium surface temperature response to CO2 doubling). Several constraints are shown to be closely related, emphasizing the importance for careful understanding of proposed constraints. A new method is presented for decomposing correlation between an emergent constraint and ECS into terms related to physical processes and geographical regions. Using this decomposition, one can determine whether the processes and regions explaining correlation with ECS correspond to the physical explanation offered for the constraint. Shortwave cloud feedback is generally found to be the dominant contributor to correlations with ECS because it is the largest source of inter-model spread in ECS. In all cases, correlation results from interaction between a variety of terms, reflecting the complex nature of ECS and the fact that feedback terms and forcing are themselves correlated with each other. For 4 of the 19 constraints, the originally-proposed explanation for correlation is borne out by our analysis. These 4 constraints all predict relatively high climate sensitivity. The credibility of 6 other constraints is called into question due to correlation with ECS coming mainly from unexpected sources and/or lack of robustness to changes in ensembles. Another 6 constraints lack a testable explanation and hence cannot be confirmed. The fact that this study casts doubt upon more constraints than it confirms highlights the need for caution when identifying emergent constraints from small ensembles."

8
A lot of climate change contrarians made a big deal that CMIP5 modeling of land-climate feedbacks from nutrient uptake do not consider nutrient competition with microbes and abiotic processes during inactive periods; which reduces the amount of associated N2O emissions (see the first linked reference).  However, such contrarians failed to note that limitations on nitrogen and phosphorus availability will likely severely limit carbon sequestration from plant growth with continued global warming (see the second two references and associated images):

Riley, W. J., Zhu, Q., & Tang, J. Y. (2018). Weaker land–climate feedbacks from nutrient uptake during photosynthesis-inactive periods. Nature Climate Change. doi:10.1038/s41558-018-0325-4, https://doi.org/10.1038/s41558-018-0325-4

https://www.nature.com/articles/s41558-018-0325-4

Abstract: "Terrestrial carbon–climate feedbacks depend on two large and opposing fluxes—soil organic matter decomposition and photosynthesis—that are tightly regulated by nutrients. Earth system models (ESMs) participating in the Coupled Model Intercomparison Project Phase 5 represented nutrient dynamics poorly, rendering predictions of twenty-first century carbon–climate feedbacks highly uncertain. Here, we use a new land model to quantify the effects of observed plant nutrient uptake mechanisms missing in most other ESMs. In particular, we estimate the global role of root nutrient competition with microbes and abiotic processes during periods without photosynthesis. Nitrogen and phosphorus uptake during these periods account for 45 and 43%, respectively, of annual uptake, with large latitudinal variation. Globally, nighttime nutrient uptake dominates this signal. Simulations show that ignoring this plant uptake, as is done when applying an instantaneous relative demand approach, leads to large positive biases in annual nitrogen leaching (96%) and N2O emissions (44%). This N2O emission bias has a GWP equivalent of ~2.4 PgCO2 yr−1, which is substantial compared to the current terrestrial CO2 sink. Such large biases will lead to predictions of overly open terrestrial nutrient cycles and lower carbon sequestration capacity. Both factors imply over-prediction of positive terrestrial feedbacks with climate in current ESMs."
&

Fu et al (2016), "Climate change impacts on net primary production (NPP) and export production (EP) regulated by increasing stratification and phytoplankton community structure in the CMIP5 models", Biogeosciences, 13, 5151–5170, doi:10.5194/bg-13-5151-2016

www.biogeosciences.net/13/5151/2016/
https://www.biogeosciences.net/13/5151/2016/bg-13-5151-2016.pdf

Abstract: " Abstract. We examine climate change impacts on net primary production (NPP) and export production (sinking particulate flux; EP) with simulations from nine Earth system models (ESMs) performed in the framework of the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Global NPP and EP are reduced by the end of the century for the intense warming scenario of Representative Concentration Pathway (RCP) 8.5. Relative to the 1990s, NPP in the 2090s is reduced by 2–16% and EP by 7–18 %. The models with the largest increases in stratification (and largest relative declines in NPP and EP) also show the largest positive biases in stratification for the contemporary period, suggesting overestimation of climate change impacts on NPP and EP. All of the CMIP5 models show an increase in stratification in response to surface–ocean warming and freshening, which is accompanied by decreases in surface nutrients, NPP and EP."
&

Title: "Weaker land–climate feedbacks from nutrient uptake during photosynthesis inactive periods"

https://e3sm.org/wp-content/uploads/2018/12/CBGC-presentation-Nov-28-2018-WJ-Riley_opt.pdf

Primary production
https://en.wikipedia.org/wiki/Primary_production

Extract: " In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide.

Net primary production is the rate at which all the plants in an ecosystem produce net useful chemical energy; it is equal to the difference between the rate at which the plants in an ecosystem produce useful chemical energy (GPP) and the rate at which they use some of that energy during respiration. Some net primary production goes toward growth and reproduction of primary producers, while some is consumed by herbivores."

9
Decision makers need to consider consequences when evaluating such 'Deep Uncertainty' even such as the possible/probable abrupt end to the current global greening negative feedback mechanism:

Title: "‘Global Greening’ Sounds Good. In the Long Run, It’s Terrible."

https://www.nytimes.com/2018/07/30/science/climate-change-plants-global-greening.html

Extract: "As temperatures rise and rainfall patterns change, plants may stop soaking up extra carbon dioxide.

“Plants are quietly scrubbing the air of one China’s worth of carbon. What frightens me is knowing this can’t go on forever,” said Dr. Campbell. “If respiration catches up with photosynthesis, this huge carbon reservoir could spill back into our air.”

“There’s a wild card out there.”"

10
The linked reference confirms that current consensus estimates of ECS do not include the influence of land ice changes, and it analyzes paleodata to indicate that ECS is substantially higher than consensus estimates when accounting for the influence of land ice changes:

Stap, L. B., Köhler, P., and Lohmann, G.: Including the efficacy of land ice changes in deriving climate sensitivity from paleodata, Earth Syst. Dynam. Discuss., https://doi.org/10.5194/esd-2018-88, in review, 2018.

https://www.earth-syst-dynam-discuss.net/esd-2018-88/

Abstract. The influence of long-term processes in the climate system, such as land ice changes, has to be compensated for when comparing climate sensitivity derived from paleodata with equilibrium climate sensitivity (ECS) calculated by climate models, which is only generated by a CO2 change. Several recent studies found that the impact these long-term processes have on global temperature cannot be quantified directly through the global radiative forcing they induce. This renders the approach of deconvoluting paleotemperatures through a partitioning based on radiative forcings inaccurate. Here, we therefore implement an efficacy factor ε
  • , that relates the impact of land ice changes on global temperature to that of CO2 changes, in our calculation of climate sensitivity from paleodata. We apply our new approach to a proxy-inferred paleoclimate dataset, and find an equivalent ECS of 5.6±1.3K per CO2 doubling. The substantial uncertainty herein is generated by the range in ε
  • we use, which is based on a multi-model assemblage of simulated relative influences of land ice changes on the Last Glacial Maximum (LGM) temperature anomaly (46±14%). The low end of our ECS estimate, which concurs with estimates from other approaches, tallies with a large influence for land ice changes. To separately assess this influence, we analyse output of the PMIP3 climate model intercomparison project. From this data, we infer a functional intermodel relation between global and high-latitude temperature changes at the LGM with respect to the pre-industrial climate, and the temperature anomaly caused by a CO2 change. Applying this relation to our dataset, we find a considerable 64% influence for land ice changes on the LGM temperature anomaly. This is even higher than the range used before, and leads to an equivalent ECS of 3.8K per CO2 doubling. Together, our results suggest that land ice changes play a key role in the variability of Late Pleistocene temperatures.

11
In general terms both AR5 and CMIP5 under-represent the significance of feedback mechanisms from plants and particularly from forests.  The two linked references demonstrate that projected changes in forests will contribute to the collapse of the Amazon Rainforest, and to increasing Arctic Amplification, respectively per reference:

Kooperman, G. J., Chen, Y., Hoffman, F. M., Koven, C. D., Lindsay, K., Pritchard, M. S., … Randerson, J. T. (2018). Forest response to rising CO2 drives zonally asymmetric rainfall change over tropical land. Nature Climate Change, 8(5), 434–440. doi:10.1038/s41558-018-0144-7

https://www.nature.com/articles/s41558-018-0144-7

Abstract: "Understanding how anthropogenic CO2 emissions will influence future precipitation is critical for sustainably managing ecosystems, particularly for drought-sensitive tropical forests. Although tropical precipitation change remains uncertain, nearly all models from the Coupled Model Intercomparison Project Phase 5 predict a strengthening zonal precipitation asymmetry by 2100, with relative increases over Asian and African tropical forests and decreases over South American forests. Here we show that the plant physiological response to increasing CO2 is a primary mechanism responsible for this pattern. Applying a simulation design in the Community Earth System Model in which CO2 increases are isolated over individual continents, we demonstrate that different circulation, moisture and stability changes arise over each continent due to declines in stomatal conductance and transpiration. The sum of local atmospheric responses over individual continents explains the pan-tropical precipitation asymmetry. Our analysis suggests that South American forests may be more vulnerable to rising CO2 than Asian or African forests."
&

Abigail L. Swann, Inez Y. Fung, Samuel Levis, Gordon B. Bonan, and Scott C. Doney (January 26, 2010), "Changes in Arctic vegetation amplify high-latitude warming through the greenhouse effect", PNAS, 107 (4) 1295-1300; https://doi.org/10.1073/pnas.0913846107

https://www.pnas.org/content/107/4/1295

Abstract: "Arctic climate is projected to change dramatically in the next 100 years and increases in temperature will likely lead to changes in the distribution and makeup of the Arctic biosphere. A largely deciduous ecosystem has been suggested as a possible landscape for future Arctic vegetation and is seen in paleo-records of warm times in the past. Here we use a global climate model with an interactive terrestrial biosphere to investigate the effects of adding deciduous trees on bare ground at high northern latitudes. We find that the top-of-atmosphere radiative imbalance from enhanced transpiration (associated with the expanded forest cover) is up to 1.5 times larger than the forcing due to albedo change from the forest. Furthermore, the greenhouse warming by additional water vapor melts sea-ice and triggers a positive feedback through changes in ocean albedo and evaporation." Land surface albedo change is considered to be the dominant mechanism by which trees directly modify climate at high-latitudes, but our findings suggest an additional mechanism through transpiration of water vapor and feedbacks from the ocean and sea-ice.

12
The linked article indicates that both Arctic Amplification and GMSTA warmed fasted from 1998-2012 than AR5 reported due to missing data on the rate of warming from Arctic:

Title: "Recent Study Shows Amplified Arctic Temperature Increase during Perceived Haitus in Global Warming"

https://www.arcus.org/witness-the-arctic/2018/1/article/28403

Extract: "By analyzing this newly reconstructed global SAT dataset, we found that the Arctic warming amplification has significantly contributed to the global warming trend, causing a continued and even accelerated increase in global mean SAT, rather than a hiatus or slowdown of the trend as earlier studies suggested. The newly estimated rate of global SAT for 1998–2012 is around 0.112℃/decade, instead of 0.05℃/decade from IPCC AR5 (see Figure 1). In extending the analysis time period to the beginning of the twentieth century, we found a global warming rate at 0.091℃/decade from 1900–2014. A comparison with the rate of 0.075℃/decade and 0.089℃/decade from 1900–1998 and 1900–2012, respectively, suggests that global warming has even accelerated. Further analysis indicates that the Arctic SAT has increased at a rate of 0.755℃/decade during 1998–2012, which is more than six times the global average for the same time period. This indicates that the Arctic amplification has also been further enhanced."

Caption for the attached images: "Figure 1. Annual mean SAT anomalies (solid lines) relative to 1979–2004 climatology and their linear trends (dashed lines) over 1998–2012 for (a) the Arctic region (60–90ºN) and (b) the globe. In (a), the black lines are the results using the conventional Kriging interpolation, the red lines are the mean of two reconstructed datasets using the new method of DINEOF, and the blue shading represents the range of the two reconstructed datasets. In (b), the black lines show the results using global SATs from Karl et al. (2015), and the red lines and blue shading are the same as in (a) but for the globe. The numbers in the panels are the trends corresponding to the two dashed lines in the same colors. Figure courtesy of Huang et al. (2017)."

13
The linked reference indicates that warming sea surface conditions in the Southern Ocean, associated with continued global warming, will increase coastal precipitation (currently as snowfall).  Such increased coast snowfall, over the coming decades, can increase the surface gradient of marine glaciers, which increases the risk of ice cliff failures if/when the buttressing from adjoining ice shelves degrades.

Kittel, C., Amory, C., Agosta, C., Delhasse, A., Doutreloup, S., Huot, P.-V., Wyard, C., Fichefet, T., and Fettweis, X.: Sensitivity of the current Antarctic surface mass balance to sea surface conditions using MAR, The Cryosphere, 12, 3827-3839, https://doi.org/10.5194/tc-12-3827-2018, 2018.

https://www.the-cryosphere.net/12/3827/2018/

Abstract
Estimates for the recent period and projections of the Antarctic surface mass balance (SMB) often rely on high-resolution polar-oriented regional climate models (RCMs). However, RCMs require large-scale boundary forcing fields prescribed by reanalyses or general circulation models (GCMs). Since the recent variability of sea surface conditions (SSCs, namely sea ice concentration, SIC, and sea surface temperature, SST) over the Southern Ocean is not reproduced by most GCMs from the 5th phase of the Coupled Model Intercomparison Project (CMIP5), RCMs are then subject to potential biases. We investigate here the direct sensitivity of the Antarctic SMB to SSC perturbations around the Antarctic. With the RCM “Modèle Atmosphérique Régional” (MAR), different sensitivity experiments are performed over 1979–2015 by modifying the ERA-Interim SSCs with (i) homogeneous perturbations and (ii) mean anomalies estimated from all CMIP5 models and two extreme ones, while atmospheric lateral boundary conditions remained unchanged. Results show increased (decreased) precipitation due to perturbations inducing warmer, i.e. higher SST and lower SIC (colder, i.e. lower SST and higher SIC), SSCs than ERA-Interim, significantly affecting the SMB of coastal areas, as precipitation is mainly related to cyclones that do not penetrate far into the continent. At the continental scale, significant SMB anomalies (i.e greater than the interannual variability) are found for the largest combined SST/SIC perturbations. This is notably due to moisture anomalies above the ocean, reaching sufficiently high atmospheric levels to influence accumulation rates further inland. Sensitivity experiments with warmer SSCs based on the CMIP5 biases reveal integrated SMB anomalies (+5 % to +13 %) over the present climate (1979–2015) in the lower range of the SMB increase projected for the end of the 21st century.

14
The linked reference provides clear analysis that the current slow-down of the AMOC should contribute input to at least 20-years of unusually rapid global warming (see the first image).  This is bad news (especially when combined with the current reduction in La Nina activity and the relatively rapid reductions in anthropogenic sulfur dioxide emissions) for the risk that the WAIS may likely begin a phase of rapid ice mass loss by 2040 (which, once started, is driven by gravity):

Chen, X., & Tung, K.-K. (2018). Global surface warming enhanced by weak Atlantic overturning circulation. Nature, 559(7714), 387–391. doi:10.1038/s41586-018-0320-y

https://www.nature.com/articles/s41586-018-0320-y

Abstract: "Evidence from palaeoclimatology suggests that abrupt Northern Hemisphere cold events are linked to weakening of the Atlantic Meridional Overturning Circulation (AMOC), potentially by excess inputs of fresh water. But these insights—often derived from model runs under preindustrial conditions—may not apply to the modern era with our rapid emissions of greenhouse gases. If they do, then a weakened AMOC, as in 1975–1998, should have led to Northern Hemisphere cooling. Here we show that, instead, the AMOC minimum was a period of rapid surface warming. More generally, in the presence of greenhouse-gas heating, the AMOC’s dominant role changed from transporting surface heat northwards, warming Europe and North America, to storing heat in the deeper Atlantic, buffering surface warming for the planet as a whole. During an accelerating phase from the mid-1990s to the early 2000s, the AMOC stored about half of excess heat globally, contributing to the global-warming slowdown. By contrast, since mooring observations began in 2004, the AMOC and oceanic heat uptake have weakened. Our results, based on several independent indices, show that AMOC changes since the 1940s are best explained by multidecadal variability, rather than an anthropogenically forced trend. Leading indicators in the subpolar North Atlantic today suggest that the current AMOC decline is ending. We expect a prolonged AMOC minimum, probably lasting about two decades. If prior patterns hold, the resulting low levels of oceanic heat uptake will manifest as a period of rapid global surface warming."

