Ice Apocalypse - Multiple Meters Sea Level Rise

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The linked reference indicates that the collapse of an ice sheet can result in the abrupt reorganization of the associated jet stream:

Juan M. Lora, Jonathan L. Mitchell & Aradhna E. Tripati (22 November 2016), "Abrupt reorganization of North Pacific and western North American climate during the last deglaciation", Geophysical Research Letters, DOI: 10.1002/2016GL071244

http://onlinelibrary.wiley.com/doi/10.1002/2016GL071244/full

Abstract: "Dramatic hydroclimate shifts occurred in western North America during the last deglaciation, but the timing and mechanisms driving these changes are uncertain and debated, and previous modeling has largely relied on linear interpolations between equilibrium snapshot simulations. Using a published transient climate simulation and a range of proxy records, we analyze the region's climate evolution in order to identify the mechanisms governing hydroclimate shifts. A rapid loss of ice around 14,000 years ago causes an abrupt reorganization of the circulation, which precipitates drying and moistening of southwestern and northwestern North America, respectively. The atmospheric circulation transitions between two states on a timescale of decades to centuries, during which time the westerly jet shifts north by about 7°. In contrast to previous studies, we find that changes in the water budget of western North America prior to this event are not attributable to variations in the position of the jet, but rather to the intensity of moisture transport into the continent."

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The linked reference evaluates the implications of more accurately considering a 3-D viscoelastic Earth models as opposed to the less accurate assumption of elastic response on the sea-level fingerprint implications of an abrupt collapse of the WAIS.  Their findings conclude that "… when viscous effects are included, the peak sea-level fall predicted in the vicinity of WAIS during a melt event will increase by ~25% and ~50%, relative to the elastic case, for events of duration 25 years and 100 years, respectively."  This is important w.r.t. global sea level rise as the further the local sea-level drops around West Antarctica, the higher sea level will raise at distance away from West Antarctica.

Carling C. Hay, Harriet C. P. Lau, Natalya Gomez, Jacqueline Austermann, Evelyn Powell, Jerry X. Mitrovica, Konstantin Latychev, and Douglas A. Wiens (2016), "Sea-level fingerprints in a region of complex Earth structure: The case of WAIS", Journal of Climate, DOI: http://dx.doi.org/10.1175/JCLI-D-16-0388.1

http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-16-0388.1

Abstract: "Sea-level fingerprints associated with rapid melting of the West Antarctic Ice Sheet (WAIS) have generally been computed under the assumption of a purely elastic response of the solid Earth. We investigate the impact of viscous effects on these fingerprints by computing gravitationally self-consistent sea-level changes that adopt a 3-D viscoelastic Earth model in the Antarctic region consistent with available geological and geophysical constraints. In West Antarctica, the model is characterized by a thin (~65 km) elastic lithosphere and sub-lithospheric viscosities that span three orders of magnitude, reaching values as low as ~4 × 1018 Pa s beneath WAIS. Our calculations indicate that sea-level predictions in the near field of WAIS will depart significantly from elastic fingerprints in as little as a few decades. For example, when viscous effects are included, the peak sea-level fall predicted in the vicinity of WAIS during a melt event will increase by ~25% and ~50%, relative to the elastic case, for events of duration 25 years and 100 years, respectively. Our results have implications for studies of sea-level change due to both ongoing mass loss from WAIS over the next century and future, large scale collapse of WAIS on century-to-millennial time scales."

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It is important to note here that the best ESMs cannot match the climate response during MIS 11c (the Holsteinian Peak), where MIS 11 extents from 424,000 to 374,000 years ago (see the first linked reference).  This means that the best CMIP5 models are currently underestimating the degree of nonlinear global warming that we are likely to experience with continued anthropogenic radiative forcing:

The Mid-Brunhes Event (MBE) coincides with MIS 11 (the Holsteinian) about 400,000 to 350,000 years ago, and marks a major transition to subsequent enhanced Arctic Amplification as discussed in the open access linked reference (see the first three attached images while the fourth image from another source help to clarify that after the MBE interglacial peak global mean peak temperatures have been higher).  Furthermore, the reference associates this change with the bipolar seesaw and episodic collapses of the WAIS.  This research clearly associates the bipolar seesaw mechanism with Hansen's ice-climate feedback and with Arctic Amplification.  This also implies that if the WAIS collapses this century, that warm Atlantic water will penetrate deep into the Arctic Ocean Basin, where it would likely have an impact on any shallow methane hydrates:

Cronin et al (2017), "Enhanced Arctic Amplification Began at the Mid-Brunhes Event ~400,000 years ago", Scientific Reports 7, Article No. 14475, doi: 10.1038/s41598-017-13821-2

https://www.nature.com/articles/s41598-017-13821-2

Extract: "Enhanced Arctic amplification at the MBE suggests a major climate threshold was reached at ~400 ka involving Atlantic Meridional Overturning Circulation (AMOC), inflowing warm Atlantic Layer water, ice sheet, sea-ice and ice-shelf feedbacks, and sensitivity to higher post-MBE interglacial CO₂ concentrations.
…
Southern Hemisphere ocean-atmosphere-sea ice processes are critical for understanding the MBE, specifically the idea that there is a bipolar seesaw operating between Northern and Southern Hemispheres on millennial timescales explain warmer interglacial condition in the Southern Hemisphere.  Barker et al. (2011) demonstrated that abrupt millennial-scale AMOC variability characterized the last 800 ka, albeit without the large amplitude shift seen in our Arctic records.  Holden et al. proposed a role for decreased stability of the West Antarctic Ice Sheet following the MBE, leading to AMOC slowdown during deglacials.  Thus, it is possible that ice sheet/ice shelf instability characterized both hemispheres providing the necessary non-linear dynamics to explain large amplitude temperature events in the Arctic Ocean.  However establishing the relationship between bottom temperature, sea ice and productivity during stadial and interstadial periods – require better sediment core resolution in the Arctic.  Nonetheless, the large shift in Arctic land ice, ice shelves and sea ice at the MBE, suggests an amplification of Arctic climate sensitivity related to higher interglacial CO₂ concentrations and associated feedbacks involving ice shelves and ice sheets, Heinrich-like events, AMOC-forcing Arctic Ocean temperature oscillations, and deeper submergence of Atlantic water in the central Arctic Basin."
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Furthermore, the linked reference studies the paleo decay of the Cordilleran ice sheet and finds that it lost most of its ice mass earlier than consensus science previously thought, and it lost much of its ice mass over a relatively short period.  CMIP5 models were not calibrated with such information which indicates that dry land ice sheets may be less stable during global warming periods.  Personally, I am concerned about the impact of rainfall at increasingly high latitudes (with warming) on both the Greenland Ice Sheet, on Arctic permafrost, and on the WAIS:

B. Menounos et al (10 Nov 2017), "Cordilleran Ice Sheet mass loss preceded climate reversals near the Pleistocene Termination", Science, Vol. 358, Issue 6364, pp. 781-784, DOI: 10.1126/science.aan3001

http://science.sciencemag.org/content/358/6364/781

Abstract: "The Cordilleran Ice Sheet (CIS) once covered an area comparable to that of Greenland. Previous geologic evidence and numerical models indicate that the ice sheet covered much of westernmost Canada as late as 12.5 thousand years ago (ka). New data indicate that substantial areas throughout westernmost Canada were ice free prior to 12.5 ka and some as early as 14.0 ka, with implications for climate dynamics and the timing of meltwater discharge to the Pacific and Arctic oceans. Early Bølling-Allerød warmth halved the mass of the CIS in as little as 500 years, causing 2.5 to 3.0 meters of sea-level rise. Dozens of cirque and valley glaciers, along with the southern margin of the CIS, advanced into recently deglaciated regions during the Bølling-Allerød and Younger Dryas."

Extract: "Disappearance of an ice sheet
The Cordilleran Ice Sheet is thought to have covered westernmost Canada until about 13,000 years ago, even though the warming and sea level rise of the last deglaciation had begun more than a thousand years earlier. This out-of-phase behavior has puzzled glaciologists because it is not clear what mechanisms could account for it. Menounos et al. report measurements of the ages of cirque and valley glaciers that show that much of western Canada was ice-free as early as 14,000 years ago—a finding that better agrees with the record of global ice volume (see the Perspective by Marcott and Shakun). Previous reconstructions seem not to have adequately reflected the complexity of ice sheet decay."