Caption for first image: " Fig. 3 | AMOC and GSTA variations. a, Mid and subpolar latitude AMOC strength, as calculated at 41° N using altimetry measurements, from  ref. 18 (red, two-year running mean, Sverdrup scale shown on the right); inferred from integrated subpolar salinity in 0–1,500 m and 45–65° N in the Atlantic as a proxy, using the ISHII (dark blue) and Scripps (purple) datasets, with a two-year running mean. The green curve is the subpolar salinity, similarly calculated but using EN4. The AMOC fingerprint6  (dark blue) and the accumulated sea-level index (turquoise) calculated from historical tide gauge measurements22 were smoothed with 10-year and 7-year low-pass filters, respectively, from their sources. The subpolar gyre
SST index21 in orange is also a two-year running mean. See Methods for details. The inset shows RAPID-measured AMOC at 26° N. b, Shown are GSTA from HadCRUT4.6 (black), the nonlinear secular trend (close to the 100-year linear trend) (brown) and variation about the trend for timescales longer than decadal (multidecadal variability (MDV), red). The inset shows the SST spatial pattern associated with MDV obtained by regressing SST onto its time series. The blue curve is the smoothed version of GSTA obtained as the sum of the secular trend and MDV. The faint lines around the solid lines are from 100 ensemble members of the HadCRUT4.6, which assess the range of uncertainty of the data used in the solid lines."

See also (and the associated second image) which suggests that the bipolar seesaw mechanism may amplify still further the identified acceleration of global warming due to the current slow-down of the AMOC:

Title: "“Sluggish” Atlantic Circulation Could Cause Accelerated Global Warming"

https://cleantechnica.com/2018/07/19/sluggish-atlantic-circulation-could-cause-accelerated-global-warming/

Extract: "The AMOC does not run at the same speed and intensity, however, and as such a strong AMOC is typically associated with warming across the Northern Hemisphere — an association that is backed up by paleoclimatology studies (the study of climate changes over Earth’s history) that show warmer periods coincided with a strong or vigorous AMOC and colder periods coincided with a weaker AMOC.

The new study by Chen and Tung, however, presents a different way of looking at the role of the AMOC. Specifically, when taking into account the increased levels of atmospheric greenhouse gases, they suggest that climate mechanisms of the past might not necessarily provide a suitable gauge of how they would act in the present or the future. Specifically, Chen and Tung argue that half of the heat that stems from the ever-increasing atmospheric greenhouse gas levels is being stored in the deep waters of the North Atlantic by the AMOC, and that these levels are increasing and reducing global surface warming (as seen below).

Chen and Tung also explain that much work needs to be done to understand how the AMOC affects surface temperatures in other oceans and on different timescales. For example, they highlight the potential role of the mammoth Southern Ocean in heat uptake in the period since 2005 which could be part of a see-saw pattern of alternating heat uptake between the North Atlantic and Southern Oceans."

15
The total radiative forcings, RFs, from the linked ORNL website article by Blasing, T.J. (that updates such RF values reported in April 2016, see the first attached image/table) are used in the linked Wikipedia article to calculate a CO2e value of 526.6ppm; which assuming the current rate of annual increase in CO2e of about 3.5ppm indicates that late in 2018 CO2e exceeded 534ppm:

https://en.wikipedia.org/wiki/Carbon_dioxide_equivalent


Extract: "To calculate the CO2e of the additional radiative forcing calculated from April 2016's updated data: ∑ RF(GHGs) = 3.3793, thus CO2e = 280 e3.3793/5.35 ppmv = 526.6 ppmv."

http://cdiac.ornl.gov/pns/current_ghg.html


This relatively high value of 534ppm for CO2e appears to be associated with RF associated with tropospheric ozone and its chemical interaction in the atmosphere with GHGs like methane, as discussed in the following linked references.

Stevenson et al (2013), "Tropospheric ozone changes, radiative forcing and attribution to emissions in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP),"Atmos. Chem. Phys., 13, 3063–3085, doi:10.5194/acp-13-3063-2013

https://core.ac.uk/download/pdf/9666974.pdf

Abstract. Ozone (O3) from 17 atmospheric chemistry models taking part in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) has been used to calculate tropospheric ozone radiative forcings (RFs). All models applied a common set of anthropogenic emissions, which are better constrained for the present-day than the past. Future anthropogenic emissions follow the four Representative Concentration Pathway (RCP) scenarios, which define a relatively narrow range of possible air pollution emissions. We calculate a value for the pre-industrial (1750) to present-day (2010) tropospheric ozone RF of 410mWm−2. The model range of pre-industrial to present-day changes in O3 produces a spread (±1 standard deviation) in RFs of ±17 %. Three different radiation schemes were used – we find differences in RFs between schemes (for the same ozone fields) of ±10 %. Applying two different tropopause definitions gives differences in RFs of ±3 %. Given additional (unquantified) uncertainties associated with emissions, climate-chemistry interactions and land-use change, we estimate an overall uncertainty of ±30% for the tropospheric ozone RF. Experiments carried out by a subset of six models attribute tropospheric ozone RF to increased emissions of methane (44±12 %), nitrogen oxides (31±9 %), carbon monoxide (15±3 %) and non-methane volatile organic compounds (9±2 %); earlier studies attributed more of the tropospheric ozone RF to methane and less to nitrogen oxides. Normalising RFs to changes in tropospheric column ozone, we find a global mean normalised RF of 42mWm−2 DU−1, a value similar to previous work. Using normalised RFs and future tropospheric column ozone projections we calculate future tropospheric ozone RFs (mWm−2; relative to 1750) for the four future scenarios (RCP2.6, RCP4.5, RCP6.0 and RCP8.5) of 350, 420, 370 and 460 (in 2030), and 200, 300, 280 and 600 (in 2100). Models show some coherent responses of ozone to climate change: decreases in the tropical lower troposphere, associated with increases in water vapour; and increases in the sub-tropical to mid-latitude upper troposphere, associated with increases in lightning and stratosphere-to-troposphere transport. Climate change has relatively small impacts on global mean tropospheric ozone RF."

See also:

https://tes.jpl.nasa.gov/mission/climateO3/

Extract: "Tropospheric O3 is also the source of the hydroxyl radical (OH), which controls the abundance and distribution of many atmospheric constituents (including greenhouse gases such as methane and hydrochlorofluorocarbons). Ozone makes a significant contribution to the radiative balance of the upper troposphere and lower stratosphere, such that changes in the distribution of O3 in these atmospheric regions affect the radiative forcing of climate.

Climate Feedback and Forcing for Tropospheric Ozone

Climate forcing by O3 remains uncertain because O3 change as a function of altitude has been under-measured. In order to better understand the role of tropospheric O3 in climate, accurate temperature measurements are needed along with co-located O3 and CO profiles."

To point out the obvious, the ACCMIP reference shows significant increases in ozone RF by 2030 following a BAU pathway, i.e. "… we calculate future tropospheric ozone RFs (mWm−2; relative to 1750) for the four future scenarios (RCP2.6, RCP4.5, RCP6.0 and RCP8.5) of 350, 420, 370 and 460 (in 2030) …".

Finally, by comparing the major well mixed GHG concentrations from 1979 thru 2017 shown in the second attached image; with the major well mixed GHG concentrations over the past 800,000 years (in Antarctica) shown in the third image; one can readily see that not only CO₂ but also CH4 and N2O are driving us rapidly towards Mid-Pliocene conditions.

16
I have not had time to review the linked reference, so I am just posting here without comment:

Qin Wen and Jie Yao (2018), "Decoding Hosing and Heating Effects on Global Temperature and Meridional Circulations in a Warming Climate", Journal of Climate, https://doi.org/10.1175/JCLI-D-18-0297.1

https://journals.ametsoc.org/doi/full/10.1175/JCLI-D-18-0297.1

...

I have now reviewed Wen et al (2018), and perhaps I am over reacting to the fact that several media reports that I saw headlined the finding that: '… freshwater change has a significant cooling effect that can mitigate the global surface warming by as much as ~30%. … In terms of global temperature and Earth’s energy balance, the freshwater change plays a stabilizing role in a warming climate.'.  Nevertheless, I first note that 'All models are wrong, but some models are useful', and I offer the following comments about the following extracts and first two images from Wen et al (2018), as related to the following three linked references:

1. The extract and the second linked reference shows that the authors used an older version of CESM that has been superseded since June 2018 by a version with many improvements/corrections including improved subroutines for ice sheet behavior and for cloud feedback, which are both important when talking about 'Decoding Hosing and Heating Effects on Global Temperature and Meridional Circulations in a Warming Climate'.

2. The first two images make it clear that the authors' model considers a relatively slow rate of ice mass loss from the GIS and the AIS over many hundreds of years, and thus clearly does not consider abrupt ice mass loss such as that induced by ice-cliff failures and/or hydrofracturing.  Thus, the estimated 30% reduction in global warming occurs over a 2,000 year period, while Hansen et al (2016)'s larger reduction in global warming occurs over several decades and then this cooling dissipates in subsequent decades.  Furthermore, Wen et al (2018)'s findings that freshwater hosing stabilizes the Earth's energy balances is likely similarly related to the hosing scenario that they assume [which is radically less dynamic than that of Hansen et al (2016)].

3. Wen et al. (2018) acknowledge that they only consider two feedback mechanisms (i.e. the long wave and short wave radiation due to surface temperature and the impacts on the MOC) of the may involved in the net ice-climate feedback mechanism. and they ignore other such hosing feedback mechanisms as the influence of: '… the wind-driven circulations and subduction in the midlatitudes, the intermediate water formation in the subpolar Antarctic, and the atmospheric monsoon system? How are climate variabilities, such as El Niño–Southern Oscillation, the Pacific decadal oscillation, and the Atlantic multidecadal oscillation, modulated by freshwater change and radiation forcing?'. In this regards the authors' findings would have more meaning if they had bothered to calibrate their models response to match paleo data [see the third linked reference, Maier et al (2018)], as E3SM has done and I note that CESM 2.0 (which superseded the version used by the authors) adopted many of the calibrations identified by E3SM.

4. The authors define numerous terms that are consistent within the reference (and thus acceptable in peer review), but that differ from common usage, e.g.: their definition of 'sea ice' includes marine glacial ice.

5. I could go on with my critiques but leave it to say that within the author's limited set of assumptions their work appears to be consistent with other CMIP5 era of projections, but in my option they fall behind the CMIP6 level of sophistication, so I look forward to seeing the CMIP6 (and AR6) findings for freshwater hosing impact (while noting that none of them are currently considering ice-cliff failures nor hydrofracting of ice sheets).

Extract: "The hosing effect, in this work, refers to ocean freshwater flux change, which can directly change ocean salinity (Durack and Wijffels 2010) and thus upper-ocean buoyancy, affecting mainly the thermohaline circulation and thus the oceanic heat transport (OHT) (Swingedouw et al. 2007, 2009; Yang et al. 2013, 2017).

In this study, the heating and hosing effects are separated in a coupled climate model through two groups of global warming experiments. It is shown that the freshwater change has a significant cooling effect that can mitigate the global surface warming by as much as ~30%. Two significant regional cooling centers appear: one in the subpolar Atlantic and one in the Southern Ocean; both are triggered by sea ice melting but are sustained by different mechanisms. The subpolar Atlantic cooling is maintained by the weakened AMOC in the NH, while the Southern Ocean surface cooling is maintained by the enhanced northward Ekman flow related to strengthened westerly wind (Ferreira et al. 2015; Kostov et al. 2017). In these two regions, the effect of freshwater flux change dominates over that of radiation flux change, controlling the SST change in the warming climate.

The model used in this study is the Community Earth System Model (CESM) of the National Center for Atmospheric Research (NCAR), which was used in our previous studies (e.g., Dai et al. 2017). CESM is a fully coupled global climate model that provides state-of-the-art simulations of Earth’s past, present, and future climate states (http://www.cesm.ucar.edu/). CESM (version 1.0) consists of five components and one coupler: The Community Atmosphere Model, version 5 (CAM5; Park et al. 2014); the Community Land Model, version 4 (CLM4; Lawrence et al. 2012); the Community Ice Code, version 4 (CICE4; Hunke and Lipscomb 2008); the Parallel Ocean Program, version 2 (POP2; Smith et al. 2010); the Community Ice Sheet Model (Glimmer-CISM); and the CESM coupler, version 7 (CPL7). CESM1.0 is widely used and validated by researchers in the community.

This work is the first step toward quantifying the individual contributions of the heating and hosing effects to an evolving climate. Only the surface temperature and large-scale circulations are examined in this paper. Many other aspects have not been considered in the present study. For example, how do the hosing effect and the heating effect influence the wind-driven circulations and subduction in the midlatitudes, the intermediate water formation in the subpolar Antarctic, and the atmospheric monsoon system? How are climate variabilities, such as El Niño–Southern Oscillation, the Pacific decadal oscillation, and the Atlantic multidecadal oscillation, modulated by freshwater change and radiation forcing? It should be recognized that climate models have many limitations and that a climate shift in the model experiments may also exist. Many questions remain about the roles of the hydrological cycle in global change."

Caption for the first image: "FIG. 1. (a) Temporal evolutions of globally integrated net radiative flux (black), net downward SW (blue), and outgoing LW (red) at TOA (PW; positive for downward anomaly) under 2CO2 forcing. The red line at the top denotes the CO2 forcing. (b),(c) As in (a), but showing the hosing effect and heating effect, respectively. Each curve is smoothed with a 20-yr running mean. (d) Radiative flux change at the TOA in stage I of global warming, with net radiative flux (black), SW (blue), and LW (red). Stage I spans the years 200–500. (e),(f) As in (d), but showing the hosing effect and heating effect, respectively."

Caption for the second image: "FIG. 2. Temporal evolutions of (a)–(c) SST (thick solid curves) and SAT (thin dashed curves) averaged over the globe (black), NH (red), and SH (blue) (°C); (d)–(f) percentage changes of the AMOC (black), the Indo-Pacific STC (blue), and HC (red; %); and (g)–(i) AHT (red), global OHT (blue), Atlantic OHT (green), and Indo-Pacific OHT (light blue) averaged over 30°–70°N (PW). The AMOC index is defined as the maximum of the streamfunction in the range of 0°–10°C isotherms over 20°–70°N in the Atlantic. The Indo-Pacific STC is similarly defined, but in the range of 20°–30°C isotherms over 0°–30°N. The HC index is defined as the maximum streamfunction between 200 and 1000 hPa over 0°–30°N. All indexes are normalized by their time-mean values in CTRL, which are 18, 36, and 92 Sv, respectively (1 Sv = 106 m3 s−1 for the ocean; 1 Sv = 109 kg s−1 for the atmosphere). Each curve is smoothed with a 20-yr running mean. Stage I spans the years 200–500 and represents an earlier quasi-equilibrium stage of global warming, based on the AMOC evolution. Stage II spans the years 800–1100 and represents the recovery stage of the AMOC. Stage III spans the years 1700–2000 and represents the equilibrium stage of global warming for (a),(d),(g) 2CO2 forcing; (b),(e),(h) hosing effect; and (c),(f),(i) heating effect."