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The linked reference indicates that the PIIS is calving further upstream (thru 2015) than in the past due to various oceanic driven factors including intensified melting with basal crevasses.  This means that the  calving face of the PIIS is likely to continue to retreat, even before GMSTA warms sufficiently for hydrofracturing to become a significant failure mechanism for the PIIS.

Seongsu Jeong, Ian M. Howat & Jeremy N. Bassis (28 November 2016), "Accelerated ice shelf rifting and retreat at Pine Island Glacier, West Antarctica", Geophysical Research Letters, DOI: 10.1002/2016GL071360

http://onlinelibrary.wiley.com/doi/10.1002/2016GL071360/full

Abstract: "Pine Island Glacier has undergone several major iceberg calving events over the past decades. These typically occurred when a rift at the heavily fractured shear margin propagated across the width of the ice shelf. This type of calving is common on polar ice shelves, with no clear connection to ocean-ice dynamic forcing. In contrast, we report on the recent development of multiple rifts initiating from basal crevasses in the center of the ice shelf, resulted in calving further upglacier than previously observed. Coincident with rift formation was the sudden disintegration of the ice mélange that filled the northern shear margin, resulting in ice sheet detachment from this margin. Examination of ice velocity suggests that this internal rifting resulted from the combination of a change in ice shelf stress regime caused by disintegration of the mélange and intensified melting within basal crevasses, both of which may be linked to ocean forcing."

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Most climate models do not include wind-albedo interaction that is currently inducing surface melting in portions of East Antarctica, but the linked reference documents field evidence for this mechanism that could contribute to a faster rate of AIS destabilization with global warming then currently projected:

J. T. M. Lenaerts, et. al.  (2016), "Meltwater produced by wind–albedo interaction stored in an East Antarctic ice shelf", Nature Climate Change, doi:10.1038/nclimate3180

http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3180.html

Abstract: "Surface melt and subsequent firn air depletion can ultimately lead to disintegration of Antarctic ice shelves causing grounded glaciers to accelerate and sea level to rise. In the Antarctic Peninsula, foehn winds enhance melting near the grounding line, which in the recent past has led to the disintegration of the most northerly ice shelves. Here, we provide observational and model evidence that this process also occurs over an East Antarctic ice shelf, where meltwater-induced firn air depletion is found in the grounding zone. Unlike the Antarctic Peninsula, where foehn events originate from episodic interaction of the circumpolar westerlies with the topography, in coastal East Antarctica high temperatures are caused by persistent katabatic winds originating from the ice sheet’s interior. Katabatic winds warm and mix the air as it flows downward and cause widespread snow erosion, explaining >3 K higher near-surface temperatures in summer and surface melt doubling in the grounding zone compared with its surroundings. Additionally, these winds expose blue ice and firn with lower surface albedo, further enhancing melt. The in situ observation of supraglacial flow and englacial storage of meltwater suggests that ice-shelf grounding zones in East Antarctica, like their Antarctic Peninsula counterparts, are vulnerable to hydrofracturing."

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The linked reference provides field evidence supporting Hansen's ice-climate interaction mechanism.

Pepijn Bakker et al, Centennial-scale Holocene climate variations amplified by Antarctic Ice Sheet discharge, Nature (2016). DOI: 10.1038/nature20582

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature20582.html

Abstract: "Proxy-based indicators of past climate change show that current global climate models systematically underestimate Holocene-epoch climate variability on centennial to multi-millennial timescales, with the mismatch increasing for longer periods. Proposed explanations for the discrepancy include ocean–atmosphere coupling that is too weak in models, insufficient energy cascades from smaller to larger spatial and temporal scales, or that global climate models do not consider slow climate feedbacks related to the carbon cycle or interactions between ice sheets and climate. Such interactions, however, are known to have strongly affected centennial- to orbital-scale climate variability during past glaciations, and are likely to be important in future climate change. Here we show that fluctuations in Antarctic Ice Sheet discharge caused by relatively small changes in subsurface ocean temperature can amplify multi-centennial climate variability regionally and globally, suggesting that a dynamic Antarctic Ice Sheet may have driven climate fluctuations during the Holocene. We analysed high-temporal-resolution records of iceberg-rafted debris derived from the Antarctic Ice Sheet, and performed both high-spatial-resolution ice-sheet modelling of the Antarctic Ice Sheet and multi-millennial global climate model simulations. Ice-sheet responses to decadal-scale ocean forcing appear to be less important, possibly indicating that the future response of the Antarctic Ice Sheet will be governed more by long-term anthropogenic warming combined with multi-centennial natural variability than by annual or decadal climate oscillations."