See also:

Qianzi Yang et al. (2018), "Understanding Bjerknes Compensation in Meridional Heat Transports and the Role of Freshwater in a Warming Climate", Journal of Climate, https://doi.org/10.1175/JCLI-D-17-0587.1

https://journals.ametsoc.org/doi/full/10.1175/JCLI-D-17-0587.1

Abstract: "The Bjerknes compensation (BJC) under global warming is studied using a simple box model and a coupled Earth system model. The BJC states the out-of-phase changes in the meridional atmosphere and ocean heat transports. Results suggest that the BJC can occur during the transient period of global warming. During the transient period, the sea ice melting in the high latitudes can cause a significant weakening of the Atlantic meridional overturning circulation (AMOC), resulting in a cooling in the North Atlantic. The meridional contrast of sea surface temperature would be enhanced, and this can eventually enhance the Hadley cell and storm-track activities in the Northern Hemisphere. Accompanied by changes in both ocean and atmosphere circulations, the northward ocean heat transport in the Atlantic is decreased while the northward atmosphere heat transport is increased, and the BJC occurs in the Northern Hemisphere. Once the freshwater influx into the North Atlantic Ocean stops, or the ocean even loses freshwater because of strong heating in the high latitudes, the AMOC would recover. Both the atmosphere and ocean heat transports would be enhanced, and they can eventually recover to the state of the control run, leading to the BJC to become invalid. The above processes are clearly demonstrated in the coupled model CO2 experiment. Since it is difficult to separate the freshwater effect from the heating effect in the coupled model, a simple box model is used to understand the BJC mechanism and freshwater’s role under global warming. In a warming climate, the freshwater flux into the ocean can cool the global surface temperature, mitigating the temperature rise. Box model experiments indicate clearly that it is the freshwater flux into the North Atlantic that causes out-of-phase changes in the atmosphere and ocean heat transports, which eventually plays a stabilizing role in global climate change."
&

Title: "CESM - Community Earth System Model"

http://www.cesm.ucar.edu/management/CSL/files/cesm_csl_accomplishments_2017-2018.pdf

Extract: "Subsequently, a major milestone was the long-waited, community release of the CESM version 2.0, CESM2.0, in early June 2018. This new version contains many substantial science and infrastructure improvements and capabilities for use of the broader CESM and international community. These new advancements include: an atmospheric model component that incorporates significant improvements to its turbulence and convection representations, opening the way for an analysis of how these small-scale processes can impact the climate; improved ability to simulate modes of tropical variability that can span seasons and affect global weather patterns; a land ice sheet model component for Greenland that can simulate the complex way the ice sheet moves – sluggish in the middle and much more quickly near the coast – and does a better job of simulating calving of the ice into the ocean; …

AMWG CSL resources were used to address a serious defect, i.e., cooling of the global mean surface temperatures over the mid- to late 20th century, that appeared in CESM2 with the introduction of CMIP6 emissions data. A number of explanations were considered for this behavior. Studies of cloud properties influenced by volcanic sulfate emissions (Malavelle et al. 2017) suggested that CAM6 microphysics may be overestimating the 2nd indirect aerosol effect (the ‘lifetime’ effect). The lifetime effect is essentially a consequence of a droplet number effect on auto conversion rates that goes as ~ N− where N is the cloud droplet number concentration, which is directly related to aerosol concentrations. As the exponent  becomes larger, auto conversion rates become slower so that “dirtier” air leads to longer lived cloud. Removing the N dependence altogether, i.e., =0, appeared to resolve the cooling. However, this is not considered a physically plausible option. Extensive experimentation with alternate formulations of the auto conversion dependence on N was conducted. A compromise formulation with =1.1 similar to that used in DOE’s E3SM was adopted."
&

E. Maier et al. (2018), "North Pacific freshwater events linked to changes in glacial ocean circulation", Nature, Vol. 559, https://doi.org/10.1038/s41586-018-0276-y

https://www.geo.umass.edu/climate/papers2/Maier_2018.pdf

Abstract: "There is compelling evidence that episodic deposition of large volumes of freshwater into the oceans strongly influenced global ocean circulation and climate variability during glacial periods. In the North Atlantic region, episodes of massive freshwater discharge to the North Atlantic Ocean were related to distinct cold periods known as Heinrich Stadials. By contrast, the freshwater history of the North Pacific region remains unclear, giving rise to persistent debates about the existence and possible magnitude of climate links between the North Pacific and North Atlantic oceans during Heinrich Stadials4,5. Here we find that there was a strong connection between changes in North Atlantic circulation during Heinrich Stadials and injections of freshwater from the North American Cordilleran Ice Sheet to the northeastern North Pacific. Our record of diatom δ18O (a measure of the ratio of the stable oxygen isotopes 18O and 16O) over the past 50,000 years shows a decrease in surface seawater δ18O of two to three per thousand, corresponding to a decline in salinity of roughly two to four practical salinity units. This coincided with enhanced deposition of ice-rafted debris and a slight cooling of the sea surface in the northeastern North Pacific during Heinrich Stadials 1 and 4, but not during Heinrich Stadial 3. Furthermore, results from our isotope-enabled model suggest that warming of the eastern Equatorial Pacific during Heinrich Stadials was crucial for transmitting the North Atlantic signal to the northeastern North Pacific, where the associated subsurface warming resulted in a discernible freshwater discharge from the Cordilleran Ice Sheet during Heinrich Stadials 1 and 4. However, enhanced background cooling across the northern high latitudes during Heinrich Stadial 3—the coldest period in the past 50,000 years—prevented subsurface warming of the northeastern North Pacific and thus increased freshwater discharge from the Cordilleran Ice Sheet. In combination, our results show that nonlinear ocean– atmosphere background interactions played a complex role in the dynamics linking the freshwater discharge responses of the North Atlantic and North Pacific during glacial periods."

Extract: "The results of our data–model comparison provide compelling evidence that, during North Atlantic cold stadials characterizing the past 50,000 years, perturbations to the AMOC could have been teleconnected to the northeastern North Pacific region, triggering freshwater discharge events via interactions between low and high latitudes and between oceans and the atmosphere. Until now, such North Pacific freshwater input events have not been considered as standard forcing components in glacial climate simulations; the incorporation of this freshwater forcing scenario provides a new basis for research that could reconcile the discrepancies within proxy data regarding the responses of North Pacific ocean circulation to AMOC changes."

Caption for the third image: "Fig. 2 | Proxy data from the North Pacific and North Atlantic (50 kyr to 5 kyr bp). a–f, Data from northeastern North Pacific core SO202-27-6 (in b, e and f, data for the past 25 kyr bp are from ref. 12). a, Ice-rafted debris. b, δ18Odiat. data; error bars show the errors of replicate analyses or the long-term reproducibility of standards (1σ). c, Surface δ18Osw; dark grey and light grey envelopes show 68% and 95% confidence intervals, respectively. d, Sea-surface salinity calculated from surface δ18Osw; green envelopes show 95% confidence intervals, assuming a CIS meltwater δ18O of −20‰ (light green) or −30‰ (dark green). e, Subsurface δ18Opl.foram. data from sinistral N. pachyderma. f, Alkenone-based SSTs. g, Alkenone-based (solid line) and magnesium/calcium-based (dashed line) SSTs (from the eastern Equatorial Pacific, core MD02-2529; ref. 25). h, Sediment total reflectance (from the Cariaco Basin; ref. 24). L*, lightness; sm200, 200-point running mean. i, 231Pa/230Th ratio (Ocean Drilling Program (ODP) site 1063; ref. 3). j, NGRIP δ18O record7. EEP, eastern Equatorial Pacific; HS, Heinrich Stadial; ITCZ, Intertropical Convergence Zone. Arrows indicate the direction of proxy changes during Heinrich Stadials 1, 3 and 4."


17
I have not had time to review the linked reference, so I am just posting here without comment:

Qin Wen and Jie Yao (2018), "Decoding Hosing and Heating Effects on Global Temperature and Meridional Circulations in a Warming Climate", Journal of Climate, https://doi.org/10.1175/JCLI-D-18-0297.1

https://journals.ametsoc.org/doi/full/10.1175/JCLI-D-18-0297.1

Abstract: "The global temperature changes under global warming result from two effects: one is the pure radiative heating effect caused by a change in greenhouse gases, and the other is the freshwater effect related to changes in precipitation, evaporation, and sea ice. The two effects are separated in a coupled climate model through sensitivity experiments in this study. It is indicated that freshwater change has a significant cooling effect that can mitigate the global surface warming by as much as ~30%. Two significant regional cooling centers occur: one in the subpolar Atlantic and one in the Southern Ocean. The subpolar Atlantic cooling, also known as the “warming hole,” is triggered by sea ice melting and the southward cold-water advection from the Arctic Ocean, and is sustained by the weakened Atlantic meridional overturning circulation. The Southern Ocean surface cooling is triggered by sea ice melting along the Antarctic and is maintained by the enhanced northward Ekman flow. In these two regions, the effect of freshwater flux change dominates over that of radiation flux change, controlling the sea surface temperature change in the warming climate. The freshwater flux change also results in the Bjerknes compensation, with the atmosphere heat transport change compensating the ocean heat transport change by about 80% during the transient stage of global warming. In terms of global temperature and Earth’s energy balance, the freshwater change plays a stabilizing role in a warming climate."

18
For those who are interested, I provide the attached schedule for the IPCC AR6 WG1 authors from the linked website.  Maybe there will be leaks of the first draft after April 2019 and the final version should be made public in 2021:

https://wg1.ipcc.ch/AR6/AR6.html


19
The linked reference provides novel evidence from coral samples recovered from a few sites on Antarctica's continental shelf and slope, that indicate an intrusion of warm CDW occurred in these locations (see the attached image) around 1830 AD, which was likely associated with both a reduction of the Overturning Current and an increases in the circumpolar westerly wind.  This paleo evidence provides support for two ice-climate feedback mechanisms.

Theresa M. King et al. (24 October 2018), "Large‐Scale Intrusion of Circumpolar Deep Water on Antarctic Margin Recorded by Stylasterid Corals", Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2018PA003439

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018PA003439

Abstract: "We present centennial-scale radiocarbon (14C) records archived by deep sea stylasterid corals from the outer shelf and upper slope of the Antarctic margin.  These novel stylasterids (Errina spp.) were collected from the western Ross Sea shelf (500 m) and slope (1700 m), as well as the eastern Wilkes Land shelf (670 m).  We provide two corals from each region and document an abrupt reversal of 14C ages in the upper (younger) part of each coral.  We test the statistical robustness of each record and demonstrate the significance of the age reversals, as well as the ability of these corals to record environmental change.   We discuss a variety of possible drivers for this 14C reversal, and conclude that it is most likely an encroachment of 14C-depeleted Circumpolar Deep Water (CDW).  This water mass has regionally intruded onto the Antarctic margin in recent decades, facilitating loss of grounded Antarctic ice; this has implications for global sea level, deep water formation, and carbon sequestration in the Southern Ocean.  Thus, understanding past variability of CDW on the margin is vital to better constrain climate change trajectories in the near future.   We estimate large-scale encroachment of CDW onto the shelf likely commencing after 1830 CE (±120 yr).  We present possible drivers for the intrusion, but highlight the need for additional chronologic constraint.  This study not only demonstrates the utility of a novel coral taxon, but also presents the paleoceanographic community with testable hypothesis concerning recent, widespread CDW intrusion."

Extract: "Based on estimated calibrated calendar ages, our records suggest that the onset of the intrusion could have aligned with the termination of the Little Ice Age event.  This time period is accompanied by both reduction of deep water formation and strengthening of the westerly winds, two mechanisms that could be responsible for the shoaling of CDW onto the shelf."

20
The first image shows the Antarctic Bedmap with the ice instantly removed, while the second image shows the Antarctic Bedmap with the ice removed and all associated isostatic rebound recovered.  Also, the dotted lines on the five cross-sections thru the WAIS on the third image shows the hundreds of meters of rebound that would be recovered if the WAIS were to abruptly collapse.  This much rebound would certainly contribute to an increase in local seismic and volcanic activity.

Edit: For a plan view of the candidate seaways proposed by Vaughan in the third image, see Reply #328

21
The attached image is from the linked open access reference & provides background information relevant to different possible definitions for the Anthropocene.  This image implies that without anthropogenic radiative forcing Earth would now be headed for another glacial period; and I wonder whether this temperature trend may also act as yet another factor masking a relatively high value of ECS; also I wonder whether the upward tend of atmospheric methane concentration since 5,000 years ago, makes the Anthropocene's radiative forcing signature much different than past interglacial periods such as that for the Mid-Pliocene.

Lewis, S. L.; Maslin, M. A. (12 March 2015). "Defining the Anthropocene". Nature 519: 171–180. doi:10.1038/nature14258

http://www.nature.com/nature/journal/v519/n7542/full/nature14258.html
https://eorder.sheridan.com/3_0/app/orders/4609/article.php#171


22
As the poles continue to warm, we should not forget that blooms of microbes on the ice surface can decrease albedo and increase the rate of ice mass loss both in Greenland and Antarctica:

Title: "Guest post: Is ‘glacier carbon’ good or bad for the climate?"

https://www.carbonbrief.org/guest-post-is-glacier-carbon-good-or-bad-for-the-climate

Extract: "When microbes draw down CO2, they grow and divide, leading to increased biomass on the ice surface.
This accumulation of organic matter on the ice surface is very dark in colour and, therefore, quickly absorbs sunlight, transferring the excess energy into the ice as heat."
&

Title: "Mysterious Microbes Turning Polar Ice Pink, Speeding Up Melt"

https://www.nationalgeographic.com/environment/2018/09/news-greenland-ice-sheet-melting-arctic-algae/

Extract: "Thriving communities of red algae are doing something nefarious to the world's ice sheets: melting them more quickly."

See also:
https://www.nature.com/articles/s41467-018-03353-2


23
ASLR,

Considering that you frequently talk about the importance of ENSO events to the pace of Antarctic melt, I thought I should post this new paper (out yesterday) here:

Increased variability of eastern Pacific El Niño under greenhouse warming Wenju Cai, Guojian Wang, Boris Dewitte, Lixin Wu, Agus Santoso, Ken Takahashi, Yun Yang, Aude Carréric & Michael J. McPhaden https://www.nature.com/articles/s41586-018-0776-9
...

This is an indication of both increasingly high values of ECS and of increasingly high levels of telecommunication of energy from the Tropical Pacific directly to West Antarctica.

24
With regards to my last post, some readers may wonder what Steffen et al (2018) mean by 'Hothouse' conditions (as opposed to Pliocene or Miocene conditions).  Generally, 'Hothouse' conditions can be taken as Early Eocene Climatic Optimum (EECO; ∼52–50 Ma) conditions as discussed in the linked reference Evans et al. (2018) and the associated attached images.  Note that as the reference demonstrates that no current ESM (including FAMOUS which has been tailored for the EECO) can accurately project the full extent of Polar Amplification during the EECO; this means that if we keep following SSP5-Baseline long enough to reach atmospheric CO₂ concentrations around 560 ppm, then depending on the history of our climate change momentum, the Earth's climate could flip into an equable conditions (characterized by warm poles):

David Evans, Navjit Sagoo, Willem Renema, Laura J. Cotton, Wolfgang Müller, Jonathan A. Todd, Pratul Kumar Saraswati, Peter Stassen, Martin Ziegler, Paul N. Pearson, Paul J. Valdes, and Hagit P. Affek (January 22, 2018), "Eocene greenhouse climate revealed by coupled clumped isotope-Mg/Ca thermometry", PNAS February 6, 2018 115 (6) 1174-1179; https://doi.org/10.1073/pnas.1714744115

https://www.pnas.org/content/115/6/1174
https://www.pnas.org/content/pnas/115/6/1174.full.pdf

Significance
Reconstructing the degree of warming during geological periods of elevated CO2 provides a way of testing our understanding of the Earth system and the accuracy of climate models. We present accurate estimates of tropical sea-surface temperatures (SST) and seawater chemistry during the Eocene (56–34 Ma before present, CO2 >560 ppm). This latter dataset enables us to reinterpret a large amount of existing proxy data. We find that tropical SST are characterized by a modest warming in response to CO2. Coupling these data to a conservative estimate of high-latitude warming demonstrates that most climate simulations do not capture the degree of Eocene polar amplification.

Abstract

Past greenhouse periods with elevated atmospheric CO2 were characterized by globally warmer sea-surface temperatures (SST). However, the extent to which the high latitudes warmed to a greater degree than the tropics (polar amplification) remains poorly constrained, in particular because there are only a few temperature reconstructions from the tropics. Consequently, the relationship between increased CO2, the degree of tropical warming, and the resulting latitudinal SST gradient is not well known. Here, we present coupled clumped isotope (Δ47)-Mg/Ca measurements of foraminifera from a set of globally distributed sites in the tropics and midlatitudes. Δ47 is insensitive to seawater chemistry and therefore provides a robust constraint on tropical SST. Crucially, coupling these data with Mg/Ca measurements allows the precise reconstruction of Mg/Casw throughout the Eocene, enabling the reinterpretation of all planktonic foraminifera Mg/Ca data. The combined dataset constrains the range in Eocene tropical SST to 30–36 °C (from sites in all basins). We compare these accurate tropical SST to deep-ocean temperatures, serving as a minimum constraint on high-latitude SST. This results in a robust conservative reconstruction of the early Eocene latitudinal gradient, which was reduced by at least 32 ± 10% compared with present day, demonstrating greater polar amplification than captured by most climate models.

Caption for second image: "Fig. 2 Seawater Mg/Ca reconstruction for the Eocene and early Oligocene based on coupled Δ47-Mg/Ca LBF and ridge-flank CaCO3 vein (CCV) data, shown in the context of previous Cenozoic reconstructions (33, 34, 56, 57) and box models (refs. 35, 36, and 58; WA89, SH98, and HS15, respectively), that are commonly used for calculating planktonic and deep-benthic foraminifera Mg/Ca data. Coral-derived data younger than 20 Ma are omitted. The 95% confidence intervals on our Eocene Mg/Casw curve are derived from bootstrapping 1,000 locally weighted scatterplot smoothing (LOWESS) fits, including both geochemical and dating uncertainties."

Caption for third image: "Fig. 5. Early Eocene (48–56 Ma) model-data comparison. (A) Zonally averaged latitudinal gradients based on proxy CO2 and SST data (gray box) and climate models over a range of CO2 (circles) (12, 46–48, 60). Proxy CO2 range is from ref. 1; the gradient uncertainty is the combined 2 SE of the tropical and high-latitude proxy data (see text). Proxy-derived gradient is shown relative to present day; Eocene climate model simulations are shown relative to their preindustrial counterpart. Most model simulations do not capture the reduced latitudinal gradient within the range of proxy CO2 (<2,250 ppm). (B) Site-specific model-data comparison for both the tropics and high latitudes. Model SST competency assessed by comparing the mean difference between the model and proxy data for low and high latitudes. Quadrants reflect different overall patterns of model-data offset. Hypothetical simulations falling on the 1:1 line would reconstruct the same latitudinal gradient as the data but not the same absolute SST, except at the origin. All models fall below this line, indicating that Eocene polar amplification is underestimated."

25
I have read that AR6 will provide more guidance to policy makers on the nature and possible consequences of 'Deep Uncertainty', for example the linked reference & associated first image [which show that long-tailed ECS values represent the highest risk (probability times consequences) to society], provides one simple example of how the possible consequences of long-tailed risk (per AR5) could be conveyed better.

Rowan T. Sutton (2018), "ESD Ideas: a simple proposal to improve the contribution of IPCC WGI to the assessment and communication of climate change risks", Earth Syst. Dynam., 9, 1155–1158, https://doi.org/10.5194/esd-9-1155-2018

https://www.earth-syst-dynam.net/9/1155/2018/esd-9-1155-2018.pdf

Abstract: "The purpose of the Intergovernmental Panel on Climate Change (IPCC) is to provide policy-relevant
assessments of the scientific evidence about climate change. Policymaking necessarily involves risk assessments,
so it is important that IPCC reports are designed accordingly. This paper proposes a specific idea, illustrated with
examples, to improve the contribution of IPCC Working Group I to informing climate risk assessments."

Extract: "Some will argue that the WGII report is needed to provide information on impacts. For detailed information this is certainly the case, but the general shape of the damage function for a large basket of impacts (Fig. 1) is insensitive to such details and is all that is needed to justify WGI providing a much more thorough assessment of relevant scenarios. Other critics will suggest that for WGI to explicitly identify high-impact scenarios would constitute scaremongering; this concern is no doubt one reason why previous WGI reports have focused so much on the likely range. But it is misguided (see also Emanuel, 2014). Policymakers need to know about high-impact scenarios and WGI has a responsibility to contribute its considerable expertise to making the appropriate assessments."

Caption for the attached image: "Figure 1. A schematic representation of how climate change risk depends on equilibrium climate sensitivity (ECS). (a) A possible likelihood distribution consistent with the IPCC AR5 assessment that “Equilibrium climate sensitivity is likely in the range 1.5 to 4.5 _C (high confidence), extremely unlikely less than 1C (high confidence) and very unlikely greater than 6C (medium confidence)”. (b) A schematic illustration of the fact that, for a given emissions scenario, the cost of impacts and adaptation rises very rapidly (shown here as an exponential damage function) with ECS. (c) In this example, the resultant risk (quantified here as likelihood x impact) is highest for high ECS values. The precise shape of the risk curve is dependent on assumptions about the shape of the likelihood and damage functions at high sensitivity (Weitzman, 2011) (figure by Ed Hawkins)."

Furthermore, I believe that plots such as the second image could better convey the need to design civil features (say flood protection) to meet much higher capacity levels than the mean demand levels typically discussed in the media.

Lastly, I hope that AR6 provides much better guidance than AR5 did, with regard to such matters as the risks and consequences associated with:

1. Various ice-climate feedback mechanisms associated with rapid ice mass loss from ice sheets.

2. Risks off abrupt changes in climate state (say from Mid-Pliocene conditions to Early-Pliocene conditions) due to potential cascading of tipping points for different feedback mechanisms.

3. The potential impacts of the current very high rates of radiative forcing as compared to all paleo cases evaluated.

4. The fact that PDFs (probability density functions) shift with continued warming, so there are consequences with delaying effective climate action [see the third image from Steffen et al (2018)].

26
I note that I remember four different definitions of pre-industrial in AR5, while the summary declines to specify which definition policy makers should follow.  Furthermore, the linked reference indicates that the proper definition of the pre-industrial baseline could add up to +0.2C to IPCC projections of GMSTA; while the Paris Accord has declined to adopt Schurer et al. (2017)'s proposed definition.

Schurer et al (2017), "Importance of the pre-industrial baseline for likelihood of exceeding Paris goals", Nature Climate Change 7, 563-567, doi:10.1038/nclimate3345

http://www.nature.com/nclimate/journal/v7/n8/full/nclimate3345.html?foxtrotcallback=true

Abstract: "During the Paris conference in 2015, nations of the world strengthened the United Nations Framework Convention on Climate Change by agreeing to holding ‘the increase in the global average temperature to well below 2◦C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5◦C’ (ref. 1). However, ‘pre-industrial’ was not defined. Here we investigate the implications of different choices of the pre-industrial baseline on the likelihood of exceeding these two temperature thresholds. We find that for the strongest mitigation scenario RCP2.6 and a medium scenario RCP4.5, the probability of exceeding the thresholds and timing of exceedance is highly dependent on the pre-industrial baseline; for example, the probability of crossing 1.5◦C by the end of the century under RCP2.6 varies from 61% to88% depending on how the baseline is defined. In contrast, in the scenario with no mitigation, RCP8.5, both thresholds will almost certainly be exceeded by the middle of the century with the definition of the pre-industrial baseline of less importance. Allowable carbon emissions for threshold stabilization are similarly highly dependent on the pre-industrial baseline. For stabilization at 2◦C, allowable emissions decrease by as much as 40% when earlier than nineteenth-century climates are considered as a baseline."
Extract: "In total, spatially complete blended global temperatures from 23 simulations, from 7 different models, were analysed with the means of each model for the period 1401–1800 found to be cooler than the late-nineteenth-century baseline (1850–1900) by 0.03◦C to 0.19◦C (multi-model mean of 0.09◦C, Fig.2b). In these simulations, and in temperature reconstructions of the past millennium, there is considerable centennial variability. Some periods, such as the sixteenth century, are of comparable warmth to the late nineteenth century, while other periods have a multi-model mean nearly 0.2◦C cooler."

Caption of attached image: "Figure2 | Model-simulated difference in global mean temperature between different pre-industrial periods and 1850–1900. a, Range of ensemble means for different models, and for different forcing combinations. Model distribution fitted with a kernel density estimate (violin plot)—red, all forcings combined; green, greenhouse gas forcing alone; blue, volcanic forcing alone; yellow, solar forcing alone. Model mean: circle; 10–90% model range: bar. Differences refer to the mean of the period enclosed by the dashed lines; except on the far right, where they are means for the full period 1401–1800 (relative to 1850–1900). b–e, Model means for different forcing combinations—colours, ensemble means for individual models; black line, mean over all models."

Obviously, such definitions matter when we are trying to decide by which decade we have approached Mid-Pliocene conditions, and can also impact estimates of ECS.

27
As anthropogenic forcing is the largest source of climate change, one of the 'Deepest Uncertainties' is whether decision makers will accept that the probability of near-term abrupt climate change is serious enough to do something effective about it.  In this regard, I provide the attached image from the linked article that indicates that scientific education about climate change is insufficient to prevent tribalism among conservatives, but rather it takes 'science curiosity' to be open-minded enough to accept this risk.  It is not clear to me that decision makers exhibit sufficient 'science curiosity' to avoid ice-climate induced abrupt climate change in the coming decades:

Title: "Why Smart People Are Vulnerable to Putting Tribe Before Truth" by Dan Kahan, SciAm 2018.

https://blogs.scientificamerican.com/observations/why-smart-people-are-vulnerable-to-putting-tribe-before-truth/

Extract: "Science literacy is important, but without the parallel trait of 'science curiosity," it can lead us astray."

28
First, I note that a jökulhlaup is a glacial outburst of meltwater, and the first linked article demonstrates that with Antarctic subglacial lakes and drainage systems such event can be regulated by the nature of pressure waves passing through the system over multiple years; and that the pressures associated with such waves can regulate the glacial ice flow velocities.

C. F. Dow et al. (22 March 2018), "Dynamics of Active Subglacial Lakes in Recovery Ice Stream", JGR Earth Surface, https://doi.org/10.1002/2017JF004409

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017JF004409

Abstract
Recovery Ice Stream has a substantial number of active subglacial lakes that are observed, with satellite altimetry, to grow and drain over multiple years. These lakes store and release water that could be important for controlling the velocity of the ice stream. We apply a subglacial hydrology model to analyze lake growth and drainage characteristics together with the simultaneous development of the ice stream hydrological network. Our outputs produce a good match between modeled lake location and those identified using satellite altimetry for many of the lakes. The modeled subglacial system demonstrates development of pressure waves that initiate at the ice stream neck and transit to within 100 km of the terminus. These waves alter the hydraulic potential of the ice stream and encourage growth and drainage of the subglacial lakes. Lake drainage can cause large R‐channels to develop between basal overdeepenings that persist for multiple years. The pressure waves, along with lake growth and drainage rates, do not identically repeat over multiple years due to basal network development. This suggests that the subglacial hydrology of Recovery Ice Stream is influenced by regional drainage development on the scale of hundreds of kilometers rather than local conditions over tens of kilometers.

Plain Language Summary
Ice streams are fast‐flowing areas of the Antarctic ice sheet that drain large quantities of ice into the ocean, contributing to sea level rise. We have run a model of water flow underneath Recovery Ice Stream to examine lakes that build up and drain underneath kilometers of ice to find out whether they have an impact on the speed of the overlying ice. We find that the timing of the lake growth and drainage is determined by the hydrological conditions underneath the entirety of the ice stream, stretching over hundreds of kilometers. As the lakes drain, they melt channels that connect as sub‐ice rivers between the drainage basins. We also find that the regions of highest water pressure, and therefore the fastest‐moving overlying ice, are concentrated in the deepest parts of the trough that the ice stream flows through. This is an important finding for determining the controls on fast ice stream flow speed and therefore the stability of the Antarctic ice sheet.

Extract: "Antarctic subglacial lakes have been modeled within synthetic ice dynamics models (Pattyn, 2008; Sergienko et al., 2007) and as basins that are filled and drained by tuning with satellite altimetry data (Carter & Fricker, 2012; Carter et al., 2009, 2011). Recent work by Carter et al. (2017) suggests that Antarctic lake dynamics cannot be influenced by the formation of Röthlisberger (R-) channels that melt upward into the ice, instead arguing that sediment canals are necessary to allow lake drainage. These treatments of Antarctic subglacial lakes are different from those models that examine ice marginal lake outburst floods or subglacial jökulhlaups, where rapid (on the scale of days to weeks) drainage occurs. Models examining the latter focus on the water pressure allowing ice uplift and downstream lake drainage (e.g., Ng & Liu, 2009; Nye, 1976) or negative pressure gradients that prevent outflow of the lakes until they are reversed by hydrological development (e.g., Evatt et al., 2006; Fowler, 1999; Kingslake, 2015). In contrast, the active Antarctic subglacial lakes differ because they drain over a timescale of years and can become much larger (>10km2), although often shallower (e.g., <10m deep) than ice marginal or jökulhlaup lakes. The work of Dow et al. (2016) found that at no time were hydraulic pressure gradients reversed when applying a synthetic hydrology model to Antarctic lakes. Instead, lake dynamics were driven by spatially and temporally varying conductivity of the basal drainage system including the growth of R-channels that drained the lake. The Dow et al. (2016) study applied a synthetic, planar topography with one overdeepening, designed to emulate Recovery Ice Stream. However, until now, a 2-D approach to catchment-scale hydrology modeling with Antarctic topography including multiple lake basins has not been attempted.

This suggests that the water pressure plays a more important role in the ice stream velocity than the water thickness, which as we demonstrate with our model outputs is not always coincident with water pressure, either spatially or temporally."
&

Next, I note that the first two attached images are from the second linked reference, and they show the extensive subglacial lake and meltwater drainage systems in Antarctica (with increasing warming these systems should become more extensive and important in the future):

S. J. Livingstone, C. D. Clark, and J. Woodward (2013), "Predicting subglacial lakes and meltwater drainage pathways beneath the Antarctic and Greenland ice sheets", The Cryosphere Discuss., 7, 1177–1213, www.the-cryosphere-discuss.net/7/1177/2013/, doi:10.5194/tcd-7-1177-2013

The caption for the first image is: "In (B), the blue colour illustrates regions below the pressure melting point. This is used as a simple mask to remove all subglacial lakes that fall within the cold-bedded zones. Note, the subglacial drainage network is still treated as though the bed was wholly warm based."

The caption for the second image is:  "(B) the fraction of the grounded ice-sheet bed occupied by subglacial lakes vs ice-sheet area, with both the Antarctic and Greenland subglacial lake data plotted."
&

Next, the following link leads to findings presented at the AGU 2013 conference about new evidence characterizing the nature of the subglacial hydrological system in Antarctica:

http://www.livescience.com/41994-antarctica-new-lakes-streams.html

In the third attached image, the red dots mark surface changes that scientists think are caused by water moving beneath Antarctica's ice. The blue and magenta colors indicate ice velocity, with the magenta showing the fastest-moving ice.
&

Finally, the following linked reference by Bell discusses the importance of correctly modeling the influence of subglacial hydrology on ice mass loss from AIS:

Robin E. Bell (2008), "The role of subglacial water in ice-sheet mass balance", Nature Geoscience, doi:10.1038/ngeo186

http://www.nature.com/ngeo/journal/v1/n5/abs/ngeo186.html

Abstract: "In the coming decades, significant changes in the polar regions will increase the contribution of ice sheets to global sea-level rise. Under the ice streams and outlet glaciers that deliver ice to the oceans, water and deformable wet sediments lubricate the base, facilitating fast ice flow. The influence of subglacial water on fast ice flow depends on the geometry and capacity of the subglacial hydrologic system: water moving rapidly through a well-connected system of conduits or channels will have little impact on ice-sheet velocities, but water injected into a spatially dispersed subglacial system may reduce the effective pressure at the base of the ice sheet, and thereby trigger increased ice-sheet velocities. In Greenland, the form of the subglacial hydrologic system encountered by increasing surface melt water will determine the influence of changing atmospheric conditions on ice-sheet mass balance. In Antarctica, subglacial lakes have the capacity to both modulate velocities in ice streams and outlet glaciers and provide nucleation points for new fast ice-flow tributaries. Climate models of ice-sheet responses to global change remain incomplete without a parameterization of subglacial hydrodynamics and ice dynamics."

29
The linked refence works to try to explain why the surface temperatures for the Early Pliocene was so much warmer than for the Mid-Pliocene, and points to this nonlinear saddlenode bifurcation being associated with primarily Arctic Amplification but also probably due to both increased El Nino frequency and an expanded Hadley Cell (see the first image).  As I have previously noted that ice-climate feedback from a collapse of the WAIS would contribute to all three (Arctic Amplification, more frequent El Nino events and an expanded Hadley Cell); it is possible/probable that as early as 2060 Earth could be in conditions comparable to the Early Pliocene (with GMSTA up to +3.6C) even if we stop following SSP5-Baseline after 2035.  To emphasize this point I repost the second image of how such a bifurcation can lead to an abrupt change in climate state (due to a tipping perturbation such as abrupt ice mass loss from the WAIS).  Also as precaution, I note that Energy Balance Models are associated with inferred climate sensitivity which is lower than true climate sensitivity as shown in the third image.