See also the linked article entitled: "Antarctic Ice Sheet study reveals 8,000-year record of climate change".

http://phys.org/news/2016-12-antarctic-ice-sheet-reveals-year.html

Extract: "An international team of researchers has found that the Antarctic Ice Sheet plays a major role in regional and global climate variability - a discovery that may also help explain why sea ice in the Southern Hemisphere has been increasing despite the warming of the rest of the Earth.

…
Global climate models that look at the last several thousand years have failed to account for the amount of climate variability captured in the paleoclimate record, according to lead author Pepijn Bakker, a climate modeller from the MARUM Center for Marine Environmental Studies at the University of Bremen in Germany.

The researchers first turned their attention to the Scotia Sea. "Most icebergs calving off the Antarctic Ice Sheet travel through this region because of the atmospheric and oceanic circulation," explained Weber. "The icebergs contain gravel that drop into the sediment on the ocean floor - and analysis and dating of such deposits shows that for the last 8,000 years, there were centuries with more gravel and those with less."

The research team's hypothesis is that climate modellers have historically overlooked one crucial element in the overall climate system. They discovered that the centuries-long phases of enhanced and reduced Antarctic ice mass loss documented over the past 8,000 years have had a cascading effect on the entire climate system.

Using sophisticated computer modelling, the researchers traced the variability in iceberg calving (ice that breaks away from glaciers) to small changes in ocean temperatures."

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With continued global warming one can expect more Agulhas leakage; which per the linked reference means that one can expect the AMOC to continue slowing; which should work synergistically with Hansen's ice-climate feedback, particularly if the WAIS collapses in coming decades:

Kathryn A. Kelly, Kyla Drushka, LuAnne Thompson, Dewi Le Bars & Elaine L. McDonagh (25 July 2016), "Impact of slowdown of Atlantic overturning circulation on heat and freshwater transports", Geophysical Research Letters, DOI: 10.1002/2016GL069789

http://onlinelibrary.wiley.com/doi/10.1002/2016GL069789/abstract

Abstract: "Recent measurements of the strength of the Atlantic overturning circulation at 26°N show a 1 year drop and partial recovery amid a gradual weakening. To examine the extent and impact of the slowdown on basin wide heat and freshwater transports for 2004–2012, a box model that assimilates hydrographic and satellite observations is used to estimate heat transport and freshwater convergence as residuals of the heat and freshwater budgets. Using an independent transport estimate, convergences are converted to transports, which show a high level of spatial coherence. The similarity between Atlantic heat transport and the Agulhas Leakage suggests that it is the source of the surface heat transport anomalies. The freshwater budget in the North Atlantic is dominated by a decrease in freshwater flux. The increasing salinity during the slowdown supports modeling studies that show that heat, not freshwater, drives trends in the overturning circulation in a warming climate."

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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.

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."

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The findings of the linked reference highlight the risk that with continued global warming this century the tropical rainforests could collapse; which, would increase realized temperatures above those projected this century by CMIP5.  Such a collapse of the tropical rainforests would not only mean the loss of a major carbon sink but would also increase carbon emissions from both the decaying dead forests & the associated peatlands:

Isabel P. Montañez, Jennifer C. McElwain, Christopher J. Poulsen Joseph D. White, William A. DiMichele, Jonathan P. Wilson, Galen Griggs & Michael T. Hren (2016), "Climate, pCO2 and terrestrial carbon cycle linkages during late Palaeozoic glacial–interglacial cycles, Nature Geoscience", nature.com/articles/doi:10.1038/ngeo2822