Brady Dortmans et al. (2018), "An Energy Balance Model for Paleoclimate Transitions', Clim. Past Discuss., https://doi.org/10.5194/cp-2018-56

https://www.clim-past-discuss.net/cp-2018-56/cp-2018-56.pdf

Abstract. A new energy balance model (EBM) is presented and is used to study Paleoclimate transitions. While most previous EBMs dealt only with the globally averaged climate, this new EBM has three variants: Arctic, Antarctic and Tropical climates. This EBM incorporates the greenhouse warming effects of both carbon dioxide and water vapour, and also includes ice-albedo feedback. The main conclusion to be drawn from the EBM is that the climate system possesses multiple equilibrium states, both warm and frozen, which coexist mathematically. 5 While the actual climate can exist in only one of these states at any given time, the climate can undergo transitions between the states, via mathematical saddlenode bifurcations. This paper proposes that such bifurcations have actually occurred in Paleoclimate transitions. The EBM is applied to the study of the Pliocene Paradox, the Glaciation of Antarctica and the so-called warm, equable climate problem of both the mid-Cretaceous Period and the Eocene Epoch. In all cases, the EBM is in qualitative agreement with the geological record.

Extract: "During the early Pliocene Epoch, 3–5 Ma, the climate of the Arctic region of Earth changed abruptly from ice-free to ice-capped. The climate forcing factors then (solar constant, orbital parameters, CO2 concentration and locations of the continents) were all very similar to today. Therefore, it is difficult to explain why the early Pliocene climate was so different from that of today. That problem is known as the Pliocene Paradox, (Cronin (2010); Fedorov et al. (2006, 2010)). This paper presents a plausible explanation of the Pliocene paradox.

As stated above, a key feature of this family of mathematical models is that they incorporate physical principles that are nonlinear. As is well known, nonlinear equations can have multiple solutions, unlike linear equations which can have only one unique solution (if well-posed). In our mathematical models, the same set of equations can have two or more co-existing solutions, for example an ice-capped solution (like today’s climate) and an ice-free solution (like the Cretaceous climate), even with the same values of the forcing parameters. The determination of which solution is actually realized by the planet at a given time is dependent on past history. Changes in forcing parameters may drive the system abruptly from one stable state to another, at so-called “tipping points”. In this paper, these tipping points are investigated mathematically, and are shown to be bifurcation points, which can be investigated using mathematical bifurcation theory. Bifurcation theory tells us that the existence of bifurcation points is preserved (but the numerical values may change) under small deformations of the model equations. Thus, even though this conceptual model may not give us precise quantitative information about climate changes, qualitatively there is good reason to believe that the existence of the bifurcation points in the model will be preserved in similar more refined models and in the real world.

The change from ice-free to ice-covered in the Arctic occurred abruptly, during the Pliocene Epoch, 5.3 to 2.6 Ma. It has been a longstanding challenge for paleoclimatologists to explain this dramatic change in the climate.

During the Pliocene Epoch, all of the important forcing factors that determine climate were very similar to those of today. The Earth orbital parameters, the CO2 concentration, solar radiation intensity, position of the continents, ocean currents and atmospheric circulation all had values close to the values they have today. Yet, in the early Pliocene, 4–5 million years ago, the Arctic climate was much milder than that of today. Arctic surface temperatures were 8−19_C warmer than today and global sea levels were 15−20 m higher than today, and yet CO2 levels are estimated to have been 340−400 ppm, about the same as 20th Century values; see Ballantyne et al. (2010); Csank et al. (2011); Tedford and Harington (2003). As mentioned in the Introduction, the problem of explaining how such different climates could exist with such similar forcing parameter values has been called the Pliocene Paradox (Cronin (2010); Fedorov et al. (2006, 2010)).

Another interesting paradox concerning Polar glaciation is the fact that, although both poles have transitioned abruptly from ice-free to ice-covered, they did so at very different geological times. The climate forcing conditions of Earth are highly symmetric between the two hemispheres and for most of the history of Earth the climates of the two poles have been very similar. However, there was an anomalous period of about 30 million years, from the Eocene-Oligocene boundary (34 Ma) to the early Pliocene (4 Ma), when the Antarctic was largely ice-covered but the Arctic was ice-free.

Thus, the EBM presented here, as illustrated in Figure 7, provides a plausible explanation for the Pliocene paradox. The slowly-acting physical forcings of decreasing CO2 concentration and decreasing ocean heat transport FO were amplified by the mechanisms of ice-albedo feedback and water vapour feedback, both of which act very strongly when the temperature crosses the freezing point of water. For millions of years before the Pliocene, while the Arctic temperature remained well above freezing, the climate changed very little. However, once the freezing temperature was reached, the Arctic climate changed abruptly via a saddlenode bifurcation as in Figure 7 b), to a new frozen state. This simple mechanism suffices to explain the Pliocene paradox. No more complicated explanations are necessary.

Several other explanations have been proposed for the Pliocene paradox. There is convincing evidence that, at the beginning of the Pliocene, there was a permanent El Niño condition in the tropical Pacific ocean, see Cronin (2010); Fedorov et al. (2006, 2010). (However, some have disputed this finding, see Watanabe et al. (2011).) It has been suggested that a permanent El Niño condition could explain the warm early Pliocene, and that the onset of the El Niño – La Niña Southern Oscillation (ENSO) was the cause of sudden cooling of the Arctic during the Pliocene. Today, it is known that ENSO can influence weather patterns as far away as the Arctic.

Another suggestion is that Hadley cell feedback contributed to the abrupt cooling of the Arctic during the Pliocene. Recent work shows that an increase in pole-to-equator temperature gradient causes the Hadley cells to contract towards the equator, while increasing in circulation velocity, see Lewis and Langford (2008); Langford and Lewis (2009). This would cause a decrease in equator to pole atmospheric heat transport, which would in turn accelerate Arctic cooling; this is called Hadley cell feedback.  Further work on modelling this mechanism is in progress. It is conjectured here that Hadley cell feedback may in fact have caused the end of a permanent El Niño condition in the Pliocene, as follows. It is known that the La Niña phase of ENSO is forced in part by the Trade Winds blowing East to West across the tropical Pacific Ocean. The Trade Winds are the surface component of the Hadley circulation. Therefore, acceleration of the Hadley circulation would strengthen the Trade Winds, enhancing the conditions for La Niña and ending the permanent El Niño. Further work on this conjecture also is in progress.

In the Tropics, many of the values of the forcing parameters are different from their values in the Arctic and Antarctic, see Table 2. The geological record shows little change in the tropical climate over the past 100 million years, other than a little cooling. Even when Arctic climate changed dramatically in the Pliocene, the Tropical climate changed very little.

The new entry in this Table, one that did not appear in the polar models, is FC, which represents transport of heat away from the surface to the atmosphere, by conduction / convection / change of state of water. The most important of these is the upward transport of latent heat. Surface water evaporates, taking heat from the surface. As warm moist air rises and cools, the water vapour condenses, releasing its latent heat into the surrounding atmosphere."


Caption for the first attached image: "Figure 7. Pliocene Arctic EBM (36)(37). Parameter values δ = 0.67, FA = 115; other parameters as in Table 1. Subfigure a): CO2 takes valuesµ = 1200, 1000, 800, 600, 400, 200ppm,from top to bottom on the blue curves, with fixed FO = 50 Wm−2. The warm equilibrium state disappears as µ decreases. Subfigure b): Bifurcation Diagram for the Pliocene Paradox. Here, CO2 concentration µ and ocean heat transport FO decrease simultaneously, with increasing ν, (0≤ν ≤1), as given by equations (42). As ν increases, the warm equilibrium solution (τS > 1) disappears in a saddlenode bifurcation, at approximately ν = 0.9, corresponding to forcing parameter µ = 343 ppm and FO = 51 Wm2. To the right of this point, only the frozen equilibrium state exists. To the left of this point, the frozen and warm equilibrium states coexist, separated by the unstable intermediate state."

Edit, W.r.t. coming Arctic Amplification, see the following linked article:

Title: "New and emerging threats continue to appear in Arctic as region warms, 2018 Arctic Report Card says"

https://www.accuweather.com/en/weather-news/warming-and-melting-arctic-has-global-impacts-2018-arctic-report-card-says/70006872

Extract: "The Arctic Ocean has lost 95 percent of its oldest, thickest ice. In 2018, Arctic sea ice remained younger and thinner and covered less area than in the past. The 12 lowest extents in the satellite record have occurred in the last 12 years, according to the report."

30
For what it is worth, SSP5 will be used in the upcoming AR6, and per the linked reference & associated image), following the SSP5-Baseline scenario through at least 2035 (due to the lag in time between the forcing and the change in surface temperature), GMSTA (above pre-industrial) would be about +1.6C in 2030 and +2C in 2040, which agrees with my estimates in Reply #344:

Kriegler et al. (2017), "Fossil-fueled development (SSP5): An energy and resource intensive scenario for the 21st century", Global Environmental Change, Volume 42, January 2017, Pages 297-315, https://doi.org/10.1016/j.gloenvcha.2016.05.015

http://www.sciencedirect.com/science/article/pii/S0959378016300711?via%3Dihub

Abstract: "This paper presents a set of energy and resource intensive scenarios based on the concept of Shared Socio-Economic Pathways (SSPs). The scenario family is characterized by rapid and fossil-fueled development with high socio-economic challenges to mitigation and low socio-economic challenges to adaptation (SSP5). A special focus is placed on the SSP5 marker scenario developed by the REMIND-MAgPIE integrated assessment modeling framework. The SSP5 baseline scenarios exhibit very high levels of fossil fuel use, up to a doubling of global food demand, and up to a tripling of energy demand and greenhouse gas emissions over the course of the century, marking the upper end of the scenario literature in several dimensions. These scenarios are currently the only SSP scenarios that result in a radiative forcing pathway as high as the highest Representative Concentration Pathway (RCP8.5). This paper further investigates the direct impact of mitigation policies on the SSP5 energy, land and emissions dynamics confirming high socio-economic challenges to mitigation in SSP5. Nonetheless, mitigation policies reaching climate forcing levels as low as in the lowest Representative Concentration Pathway (RCP2.6) are accessible in SSP5. The SSP5 scenarios presented in this paper aim to provide useful reference points for future climate change, climate impact, adaption and mitigation analysis, and broader questions of sustainable development."

31
We should also keep in mind the ice-climate feedback risks associated with both nonlinear surface melting of the GIS (see the first linked reference and image); and of increasing rainfall around the Artic (see the second linked reference) and in Greenland (see the third linked reference w.r.t. atmospheric rivers):

Trusel, L. D., Das, S. B., Osman, M. B., Evans, M. J., Smith, B. E., Fettweis, X., … van den Broeke, M. R. (2018). Nonlinear rise in Greenland runoff in response to post-industrial Arctic warming. Nature, 564(7734), 104–108. doi:10.1038/s41586-018-0752-4, https://doi.org/10.1038/s41586-018-0752-4

https://www.nature.com/articles/s41586-018-0752-4

Abstract: "The Greenland ice sheet (GrIS) is a growing contributor to global sea-level rise, with recent ice mass loss dominated by surface meltwater runoff. Satellite observations reveal positive trends in GrIS surface melt extent, but melt variability, intensity and runoff remain uncertain before the satellite era. Here we present the first continuous, multi-century and observationally constrained record of GrIS surface melt intensity and runoff, revealing that the magnitude of recent GrIS melting is exceptional over at least the last 350 years. We develop this record through stratigraphic analysis of central west Greenland ice cores, and demonstrate that measurements of refrozen melt layers in percolation zone ice cores can be used to quantifiably, and reproducibly, reconstruct past melt rates. We show significant (P < 0.01) and spatially extensive correlations between these ice-core-derived melt records and modelled melt rates and satellite-derived melt duration across Greenland more broadly, enabling the reconstruction of past ice-sheet-scale surface melt intensity and runoff. We find that the initiation of increases in GrIS melting closely follow the onset of industrial-era Arctic warming in the mid-1800s, but that the magnitude of GrIS melting has only recently emerged beyond the range of natural variability. Owing to a nonlinear response of surface melting to increasing summer air temperatures, continued atmospheric warming will lead to rapid increases in GrIS runoff and sea-level contributions."

Caption for the image: "Fig. 4 | Exceptional rise in Greenland ice-sheet runoff and climate warming context. a, GrIS-integrated meltwater runoff, as simulated by regional climate models (coloured lines; 5-year smoothed) and reconstructed using the NU and CWG ice-core-derived melt records (black line; 95% confidence interval shaded; see Methods). b, Median onset of significant trends (vertical black dotted lines) and climate emergence above pre-industrial (vertical red dotted lines) for mean  Arctic temperatures (top), our ice-core-derived runoff reconstruction (middle) and two summer Arctic sea-ice extent datasets (bottom;  Methods). Median absolute deviations of trend onsets and climate  emergence shown as shaded boxes. Thin and bold black lines denote  15-year and 50-year Gaussian smoothed series. c, Recent modelled evolution of mean summer (JJA) near-surface air temperature and surface
melt (in millimetres of water equivalent per year) across CWG. Ice core sites are shown as coloured points, and a Jakobshavn basin (basin 7.1; Fig. 1) elevational transect as grey points from RACMO2.3p2 (circles) and MARv3.7 (squares). Means over the past 20 years of the ice-core records (1994–2013) at core sites are denoted by points with single black border, and peak melting in 2012 by double black borders. The evolution of CWG ice-sheet melt in response to a warming climate is well represented by an exponential function (black curve). Recent melt rates at our percolation zone core sites approach conditions where the models have recently begun to simulate meltwater runoff (blue dashed line indicates mean runoff-linked melt rate and the shaded region corresponds to ±1 s.d.; see Methods for details)."

Richard Bintanja and Olivier Andry (2017), “Towards a rain-dominated Arctic”, Geophysical Research Abstracts Vol. 19, EGU2017-4402

http://meetingorganizer.copernicus.org/EGU2017/EGU2017-4402.pdf

Abstract: “Current climate models project a strong increase in Arctic precipitation over the coming century, which has been attributed primarily to enhanced surface evaporation associated with sea-ice retreat. Since the Arctic is still quite cold, especially in winter, it is often (implicitly) assumed that the additional precipitation will fall mostly as snow. However, very little is known about future changes in rain/snow distribution in the Arctic, notwithstanding the importance for hydrology and biology. Here we use 37 state-of-the-art climate models in standardised twenty-first century (2006–2100) simulations to show that 70◦ – 90◦N average annual Arctic snowfall will actually decrease, despite the strong increase in precipitation, and that most of the additional precipitation in the future (2091– 2100) will fall as rain. In fact, rain is even projected to become the dominant form of precipitation in the Arctic region. This is because Arctic atmospheric warming causes a greater fraction of snowfall to melt before it reaches the surface, in particular over the North Atlantic and the Barents Sea. The reduction in Arctic snowfall is most pronounced during summer and autumn when temperatures are close to the melting point, but also winter rainfall is found to intensify considerably. Projected (seasonal) trends in rain/snowfall will heavily impact Arctic hydrology (e.g. river discharge, permafrost melt), climatology (e.g. snow, sea ice albedo and melt) and ecology (e.g. water and food availability).”

Also as a repost of Reply #195: continued global warming should increase the frequency with which atmospheric rivers reach Greenland, we may be in for some rude surprises in the coming decades (w.r.t. increasing rates of ice mass loss from the Greenland Ice Sheet):

William Neff (2018), "Atmospheric rivers melt Greenland", Nature Climate Change 8, 857-858, DOI: https://doi.org/10.1038/s41558-018-0297-4

http://www.nature.com/articles/s41558-018-0297-4

Abstract: "Recent years have seen increased melting of the Greenland Ice Sheet, contributing to accelerated rates of sea-level rise.  New research suggests that this melting due to an increased frequency of atmospheric rivers, narrow filaments of moist air moving polewards."

32
As a follow-on to my last post, the first two images from the first linked website entitled "Figures from the Global Carbon Budget 2018", show (respectively) that we are currently following the SSP5 baseline scenario, and that we are above the SSP scenarios required to state below the 1.5C goal.

http://folk.uio.no/roberan/GCB2018.shtml

Also, by the end of 2018 the world population will be about 7.7 million people, which, per the third & fourth images, slightly exceeds that assumed by SSP5.

33
...

It's hard to reconcile mid-Pliocene conditions beginning around 2030 with the IPCC report.

Not to repeat myself, but per the linked Gavin Schmidt tweeter thread, for a 20yr loess trend line Gavin is predicting that the GMSTA in 2019 will be 1.2+/-0.15C (see the first attached image) or 1.23C for a 15yr loess trend line (see the extract below).  I note that this prediction is in line with Hansen's prediction that I cited in Reply #220 and as is indicated by the second attached image.  So if one takes Gavin's estimate of +1.23C by the end of 2019 together with Hansen's value of 0.38C/decade one gets GMSTAs of +1.61C by 2030 and +1.99C by 2040 (note in most of my posts I take 2040 as the date when conditions for key West Antarctic marine glaciers reaching Mid-Pliocene oceanic and atmospheric conditions).

https://twitter.com/ClimateOfGavin/status/1068336654887337984

Extract: "ENSO forecast for DJF here: https://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/ … (I used 1±0.6 (95% CI)). Note there is also some dependence on the smoothing; predictions for 2019 would be 1.23 or 1.17 using a 15yr or 30yr loess smooth....1.2±0.15 ºC above the late 19th C. A warmer yr than 2018 (which will #4), almost certain >1ºC yr, and 1 in 3 chance of a new record."

Next, it is somewhat unclear what Mid-Pliocene conditions, in West Antarctica, actually means.  Per the third image, from Sweet et al. 2017) GMSTA (from pre-industrial) during the Pliocene ranges from +1.8C to +3.6C; while the fourth image from Hansen & Sato shows Pliocene GMSTA relative to the Holocene Optimum.

Thus to begin to reconcile Mid-Pliocene conditions circa 2040 with AR5, one needs to believe (at least) that IPCC underestimates:

a) ECS and negative forcing from anthropogenic aerosols,
b) the role of ENSO (& IPO) in determining GMSTA in the coming decades,
c) the role of ice-climate feedback mechanisms that have already been triggered.

Edit, there currently are 2,387 posts in the "Conservative Scientists & its Consequences" thread related to why the IPCC is likely erring of the side of least drama in its climate change projections:
 
https://forum.arctic-sea-ice.net/index.php/topic,1053.0.html

Edit2, with regard to the 2030 date, I suspect that Burke et al (2018) are likely referring to the CO2 concentration by 2030 (see the CO2 concentrations given in the third image).

34
As a follow-on to my last two posts, I note that:

1. The first image [from Wilson et al (2018)], highlights that the sea level rise during MIS 11 (the Holsteinian) was higher (6 to 13m) than for MIS5 (the Eemian, 6 to 9m), even though its radiative forcing and Antarctic temperature increase were both less than for MIS 5.  As no current ESM projection can match the sensitivity of MIS 11, this is an indication that all reported projections err on the side of least drama.

Wilson, D. J., Bertram, R. A., Needham, E. F., van de Flierdt, T., Welsh, K. J., McKay, R. M., … Escutia, C. (2018). Ice loss from the East Antarctic Ice Sheet during late Pleistocene interglacials. Nature. doi:10.1038/s41586-018-0501-8

https://www.nature.com/articles/s41586-018-0501-8

Caption for the first attached image: "Fig. 3 | Comparison of U1361A records to regional palaeoclimate and global sea level records. a, Antarctic ice core temperature difference (ΔT, difference from mean values of the last millennium) derived from deuterium isotopes at EPICA Dome C (EDC)11 plotted on EDC3 age scale. bp, before present. b, Southern Ocean bottom water temperature (BWT) from Mg/Ca at Ocean Drilling Program (ODP) Site 1123 (ref. 18).  c, Southern Ocean sea surface temperature (SST) from alkenones at ODP Site 1090 (ref. 19). d, Ba/Al ratios (XRF-scanner counts; three-point smoothed) in U1361A. e, Bulk detrital sediment Nd isotopes in U1361A (error bars are 2 s.d. external reproducibility). f, Sea level proxy from benthic oxygen isotopes28, labelled with MIS numbers and sea level estimates17 from MIS 5e and MIS 11. Shading in a–c, f represents intervals with values above modern (or late Holocene core top); red dashed line in e indicates the core top εNd value of U1361A. For chronostratigraphic constraints on U1361A, see Supplementary Table 8 and Methods."

2. The second image [from Weber et al (2014)] shows that an iceberg armada from the ASE (Amundsen Sea Embayment, say from 2040 to 2060) would be initially carried eastward by the Antarctic Coastal Current where it would provide meltwater that would disrupt AABW formation in East Antarctica until it was kicked northward in 'Iceberg Alley' in the Weddell Sea, into the ACC stream.  This parallels the scenario modeled by Fogwill et al with a figure showing impact on AABW formation in my Reply #338.

3. The third image shows the findings of a field survey of the Recovery Ice Stream, indicating the presences of subglacial lakes that could well accelerate ice mass loss from this EAIS glacier beyond that indicated by Pollard, DeConto and Alley (2018) for Pliocene conditions. The linked article talks about the IceBridge mission to investigate the Recovery Glacier area from which the third image was taken:

http://blogs.ei.columbia.edu/2012/10/29/a-recovery-mission/

Also, I note that almost all other key Antarctic marine glacier have extensive systems of subglacial lakes and streams that could accelerate ice flow in the near-term future with continued global warming.

4.  The fourth image shows the location of key gyres around Antarctica including the 'Unnamed Gyre' that is probably driving upwelling of warm CDW towards the grounding line of Totten Glacier (and thus likely which is accelerating ice mass loss from this key EAIS marine glacier beyond that accounted for in any model that I know of).

35
As a follow-on to my last post, I provide the four attached images that show:

1. The location and 2008-2009 ice velocities of key marine glaciers around Antarctica.

2. The location of the Totten Basin.

3. The ocean upwelling of CDW that is currently impacting ice mass loss from the Totten Glacier.

4. The 'ice plug' (or the ice that must be lost before either MISI or MICI occurs) for the Wilkes Basin; and which is similar to most other key Antarctic marine glaciers.

36
More Glaciers in East Antarctica Are Waking Up https://www.nasa.gov/feature/goddard/2018/more-glaciers-in-antarctica-are-waking-up
...

wdmn,

I concur that ice mass loss from the EAIS is of concern, and is included in Pollard, DeConto and Alley (2018)'s projections for Pliocene conditions (which we are likely to reach by 2040).  However, the old questions of 'how soon?' and 'how fast?' always get asked by decision makers.  In this regards:

A. The first image from Sweet et al (2017) show that after several millennia of Pliocene conditions mean global sea level might rise by as much as 30m (most of which would come from the GIS, the WAIS and the EAIS); and the second image from the same source shows a US government recommended upper bound of 2.5 m of global mean sea level rise by 2100 (including from all sources including the EAIS).

Sweet, W.V., R. Horton, R.E. Kopp, A.N. LeGrande, and A. Romanou, 2017: Sea level rise. In: Climate Science Special Report: Fourth National Climate Assessment, Volume I [Wuebbles, D.J., D.W. Fahey, K.A. Hibbard, D.J. Dokken, B.C. Stewart, and T.K. Maycock (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 333-363, doi: 10.7930/J0VM49F2.

https://science2017.globalchange.gov/downloads/CSSR_Ch12_Sea_Level_Rise.pdf

Caption for the first image: "Figure 12.2: (a) The relationship between peak global mean temperature, atmospheric CO2, maximum global mean sea level (GMSL), and source(s) of meltwater for two periods in the past with global mean temperature comparable to or warmer than present. Light blue shading indicates uncertainty of GMSL maximum. Red pie charts over Greenland and Antarctica denote fraction, not location, of ice retreat. Atmospheric CO2 levels in 2100 are shown under RCP8.5. (b) GMSL rise from −500 to 1900 CE, from Kopp et al.’s geological and tide gauge-based reconstruction (blue), from 1900 to 2010 from Hay et al.’s tide gauge-based reconstruction (black), and from 1992 to 2015 from the satellite-based reconstruction updated from Nerem et al. (magenta). (Figure source: (a) adapted from Dutton et al. 2015 and (b) Sweet et al. 2017).

Caption for the second image: "Figure 12.4: (a) Global mean sea level (GMSL) rise from 1800 to 2100, based on Figure 12.2b from 1800 to 2015, the six Interagency GMSL scenarios (navy blue, royal blue, cyan, green, orange, and red curves), the very likely ranges in 2100 for different RCPs (colored boxes), and lines augmenting the very likely ranges by the difference between the median Antarctic contribution of Kopp et al. and the various median Antarctic projections of DeConto and Pollard. (b) Relative sea level (RSL) rise (feet) in 2100 projected for the Interagency Intermediate Scenario (1-meter [3.3 feet] GMSL rise by 2100) (Figure source: Sweet et al. 2017)."

See also:
Sweet, W.V., R.E. Kopp, C.P. Weaver, J. Obeysekera, R.M. Horton, E.R. Thieler, and C. Zervas, 2017: Global and Regional Sea Level Rise Scenarios for the United States. National Oceanic and Atmospheric Administration, National Ocean Service, Silver Spring, MD. 75 pp. https://tidesandcurrents.noaa.gov/publications/techrpt83_Global_and_Regional_SLR_Scenarios_for_the_US_final.pdf

B. The third image, from Wilson et al (2018) shows the extent of ice loss from the Wilkes Basin after several millennia of Pleistocene conditions for different assumptions

Wilson, D. J., Bertram, R. A., Needham, E. F., van de Flierdt, T., Welsh, K. J., McKay, R. M., … Escutia, C. (2018). Ice loss from the East Antarctic Ice Sheet during late Pleistocene interglacials. Nature. doi:10.1038/s41586-018-0501-8

https://www.nature.com/articles/s41586-018-0501-8

Abstract: "Understanding ice sheet behaviour in the geological past is essential for evaluating the role of the cryosphere in the climate system and for projecting rates and magnitudes of sea level rise in future warming scenarios. Although both geological data and ice sheet models indicate that marine-based sectors of the East Antarctic Ice Sheet were unstable during Pliocene warm intervals, the ice sheet dynamics during late Pleistocene interglacial intervals are highly uncertain. Here we provide evidence from marine sedimentological and geochemical records for ice margin retreat or thinning in the vicinity of the Wilkes Subglacial Basin of East Antarctica during warm late Pleistocene interglacial intervals. The most extreme changes in sediment provenance, recording changes in the locus of glacial erosion, occurred during marine isotope stages, when Antarctic air temperatures were at least two degrees Celsius warmer than pre-industrial temperatures for 2,500 years or more. Hence, our study indicates a close link between extended Antarctic warmth and ice loss from the Wilkes Subglacial Basin, providing ice-proximal data to support a contribution to sea level from a reduced East Antarctic Ice Sheet during warm interglacial intervals. While the behaviour of other regions of the East Antarctic Ice Sheet remains to be assessed, it appears that modest future warming may be sufficient to cause ice loss from the Wilkes Subglacial Basin."

Extract: "The key finding from our new data set is that the Wilkes Subglacial Basin has been susceptible to ice loss not only during warm Pliocene intervals [Ref. 5] with CO2 levels of approximately 400 p.p.m., but also during the late Pleistocene despite CO2 levels [Ref. 25] remaining below 300 p.p.m. Hence, we provide data-based evidence in support of recent ice sheet models that simulate margin retreat and ice loss during late Pleistocene interglacials [Refs. 2,3,9] (Fig. 1b)"

"Based on the ice sheet response during past interglacial periods, we estimate that substantial ice loss within the Wilkes Subglacial Basin would be likely to occur with approximately 2 °C warming (above pre-industrial) if sustained for a few millennia."

Caption for the third image: "Fig. 1 | Setting of IODP Site U1361 offshore of the Wilkes Subglacial Basin. a, Map of Antarctica showing subglacial bedrock elevation above sea level12,31 and the U1361A coring location. b, Detailed map of the Wilkes Subglacial Basin, with lines illustrating positions of the ice sheet margin in different ice sheet models and scenarios: red dashed line, fully retreated state of Mengel and Levermann under 1.8 °C ocean warming; black dashed line, maximum simulated MIS 5e retreat of DeConto and Pollard, equivalent to approximately 2 °C ocean and atmospheric warming; and modelled retreat of Golledge et al. for both 2 °C ocean and atmospheric warming (ochre dotted line) and 4 °C ocean and atmospheric warming (white dotted line). C, N, and M indicate positions of Cook, Ninnis, and Mertz ice shelves, respectively."

C. The fourth image from: A dynamic early East Antarctic Ice Sheet suggested by ice-covered fjord landscapes by Young et al, 2011, doi:10.1038/nature10114; show where the Aurora Subglacial Basin (ASB) is located and how its bottom topology feed basal meltwater down towards the Totten Glacier (whose catchment basin forms a major part of the ASB), which could serve to accelerate the ice mass loss from this area.  Furthermore, the Young et al 2011 paper notes that the Aurora Basin contains several paleo-fjords; which, indicate that in the past the EAIS had on at least two occasions retreated into this subglacial basin.  This clearly raises concerns about the potential SLR contributions from this area (including the Totten and Moscow University Ice Shelf areas) during this century.

Best,
ASLR

37
As a follow-on to my last post, as to why Hansen et al (2016)'s model projections many not fully simulate the longer term impacts of a potential abrupt collapse of the WAIS beginning circa 2040, I also note that:

a. The first image shows projections from an ESM indicating that with continued BAU radiative forcing (here labeled A2), that a warm pulse of North Atlantic water would enter the Arctic Ocean Basin and rapidly reduce Arctic Sea Ice Extents, ASIE, and I note that models including freshwater hosing from an abrupt collapse of the WAIS indicate that this rapid loss of ASIE could happen in as little as 5-years after the collapse of the ASE marine glaciers.

b. The second image shows that the synergist telecommunication of warm evaporation from the Tropical Atlantic to the Tropical Pacific is strongest during periods of rising CO2 emissions, and I highly doubt that Hansen et al (2016)'s model simulated this positive forcing bipolar seesaw mechanism.

c. The third image shows how large radiative forcing from accelerated CH4 emissions (direct, indirect and from associated O3, Strat. H2O and CO2) for the indicated cases, and I note that methane emissions from the Arctic permafrost regions could accelerate rapidly with a collapse of the WAIS due to such sources as Thermokarst Lakes and increase rainfall in the Arctic.

d. The fourth image shows paleo data that ESS (Earth System Sensitivity) increased up to 7C during the Pliocene (presumably over several thousand years); and with our current radiative forcing occurring at a rate of about 100 times fast than that during the PETM, who knows how fast we might ratchet up (due to a cascade of positive feedback tipping points) from ECS to ESS conditions.


38
As a follow-on to my last post (#337), as to why Hansen et al (2016) intermediate ESM projections of ice-climate interaction dampen-out rapidly after circa 2080 (for the orange curves) I briefly note that:

a. The first image shows that currently Arctic Amplification is increasing much faster than Antarctic Amplification, and I suspect that Hansen et al (2016)'s model simulates this response into the near-term future.

b. However, most of the suppression of the rate of increase of Antarctic Amplification is due to the relatively high surface elevations of both the WAIS and the EAIS, while after a WAIS collapse (with the associated slow-down of the Overturning Current) both the surface elevation of most of the WAIS would be near sea level and the strong El Nino activity from the slow-down of the Overturning Current would continue to telecommunicate heat from the Tropical Pacific directly to West Antarctica  (the second image shows how a collapse of only the ASE marine glaciers would suppress Antarctic Bottom Water, AABW (which helps drive the Overturning Circulation current), formation for hundreds of years.

c. Also, the third image (from the following linked website) shows the location of AABW formation around Antarctica, which indicates that a collapse of the WAIS would suppress AABW formation both in the Ross and the Weddell Seas.  Furthermore, the fourth image (also from the following linked website) shows the areas of high Antarctic Sea Ice formation, & which indicates that both the Ross and the Weddell Sea regions currently produce relatively large amounts of Antarctic Sea Ice, which would be suppressed after the collapse of the WAIS [which might contribute to a slow transition to a La Nina dominated period by the Mid-Eemian (Mid-LIG)].

In my opinion these considerations help to illustrate how the nominally 20,000 year period of high frequency El Nino events occurred in the early Eemian (early LIG); while this is not indicated by Hansen et al (2016)'s model projections.

Website title: "Polar Oceanography"

http://wwwoa.ees.hokudai.ac.jp/research/polar_e.html

Caption for the third image: "The amount of Antarctic bottom water generated governs the relative strength of global deep ocean circulation. The Weddell and Ross Seas are well known regions of Antarctic bottom water formation, but in recent years it has emerged that the waters off Adelie Land in the vicinity of East 140 degrees Longitude is also an important region for bottom water generation"

Caption for the fourth image: "Mapping of annual quantity survey of sea ice production levels in Antarctic Sea (converted in thickness). & Extract of associated text: The below figure illustrates the first spatial distribution (mapping) of annual sea ice production levels in the Antarctic by combining satellite data and site survey and meteorological data. The body of water with the highest sea ice production is the Ross Sea, which corresponds well with this area being the production area of the Antarctic bottom water with the highest salinity. It is also understood that second to the Ross Sea, the region of highest sea ice production is approximately 1200 km East of Japan's Showa South Pole Base. It has been suggested that this may be an undiscovered Antarctic bottom water generation region, so intensive observation began in 2008 to confirm this possibility."