http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2822.html

Abstract: "Earth’s last icehouse, 300 million years ago, is considered the longest-lived and most acute of the past half-billion years, characterized by expansive continental ice sheets and possibly tropical low-elevation glaciation. This atypical climate has long been attributed to anomalous radiative forcing promoted by a 3% lower incident solar luminosity and sustained low atmospheric pCO2 (≤300 ppm). Climate models, however, indicate a CO2 sensitivity of ice-sheet distribution and sea-level response that questions this long-standing climate paradigm by revealing major discrepancy between hypothesized ice distribution, pCO2, and geologic records of glacioeustasy. Here we present a high-resolution record of atmospheric pCO2 for 16 million years of the late Palaeozoic, developed using soil carbonate-based and fossil leaf-based proxies, that resolves the climate conundrum. Palaeo-fluctuations on the 105-yr scale occur within the CO2 range predicted for anthropogenic change and co-vary with substantial change in sea level and ice volume. We further document coincidence between pCO2 changes and repeated restructuring of Euramerican tropical forests that, in conjunction with modelled vegetation shifts, indicate a more dynamic carbon sequestration history than previously considered and a major role for terrestrial vegetation–CO2 feedbacks in driving eccentricity-scale climate cycles of the late Palaeozoic icehouse."

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I provide the following to linked references to indicate that correctly accounting for dynamical climate sensitivity in the coming decades require the use of very sophisticated climate models that can account for bipolar seesaw mechanisms correctly and which correctly apply paleo-lessons-learned (including the impacts of freshwater hosing) to our current dynamic conditions:


Joel Pedro, Markus Jochum, Christo Buizert, Sune Rasmussen, and Feng He (2017), "The Bipolar Seesaw and Its Discontents", Geophysical Research Abstracts, Vol. 19, EGU2017-11688

http://meetingorganizer.copernicus.org/EGU2017/EGU2017-11688.pdf

Abstract: "The thermal bipolar ocean seesaw hypothesis is the prevailing explanation for the out-of-phase changes in northern and southern high-latitude climate during the Dansgaard-Oeschger (D-O) events of the last glacial period and deglaciation (Stocker and Johnsen, 2003). However the seesaw hypothesis has been challenged on several grounds: it neglects the much larger transport of heat in the atmosphere compared to ocean, and it does not specify the modes and time scales of signal propagation in the coupled ocean-atmosphere system. The purpose of this presentation is to critically review the seesaw hypothesis and address these critiques.

We use transient simulations with a coupled ocean-atmosphere-sea-ice global climate model (GCM) to trace the ocean and atmospheric heat-transport changes and pathways of inter-hemispheric signal propagation during a simulated collapse and a simulated strengthening of the Atlantic Meridional Overturning Circulation (AMOC). While the simulated AMOC perturbations result in climate variations in close agreement with palaeoclimate observations, changes to the heat budget and their propagation throughout the globe differ from the ideas of Stocker and Johnsen (2003). The key differences are as follows. (1) Changes in ocean heat transport in the Atlantic in response to AMOC perturbations are partially compensated by changes in northward heat transport in the global atmosphere and in the Pacific Ocean. (2) There is little ocean transmission of temperature anomalies between the South Atlantic and high latitude Southern Ocean, because the lack of zonal boundaries and the steeply outcropping isopycnals of the Antarctic Circumpolar Current (ACC) act as a barrier to signal propagation. (3) On the multi-centennial timescale of the simulations the heat content of the Southern Ocean to the south of the ACC is insensitive to AMOC changes, and South Atlantic temperature anomalies, rather than crossing the ACC spread at intermediate depths into the Indian and Pacific oceans. (4) The global intermediate-depth ocean to the north of the ACC thus better fits the description of being a ’heat reservoir’ for changes in the AMOC than the Southern Ocean. (5) In the simulations, signal propagation to latitudes south of the ACC (including Antarctica) is dominated by teleconnections between the Hadley Circulation, the mid-latitude westerlies and Southern Ocean sea ice extent.

We conclude with an inter-hemispheric coupling hypothesis that recognises the coupled nature of (interbasin) ocean and atmosphere heat transport, the difficulty of propagating ocean anomalies across the ACC and the role of wind-stress, sea ice and associated surface heat flux changes on temperature variations at high latitudes.