39
I just posted this in the "modelling the Anthropocene thread," but seems to fit here also (and I'm curious to hear your response ASLR).

New study published today: Pliocene and Eocene provide best analogs for near-future climatesmates https://www.pnas.org/content/early/2018/12/05/1809600115.

wdmn,

Thanks for you post about Burke et al (2018), which indicate that the Pliocene and Eocene provide the best analogs for near-future climates (see the first attached image of Figure 4 from that reference).

K. D. Burke, J. W. Williams, M. A. Chandler, A. M. Haywood, D. J. Lunt, and B. L. Otto-Bliesner (December 10, 2018), "Pliocene and Eocene provide best analogs for near-future climates", PNAS, https://doi.org/10.1073/pnas.1809600115

See also the supplemental material at:
https://www.pnas.org/content/suppl/2018/12/06/1809600115.DCSupplemental

Regarding my thoughts, I have both positive and negative opinions including the following:

Regarding positive thoughts:
a) It is valuable to nudge both the public and decision makers to better appreciate the seriousness of our situation and this paper provides such a valuable nudge.
b) I think that the statement that we will likely reach Mid-Pliocene surface conditions (beginning in the middle of continents) by 2030 is the most accurate and valuable, and it supports the position in many of my posts in this thread that it is appropriate to take the Pollard, DeConto and Alley (2018) for instant Mid-Pliocene conditions in Antarctica as beginning by (or before) 2040.

Regarding my negative thoughts:
a) I do not believe that any of the models cited use freshwater hosing appropriate to simulate ice-climate feedback mechanisms (as least that is the case when they were run for CMIP5), and the second image from Brown & Caldeira (2017) shows that the CMIP5 model projections likely underestimate ECS (with the higher ECS now being unmasked due to reduced SO2 emissions). However, this is more of a short-coming for projections after 2040.
b) The risks thru at least 2070 that Burke et al (2018) are ignoring by not accounting for ice-climate feedbacks can be approximated by the gold/orange curve in the third attached image from Hansen et al (2016).  Unfortunately, Hansen used a model of intermediate complexity with an ECS of about 3C and thus returned to this level of climate sensitivity after about 2080 (when the perturbation from the abrupt ice mass ends).
c) However, the fourth image from Zhang et al (2017), makes it clear that for the Early Eemian (or LIG) circa 129kya, the perturbation of abrupt ice mass loss prior to that time (i.e. before 129kya) cause a period of at least 20,000 years (see Reply #332) with frequent El Nino activity which is indicative of higher values of ECS (say at least 4.5C).
d) Paleo-radiative forcings all occurred many times slower than during modern times so it is likely that all comparisons of the near-term future to paleo cases all err on the side of least drama.

Best,
ASLR


40
...
You would think that cold surface water (also low salinity with a higher freezing temperature) would encourage sea ice freeze as winter approaches and discourage sea ice melt as summer commences. Since 1979 up to recently, there has been a slow but measurable increase in Antarctic sea ice extent (maximum extent in 2014). Hypothesis confirmed.

BUT since then the opposite. Antarctic sea ice extent is in decline, not just at max and min but during the melt season. Temporary aberration? Or is something  extra going on?
The ice-climate feedback mechanism is not dependent on the presences of Antarctic sea ice, as the linked reference and associated image show that this feedback mechanism works with low salinity surface water (and/or sea ice) along the Antarctic coastline:

Bronselaer, B. et al. (2018) Change in future climate due to Antarctic meltwater, Nature, doi:s41586-018-0712-z

https://www.nature.com/articles/s41586-018-0712-z


41
The linked reference provides a mathematical framework for modeling cascading tipping mechanisms resulting in abrupt climate change; and as an illustration of this methodology it provides a conceptual model for coupling the North Atlantic Ocean Overturning Current and the ENSO system in the Pacific.  Consensus climate science should use such a methodology to better evaluate the risks associated with Hansen's ice-climate feedback mechanism:

Dekker, M. M., von der Heydt, A. S., and Dijkstra, H. A.: Cascading transitions in the climate system, Earth Syst. Dynam. Discuss., https://doi.org/10.5194/esd-2018-26, 2018.

https://www.earth-syst-dynam-discuss.net/esd-2018-26/
https://www.earth-syst-dynam.net/9/1243/2018/esd-9-1243-2018.pdf

Abstract. We provide a theory of cascading tipping, i.e., a sequence of abrupt transitions occurring because a transition in one subsystem changes the background conditions for another subsystem. A mathematical framework of elementary deterministic cascading tipping points in autonomous dynamical systems is presented containing the double-fold, fold-Hopf, Hopf-fold and double-Hopf as most generic cases. Statistical indicators which can be used as early warning indicators of cascading tipping events in stochastic, non-stationary systems are suggested. The concept of cascading tipping is illustrated through a conceptual model of the coupled North Atlantic Ocean – El-Niño Southern Oscillation (ENSO) system, demonstrating the possibility of such cascading events in the climate system.

42
The linked reference provides paleo data (from the past 360,000 years) that the ENSO assumes a La Nina like pattern during glacial periods and assumes an El Nino like pattern during rapidly changing portions of interglacial periods.  As we are in the most rapidly changing interglacial period on record, this is not good news (as El Nino like Earth System patterns can result in effective ECS values in the range of 5C):

Zhang, S., Li, T., Chang, F. et al. Chin. J. (2017), "Correspondence between the ENSO-like state and glacial-interglacial condition during the past 360 kyr", Ocean. Limnol., 35: 1018. https://doi.org/10.1007/s00343-017-6082-9

https://link.springer.com/article/10.1007/s00343-017-6082-9#citeas

Abstract: "In the warming world, tropical Pacific sea surface temperature (SST) variation has received considerable attention because of its enormous influence on global climate change, particularly the El Niño-Southern Oscillation process. Here, we provide new high-resolution proxy records of the magnesium/calcium ratio and the oxygen isotope in foraminifera from a core on the Ontong-Java Plateau to reconstruct the SST and hydrological variation in the center of the Western Pacific Warm Pool (WPWP) over the last 360 000 years. In comparison with other Mg/Ca-derived SST and δ18O records, the results suggested that in a relatively stable condition, e.g., the last glacial maximum (LGM) and other glacial periods, the tropical Pacific would adopt a La Niña-like state, and the Walker and Hadley cycles would be synchronously enhanced. Conversely, El Niño-like conditions could have occurred in the tropical Pacific during fast changing periods, e.g., the termination and rapidly cooling stages of interglacial periods. In the light of the sensitivity of the Eastern Pacific Cold Tongue (EPCT) and the inertia of the WPWP, we hypothesize an inter-restricted relationship between the WPWP and EPCT, which could control the zonal gradient variation of SST and affect climate change."

Extract: "Previous research has discussed super-ENSO events in interglacial periods (Beaufort et al., 2001; Rincón-Martínez et al., 2010; Zhang et al., 2015). Nevertheless, interglacial periods defined by marine isotopes are not consistent with SST variations in the tropical Pacific (Fig.4), i.e., tropical SSTs during such periods are not as stable as in glacial periods. The real warm time in an interglacial period generally persists for 10–30 kyr, and it is always combined with a subsequent cooling process that involves a sequence of global fluctuations. Accordingly, an interglacial period should not be regarded as a single entity, as discussed in previous studies."

Edit: I think that this paper is important and verifies the ice-climate feedback mechanism associated with the ENSO cycles, and may explain how the Eemain got a double bump in sea level rise.  The attached image is Figure 4 from Zhang et al. (2017).

43
If the WAIS were to collapse abruptly it would produce an armada of icebergs from the WAIS in the Southern Ocean that would last for decades.  I note that debris fields in Drake Passage have shown that during Meltwater Pulse 1A (with different conditions than today) such iceberg armadas did exist and circled around the Southern Ocean.  The attached image shows how the iceberg rafted debris mechanism works:

Weber, M. E., Clark, P. U., Kuhn, G., Timmermann, A., Sprenk, D., Gladstone, R., … Ohlwein, C. (2014). Millennial-scale variability in Antarctic ice-sheet discharge during the last deglaciation. Nature, 510(7503), 134–138. doi:10.1038/nature13397

http://www.nature.com/nature/journal/v510/n7503/abs/nature13397.html

Abstract: "Our understanding of the deglacial evolution of the Antarctic Ice Sheet(AIS) following the Last Glacial Maximum (26,000–19,000years ago) is based largely on a few well-dated but temporally and geographically restricted terrestrial and shallow-marine sequences. This sparseness limits our understanding of the dominant feedbacks between the AIS, Southern Hemisphere climate and global sea level. Marine records of iceberg-rafted debris (IBRD) provide an early continuous signal of ice-sheet dynamics and variability. IBRD records from the North Atlantic Ocean have been widely used to reconstruct variability in Northern Hemisphere ice sheets, but comparable records from the Southern Ocean of the AIS are lacking because of the low resolution and large dating uncertainties in existing sediment cores. Here we present two well-dated, high-resolution IBRD records that capture a spatially integrated signal of AIS variability during the last deglaciation. We document eight events of increased iceberg flux from various parts of the AIS between 20,000 and 9,000 years ago, in marked contrast to previous scenarios which identified the main AIS retreat as occurring after meltwater pulse 1A and continuing into the late Holocene epoch. The highest IBRD flux occurred 14,600 years ago, providing the first direct evidence for an Antarctic contribution to meltwater pulse 1A. Climate model simulations with AIS freshwater forcing identify a positive feedback between poleward transport of Circumpolar Deep Water, subsurface warming and AIS melt, suggesting that small perturbations to the ice sheet can be substantially enhanced, providing a possible mechanism for rapid sea-level rise."

See also the research published in Nature Communications showing that current Southern Ocean waters are becoming more layered with cold water on top and warm water below; which promotes ice melting near the grounding lines of Antarctic marine glaciers, as occurred 14,000 years ago during the Meltwater Pulse 1A.  This clearly indicates an increasing risk of multiple meters of SLR this century:

http://www.sciencecodex.com/changing_antarctic_waters_could_trigger_steep_rise_in_sea_levels-142713

Extract: "The research published in Nature Communications found that in the past, when ocean temperatures around Antarctica became more layered - with a warm layer of water below a cold surface layer - ice sheets and glaciers melted much faster than when the cool and warm layers mixed more easily.

This defined layering of temperatures is exactly what is happening now around the Antarctic.
"The reason for the layering is that global warming in parts of Antarctica is causing land-based ice to melt, adding massive amounts of freshwater to the ocean surface," said ARC Centre of Excellence for Climate System Science researcher Prof Matthew England an author of the paper.
"At the same time as the surface is cooling, the deeper ocean is warming, which has already accelerated the decline of glaciers on Pine Island and Totten. It appears global warming is replicating conditions that, in the past, triggered significant shifts in the stability of the Antarctic ice sheet.""

44
For those who want to know more about the International Thwaites Glacier Collaboration's field investigation program, I provide the following linked website and the associated attached image:

Title: "The International Thwaites Glacier Collaboration"

https://thwaitesglacier.org/projects

Extract: "Disintegration of Marine Ice-sheets Using Novel Optimised Simulations
Acronym/nickname
DOMINOS

 Projected rates of sea level rise from the West Antarctic Ice Sheet (and Thwaites Glacier in particular) have large uncertainties due to difficulties in understanding and projecting the calving and dynamic processes that control the ice sheet stability. This uncertainty is magnified by the poorly understood connection between calving processes, ice sheet stability and climate. To address these uncertainties, our proposal seeks to explicitly resolve the processes that could cause retreat and collapse of Thwaites Glacier using a novel ice-dynamics model suite. This model suite includes a discrete element model capable of simulating coupled fracture and ice-flow processes, a 3D full Stokes continuum model, and the continental scale ice-dynamics model BISICLES. Ice dynamics models will be coupled to an ocean forcing model suite including simple plume models, intermediate complexity 2-layer ocean models and fully 3D regional ocean models. This hierarchical approach will use high-fidelity process models to inform and constrain the sequence of lower-order models needed to extrapolate improved understanding to larger scales and has the potential to radically reduce uncertainty of rates of marine ice sheet collapse and associated sea level rise. The large-scale modeling approach will be tested and implemented within the open source BISICLES ice dynamics model and made publicly available to other researchers via a “calving package.”

45
If the rate of potential collapse of the WAIS is relatively fast (once initiated) then within a few decades (say starting 2060) potential new seaways through the WAIS may become increasingly important (see the linked reference and attached image):

David G. Vaughan et al. (07 October 2011), "Potential seaways across West Antarctica", Geochemistry, Geophysics, Geosystems, https://doi.org/10.1029/2011GC003688

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2011GC003688

Abstract
The West Antarctic ice sheet (WAIS) has long been considered vulnerable to rapid retreat and today parts are rapidly losing ice. Projection of future change in WAIS is, however, hampered by our poor understanding of past changes, especially during interglacial periods that could be analogs for the future, but which undoubtedly provide an opportunity for testing predictive models. We consider how ice‐loss would open seaways across WAIS; these would likely alter Southern Ocean circulation and climate, and would broadly define the de‐glacial state, but they may also have left evidence of their existence in the coastal seas they once connected. We show the most likely routes for such seaways, and that a direct seaway between Weddell and Ross seas, which did not pass through the Amundsen Sea sector, is unlikely. Continued ice‐loss at present rates would open seaways between Amundsen and Weddell seas (A‐W), and Amundsen and Bellingshausen seas (A‐B), in around one thousand years. This timescale indicates potential future vulnerability, but also suggests seaways may have opened in recent interglacial periods. We attempt to test this hypothesis using contemporary bryozoan species assemblages around Antarctica, concluding that anomalously high similarity in assemblages in the Weddell and Amundsen seas supports recent migration through A‐W. Other authors have suggested opening of seaways last occurred during Marine Isotope Stage 7a (209 ka BP), but we conclude that opening could have occurred in MIS 5e (100 ka BP) when Antarctica was warmer than present and likely contributed to global sea levels higher than today.

Caption: "Figure 1. Map of Antarctica (with inset of West Antarctica) showing the thickness of ice that would need to be removed before flotation would occur, calculated assuming an ice‐density of 910 kg m−3,seawater‐density of 1030 kg m−3, a satellite‐ derived ice‐surface elevation model [Bamber et al., 2009a] and sub‐glacial bed elevation [Le Brocq et al., 2010] supplemented with unpublished data collected inland of Eltanin Bay in 2009/10. Elevationsreferenced to the EIGEN‐GL04c geoid and current sea level. The labeled sections, defining our hypothesized seaways,were chosen as the routes requiring least ice loss. The location of Up‐ B is shown according to the position given by Whillans et al. [1987]."

46
Re: "Jakobshavn is currently retreating up a positively sloped ice bed"

Wait, what ? There's a big hole behind the current grounding line ...

sidd

As you point out yourself the grounding line has not yet reached the retrograde slope, thus the measured rate of retreat of the Jakobshavn grounding line of 13 km/yr (per averaged calving event) only includes its retreat up the positive slope of the ice bed.

Edit: I further note that as the Jakobshavn calving face can advance by about 12 km/yr, the actual net retreat of the Jakobshavn grounding line is only about 1 km/yr.

Edit2: The attached image makes it clearer that the Jakobshavn calving face (issued 2017) is still climbing a positive bed slope.  But perhaps sidd's point is that after this calving face retreats down the retrograde (negative) bed slope, then Pollard, DeConto and Alley can take new measurements of the average rate of calving face retreat per year and then update their model projections with rates above their current maximum assumed rate of retreat of 13 km/yr.

47
While my last two posts addressed various issues related to the potential initiation of rapid ice mass loss from the AIS; so in this post I briefly touch upon the other primary issue about ice mass loss from the AIS; which is once initiated 'How fast will it proceed".  As Lennart van der Linde states in Reply # 278:

"Thanks for the Rob DeConto presentation of March 30th 2018, ASLR.

From 56m-66m I find particularly interesting, where he talks about (quite arbitrary) speed limits for cliff failure in his model, and stretching these limits in newer versions, as yet unpublished, if I understand correctly. Also atmospheric modelling would seem to slow melt in the first decades (compared to an earlier version), but cliff failure could speed it up more later, it seems from what he says here."


First, regarding Lennart's observation that the Pollard/DeConto AIS model now uses less atmospherically induced surface temperature increases than their earlier work; I believe, that the true value of ECS is actually higher than any of their models have assumed and that masking factors (like anthropogenic aerosols, etc.) have biased their (and most CMIP5 models) projections to err on the side of least drama.  Thus if these various masking factors are reduced (or eliminated) faster than expected by 2040; then the ice mass loss from Antarctica will proceed faster after 2040 than projected by Pollard, DeConto and Alley (2018).

Second, regarding Lennart's observation that ice-cliff failures may occur faster than project by the Pollard/DeConto AIS model indicates, I note that Pollard/DeConto/Alley limited the rate of ice-cliff retreat to the maximum observed for the Jakobshavn Glacier of about 13 km/a (see the first attached image); however:

a. Jakobshavn ice flow is restrained on both sides by the wall of the fjord; while the Thwaites Glacier does not have comparable side restaints.
b. Jakobshavn is currently retreating up a positively sloped ice bed; while the Thwaites Glacier may very soon be retreating down a negatively sloped ice bed.
c. The current height of the Jakobshavn ice face is in the 100 to 120m range, while after a few tens of kilometers of retreat, the ice face for the Thwaites Glacier could be several hundred meters high and with a relative water depth w = D/H (water dept/ice face height) of 0.6 to 0.8; and the second attached image from Schlemm & Levermann (2018) indicates that the actual retreat rate could be well over 60 km/a.

Tanja Schlemm and Anders Levermann (2018), "A simple stress-based cliff-calving law", The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-205

https://www.the-cryosphere-discuss.net/tc-2018-205/tc-2018-205.pdf

Abstract. Over large coastal regions in Greenland and Antarctica the ice sheet calves directly into the ocean. In contrast to ice-shelf calving, an increase in cliff calving directly contributes to sea-level rise and a monotonously increasing calving rate with ice thickness can constitute a self-amplifying ice loss mechanism that may significantly alter sea-level projections both of Greenland and Antarctica. Here we seek to derive a minimalistic stress-based parameterization for cliff calving. To this end we compute the stress field for a glacier with a simplified two-dimensional geometry from the two-dimensional Stokes equation. First we assume a constant yield stress to derive the failure region at the glacier front from the stress field within the ice sheet. Secondly, we assume a constant response time of ice failure due to exceedance of the yield stress. With this strongly constraining but very simple set of assumption we propose a cliff-calving law where the calving rate follows a power-law dependence on the freeboard of the ice with exponents between 2 and 3 depending on the relative water depth at the calving front. The critical freeboard below which the ice front is stable decreases with increasing relative water depth of the calving front. For a dry water front it is, for example, 75m. The purpose of this study is not to provide a comprehensive calving law, but to derive a particularly simple equation with a transparent and minimalistic set of assumptions.

48
My last post focused on potential impacts of events not accounted for by Pollard, DeConto & Alley (2018) ice mass loss projections for Antarctica subjected to Pliocene conditions (which I had previously might occur as early as 2040), on potentially initiating rapid ice mass loss from the WAIS before 2040.  In this post, I briefly cite some initial conditions not assumed by Pollard, DeConto & Alley (2018), that might similarly advance the date for initiating rapid ice mass loss from the WAIS if they were considered in appropriate ice sheet model projections.

The first example is that the AIS may be currently losing more ice mass then is currently being measured even by data such as that reported by Slater & Shepherd (2018) (see the first image); because such ice mass loss data has primarily been gathered by the GRACE satellite; and this data is corrected to account for an assumed amount of glacial isostatic rebound; which may very well result in ice mass lose estimates that err on the side of least drama.  This is relevant because if ice mass is being lost from the AIS faster than expected, then Bronselaer et al (2018)'s projections of ice-climate feedback by 2040 may also err on the side of least drama, and the ocean temperatures at the grounding lines of key Antarctic marine glaciers may well reach Pliocene-levels prior to 2040.

Slater & Shepherd (2018), "Antarctic ice losses tracking high", Nature Climate Change. doi:10.1038/s41558-018-0284-9, https://doi.org/10.1038/s41558-018-0284-9


Extract: "To the Editor — Satellite observations show that ice losses from Antarctica have accelerated over the past 25 years. Since 1992, the continent has contributed 7.6 mm to global sea levels, with 40% of this occurring in the past 5 years. Glaciers draining West Antarctica have retreated, thinned and accelerated due to ocean-driven melting at their termini, and the collapse of ice shelves at the Antarctic Peninsula has led to reduced buttressing and increased ice discharge. Of the 3.2 mm yr−1 sea-level rise (SLR) measured during the satellite era, Antarctica has contributed 0.27 mm yr−1. The magnitude of SLR from Antarctica is the largest source of uncertainty in global sea-level projections, which are key to appropriate climate change policy."

Caption: "Fig. 1 | Observed and predicted SLR due to Antarctica. The global sea-level contribution from Antarctica according to the IMBIE satellite record (shaded envelope indicates 1σ) and IPCC AR5 upper, mid, and lower projections is shown from 1992–2040 (left) and 2040–2100 (right; values on the right-hand side indicate the average sea-level contribution predicted at 2100). Darker coloured lines represent pathways from the five scenarios used in AR5 in order of increasing emissions: RCP2.6, RCP4.5, RCP6.0, SRES A1B and RCP8.5. The circle plot (inset) shows the rate of SLR (in mm yr−1) during the overlap period 2007–2017 (vertical dashed lines). All AR5 projections have been offset by 0.66 ±  0.21 mm (range is 1σ) on average, to make them equal to the observational record at their start date (2007)."

Second, Pollard, DeConto and Alley (2018) assume that for ice-cliff failures to initiate, the cliff-face needs to extend from 90m to 110m above sea level; however, if the ice upstream of the ice face has more crevasses than assumed by Pollard, DeConto and Alley (2018), then ice-cliff failures could occur with lower heights of ice faces.  In this regard the second attached image shows that upstream of the current grounding line of the ice in the Thwaites Glacier threshold, the ice has an unusually large extent of crevassing, due to the bed topology.  Thus locally, ice-cliff failure mechanisms may develop sooner than Pollard, DeConto and Alley (2018) indicate.

Third, the Thwaites Glacier Ice Tongue and Eastern Ice Shelf may be more degraded than assumed in Pollard, DeConto and Alley (2018)'s model.  In this regards, the third image shows a Sentinel 1a image of the Thwaites Glacier Ice Tongue and Eastern Ice Shelf on Dec 8 2018; and while it is subjective, in my opinion it is highly likely that both the Thwaites Ice Tongue and Ice Shelf and the Pine Island Ice Shelf are currently both in more degraded conditions than assumed by Pollard, DeConto and Alley (2018); which indicates that by might break-up sooner than assumed by Pollard, DeConto and Alley (2018).

Finally, I have previously noted that the Beaufort Sea Gyre may well release a major discharge of relatively freshwater into the North Atlantic before 2040 and if so, the Ocean Overturning Current may soon be moving slower than assumed by Pollard, DeConto and Alley (2018), and if so they may well be erring on the side of least drama with regard to the impact of the ocean on grounding line retreat around the AIS.

49
In Replies #219 & #220 I noted that Bronselaer et al (2018); Hansen (2018); and Pollard, DeConto & Alley (2018) could be taken together to support the idea that the WAIS could start to exhibit rapid ice mass loss beginning about 2040.  However, in this post I note that these three references all deal with trends rather than with the possible impacts of episodic events and chaotic variability; which could trigger a rapid ice mass loss from the WAIS earlier than 2040.

For example, currently extreme El Nino events occur about every 20-years, with the last such occurrence being the 2015-16 event; which on average would put such another extreme event around 2035-36.  However, the linked article (and associated research) indicates that such extreme events will occur about every 10-years when GMSTA reaches about 1.5C, while Gavin Schmidt projects that GMSTA will be about 1.23C in 2019; which raises the probability that we will experience another extreme El Nino event sometime between 2030 and 2035.  Furthermore, the 2015-16 event resulted in significant amounts of surface ice melting on many West Antarctic ice shelves.  Thus, it is possible that hydrofracturing could lead to a collapse of the Thwaites Glacier residual Ice Tongue and Eastern Ice Shelf between 2031 and 2036.

Title: "‘Extreme’ El Niños to double in frequency under 1.5C of warming, study says"

https://www.carbonbrief.org/extreme-el-ninos-double-frequency-under-one-point-five-celsius-warming-study

Extract: "If global warming reaches 1.5C above pre-industrial levels – the aspirational limit of the Paris Agreement – extreme El Niño events could happen twice as often, the researchers find.

That means seeing an extreme El Niño on average every 10 years, rather every 20 years."

Also, in Replies #242 and #243, I discuss a subglacial lake drainage event beneath the Thwaites Glacier that occurred from June 2013 to January 2014; which may have been triggered by a September 2012 event [see Kim et al (2018)] that resulted in the formation of an abrupt drop in the local surface elevation in the trough shown in the first image by Tinto & Bell and the second and third images by Kim et al. (2018).  The September 2012 event also triggered a surge in the ice flow of the Thwaites Ice Tongue.

Seung Hee Kim, Duk-jin Kim and Hyun-Cheol Kim (2018), "Progressive Degradation of an Ice Rumple in the Thwaites Ice Shelf, Antarctica, as Observed from High-Resolution Digital Elevation Models", Remote Sens, 9, 1236; doi:10.3390/rs1008123

Abstract: "Ice rumples are locally-grounded features of flowing ice shelves, elevated tens of meters above the surrounding surface. These features may significantly impact the dynamics of ice-shelf grounding lines, which are strongly related to shelf stability. In this study, we used TanDEM-X data to construct high-resolution DEMs of the Thwaites ice shelf in West Antarctica from 2011 to 2013. We also generated surface deformation maps which allowed us to detect and monitor the elevation changes of an ice rumple that appeared sometime between the observations of a grounding line of the Thwaites glacier using Double-Differential Interferometric SAR (DDInSAR) in 1996 and 2011. The observed degradation of the ice rumple during 2011–2013 may be related to a loss of contact with the underlying bathymetry caused by the thinning of the ice shelf. We subsequently used a viscoelastic deformation model with a finite spherical pressure source to reproduce the surface expression of the ice rumple. Global optimization allowed us to fit the model to the observed deformation map, producing reasonable estimates of the ice thickness at the center of the pressure source. Our conclusion is that combining the use of multiple high-resolution DEMs and the simple viscoelastic deformation model is feasible for observing and understanding the transient nature of small ice rumples, with implications for monitoring ice shelf stability."

Extract: "…  we monitored the surface features of the ice rumple using Landsat 7 ETM+ images from 2003–2014 (Figure 7). The images in this time series showed a gradual dissipation of the ice rumple, strongly indicating continuous thinning of the Thwaites ice shelf. Furthermore, the disappearance of surface features (e.g., crevasses and surface gradient) from 2013 onwards suggests that the ice shelf has been ungrounded, removing the pressure point that had been maintaining the ice rumple. Nonetheless, the ice shelf might have been in contact with the pinning point even after the disappearance of surface features, as intermittent ice contact to the pinning point could be possible due to ice shelf thickness fluctuations [29].

According to the optimization result, the ice thickness was 711.64 ± 14.25 m and 683.76 ± 12.48 m in 2011 and 2012, respectively. This is quite different from the known ice thickness of the Thwaites ice shelf near the grounding line (~1 km), despite the low vertical resolution of the radar sounder used for such measurements [7]. This indicates a thickness decrease of 36.17 ± 17.27 m during that one-year period. However, as the center of the pressure source in 2012 was located 341.96 m upstream and 210.10 m to the west from that in 2011, it is difficult to substantiate the thickness and thickness change with the obtained datasets. Our results for the surface depression and thinning of the ice rumple in the Thwaites ice shelf were much higher than that previously reported. It is rather surprising to observe such high (>10 m) surface depressions in an ice shelf in such a short time; Rignot et al. [30] and Paolo et al. [14] reported that the thinning rate of the Thwaites ice shelf was 6.13 and 2.80 m/year, respectively. …
From 2011–2013, our deformation maps showed the recent fading of a small ice rumple in the surface of the Thwaites ice shelf, West Antarctica. The pinning point was located nearly 5 km offshore from the previously estimated grounding line in 2011, and appeared sometime between 1996 and 2011 when the grounding line of Thwaites Glacier retreated. The deformation pattern we found, along with a time series of Landsat 7 ETM+ imagery, showed that the ice was still in contact with the basal topography as late as 2013 but is likely to have since been unpinned. We then used the deformation maps with the simple viscoelastic deformation model (widely used in volcanic studies) to interpret the surface changes in terms of pressure changes at the bottom of the ice shelf by applying an idealized spherical pressure source. The estimated numbers were reasonable and the ice shelf thickness at the center of the spherical pressure source was also estimated using the depth and radius. The surface depression and thinning of this ice rumple were found to be much higher than those of previously reported levels for the broader region, …"

Caption for second image: "Figure 1. Grounding lines of Thwaites Glacier in 1996 (green) and 2011 (red) estimated using the DDInSAR method with European remote sensing (ERS) satellites [13,15]. The orange dotted rectangle in the eastern shelf indicates a larger ice rumple previously discussed by Tinto and Bell [7]. A newly generated digital elevation model derived from TanDEM-X data on 10 June 2011 is shown within the yellow rectangle. The small red feature inside the yellow dotted square indicates the smaller ice rumple considered in this study. The background image is the MODIS Mosaic of Antarctica (MOA) image map [16]. The overlaid ice velocity map was extracted from Rignot et al. [17].

Caption for the third image: "Figure 7. Landsat 7 ETM+ images from 2003–2014 showing the gradual disappearance of the studied ice rumple in the Thwaites ice shelf. Crevasses and surface gradients are generally created atop an ice rumple due to surface extension and elevation increase. Such features were visible as late as 2011 but disappeared by 2013, indicating gradual ice thinning. Larger images are magnifications of selected areas indicated by red boxes. The yellow dotted line was extracted from the grounding line of the MEaSUREs dataset [13,15]."

Regarding the possible implications of both event driven events cited previously in this post, if the Thwaites Glacier Ice Tongue and Ice Shelf collapse due to hydrofracturing circa 2031 to 2036 followed by the loss of an ice rumple in the trough identified by Tinto & Bell together with a subglacial lake draining event that drains through the very same trough, this might flush-out any floating icebergs within the trough; which might well move the location of the local grounding line towards the upstream in of the trough where ice cliff failure mechanisms might possibly occur before 2040.

50
Future freshwater exports from the Arctic into the North Atlantic can come several sources including: a) the Beaufort Gyre, b) melting Arctic Sea Ice and c) ice mass loss from the Greenland Ice Sheet.  Furthermore, this Arctic freshwater can follow different pathways, and the cited reference indicates that these different pathways would have different (but significant) impacts on both the North Atlantic Convection and on the AMOC.  This research provide insights into Hansen's ice-climate feedback mechanism:

Wang, He, Sonya Legg, and Robert Hallberg, July 2018: The Effect of Arctic Freshwater Pathways on North Atlantic Convection and the Atlantic Meridional Overturning Circulation. Journal of Climate, 31(13), DOI:10.1175/JCLI-D-17-0629.1 .

https://journals.ametsoc.org/doi/10.1175/JCLI-D-17-0629.1
&
https://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-17-0629.1

Abstract: "This study examines the relative roles of the Arctic freshwater exported via different pathways on deep convection in the North Atlantic and the Atlantic meridional overturning circulation (AMOC). Deep water feeding the lower branch of the AMOC is formed in several North Atlantic marginal seas, including the Labrador Sea, Irminger Sea, and the Nordic seas, where deep convection can potentially be inhibited by surface freshwater exported from the Arctic. The sensitivity of the AMOC and North Atlantic to two major freshwater pathways on either side of Greenland is studied using numerical experiments. Freshwater export is rerouted in global coupled climate models by blocking and expanding the channels along the two routes. The sensitivity experiments are performed in two sets of models (CM2G and CM2M) with different control simulation climatology for comparison. Freshwater via the route east of Greenland is found to have a larger direct impact on Labrador Sea convection. In response to the changes of freshwater route, North Atlantic convection outside of the Labrador Sea changes in the opposite sense to the Labrador Sea. The response of the AMOC is found to be sensitive to both the model formulation and mean-state climate."

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