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The linked reference models the influence of surface melt lakes on the Larsen C Ice Shelf, and finds that not only do such melt lakes increase the risk of abrupt hydrofracturing failure of the ice shelf, but also: a) decreases surface albedo which increases surface ice melting and b) softens the ice associated with crevasses in the ice.  Such feedback mechanisms may soon become applicable to more southerly ice shelves around Antarctica in the coming decades:

Buzzard, S., Feltham, D., and Flocco, D.: Modelling the fate of surface melt on the Larsen C Ice Shelf, The Cryosphere, 12, 3565-3575, https://doi.org/10.5194/tc-12-3565-2018, 2018.

https://www.the-cryosphere.net/12/3565/2018/
https://www.the-cryosphere.net/12/3565/2018/tc-12-3565-2018.pdf

Abstract: "Surface melt lakes lower the albedo of ice shelves, leading to additional surface melting. This can substantially alter the surface energy balance and internal temperature and density profiles of the ice shelf. Evidence suggests that melt lakes also played a pivotal role in the sudden collapse of the Larsen B Ice Shelf in 2002.

Here a recently developed, high-physical-fidelity model accounting for the development cycle of melt lakes is applied to the Larsen C Ice Shelf, Antarctica's most northern ice shelf and one where melt lakes have been observed. We simulate current conditions on the ice shelf using weather station and reanalysis data and investigate the impacts of potential future increases in precipitation and air temperature on melt lake formation, for which concurrent increases lead to an increase in lake depth.

Finally, we assess the viability in future crevasse propagation through the ice shelf due to surface meltwater accumulation."

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The linked reference indicates that 'Arctic Browning' is currently reducing the Arctic's ability to absorb CO₂.

Rachael Treharne et al. (25 November 2018), "Arctic browning: Impacts of extreme climatic events on heathland ecosystem CO2 fluxes", Global Change Biology, https://doi.org/10.1111/gcb.14500

https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.14500

Abstract: "Extreme climatic events are among the drivers of recent declines in plant biomass and productivity observed across Arctic ecosystems, known as “Arctic browning.” These events can cause landscape‐scale vegetation damage and so are likely to have major impacts on ecosystem CO2 balance. However, there is little understanding of the impacts on CO2 fluxes, especially across the growing season. Furthermore, while widespread shoot mortality is commonly observed with browning events, recent observations show that shoot stress responses are also common, and manifest as high levels of persistent anthocyanin pigmentation. Whether or how this response impacts ecosystem CO2 fluxes is not known. To address these research needs, a growing season assessment of browning impacts following frost drought and extreme winter warming (both extreme climatic events) on the key ecosystem CO2 fluxes Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), ecosystem respiration (Reco) and soil respiration (Rsoil) was carried out in widespread sub‐Arctic dwarf shrub heathland, incorporating both mortality and stress responses. Browning (mortality and stress responses combined) caused considerable site‐level reductions in GPP and NEE (of up to 44%), with greatest impacts occurring at early and late season. Furthermore, impacts on CO2 fluxes associated with stress often equalled or exceeded those resulting from vegetation mortality. This demonstrates that extreme events can have major impacts on ecosystem CO2 balance, considerably reducing the carbon sink capacity of the ecosystem, even where vegetation is not killed. Structural Equation Modelling and additional measurements, including decomposition rates and leaf respiration, provided further insight into mechanisms underlying impacts of mortality and stress on CO2 fluxes. The scale of reductions in ecosystem CO2 uptake highlights the need for a process‐based understanding of Arctic browning in order to predict how vegetation and CO2 balance will respond to continuing climate change."

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The first linked reference determines the paleoclimate sensitivity during the Middle Eocene at a latitude of 64 degrees 48 minutes in Canada (see the first image), and finds a regional climate sensitivity of about 13C.  The second image illustrates that these findings indicate exceptionally high values of Arctic Amplification during this period as the atmospheric carbon dioxide concentrations were found to be only about 490 ppm.  While the mean global surface temperature was warmer during the Middle Eocene than today, we are warming at a much faster rate and C02e is already above 530 ppm.  This does not bode well for our collective future:

Alexander P. Wolfe, Alberto V. Reyes, Dana L. Royer, David R. Greenwood, Gabriela Doria, Mary H. Gagen, Peter A. Siver and John A. Westgate (May 2017), "Middle Eocene CO2 and climate reconstructed from the sediment fill of a subarctic kimberlite maar", GEOLOGY, July 2017; v. 45; no. 7; p. 619–622, doi:10.1130/G39002.1


https://gsw.silverchair-cdn.com/gsw/Content_public/Journal/geology/45/7/10.1130_G39002.1/1/619.pdf?Expires=1502222555&Signature=bqBU8Y3KgwV619Rh98~HEPqPp~aWdJ3w9x893T75q0T5Bn70XB~7Xvjub8K7QrFGN5OhK1RYvai3Aw5yfCYLSjKnKMt7KIMCoZnbo8drd9wtDSqrfEqLJJYFd6X7WWR~nBW9BCmhI0t2QOV2QqS7xkvQPDLc~saDe8e9-V8rrwXRI~WR-KsTvbGe2wz~XUmEU3c-lt~TD1TLajAj4Cb5EVeLNGjtF~0pt2fdKtvHMbl8C9~r5TimyGysbu5vExwPrbZvpLvfjxzipB-l5fiD7QH9qCslsthuwWOPIGGCUquL0tI6lMHQZXugcX5ix1ge4Uj7Ed6RQVvB07liZCi7mA__&Key-Pair-Id=APKAIUCZBIA4LVPAVW3Q
&
http://www.geosociety.org/datarepository/2017/2017202.pdf

Abstract: "Eocene paleoclimate reconstructions are rarely accompanied by parallel estimates of CO2 from the same locality, complicating assessment of the equilibrium climate response to elevated CO2. We reconstruct temperature, precipitation, and CO2 from latest middle Eocene (ca. 38 Ma) terrestrial sediments in the post eruptive sediment fill of the Giraffe kimberlite in subarctic Canada. Mutual climatic range and oxygen isotope analyses of botanical fossils reveal a humid temperate forest ecosystem with mean annual temperatures (MATs) more than 17 °C warmer than present and mean annual precipitation ~4× present. Metasequoia stomatal indices and gas-exchange modeling produce median CO2 concentrations of ~630 and ~430 ppm, respectively, with a combined median estimate of ~490 ppm. Reconstructed MATs are more than 6 °C warmer than those produced by Eocene climate models forced at 560 ppm CO2. Estimates of regional climate sensitivity, expressed as ΔMAT per CO2 doubling above preindustrial levels, converge on a value of ~13 °C, underscoring the capacity for exceptional polar amplification of warming and hydrological intensification under modest CO2 concentrations once both fast and slow feedbacks become expressed."

As a follow-on about Arctic Amplification, the second linked reference indicates that freshwater hosing events in the North Atlantic can result in warming of the Nordic Seas (see the third image); which can accelerate Arctic Amplification & which is another example of dynamical climate sensitivity:

Mélanie Wary et. al. (2017), "Regional seesaw between North Atlantic and Nordic Seas
during the last glacial abrupt climate events", Clim. Past Discuss., doi:10.5194/cp-2017-14

http://www.clim-past-discuss.net/cp-2017-14/cp-2017-14.pdf

Abstract. Dansgaard-Oeschger oscillations constitute one of the most enigmatic features of the last glacial cycle.  Their cold atmospheric phases have been commonly associated with cold sea-surface temperatures and expansion of sea ice in the North Atlantic and adjacent seas. Here, based on dinocyst analyses from the 48-30 ka BP interval of four sediment cores from the northern Northeast Atlantic and southern Norwegian Sea, we provide direct and quantitative evidence of a regional paradoxical seesaw pattern: cold Greenland and North Atlantic phases coincide with warmer sea-surface conditions and shorter seasonal sea-ice cover durations in the Norwegian Sea as compared to warm phases. Combined with additional paleorecords and multi-model hosing simulations, our results suggest that during cold Greenland phases, reduced Atlantic meridional overturning circulation and cold North Atlantic sea-surface conditions were accompanied by the subsurface propagation of warm Atlantic waters that re-emerged in the Nordic Seas and provided moisture towards Greenland summit.

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The first linked article entitled: "SkS Analogy 4 - Ocean Time Lag" illustrates how consensus based 'scientific' call to action can greatly underplay the risks associated with regard to dynamical climate sensitivity as illustrated by the second linked reference associated with the influences that the IPO as short-term GMSTA.  The first attached image is from the first reference & indicates that due to a 30-year lag we will not reach 2C warming until 2035 + 30 – 2065.  However, the second & third images, from the second reference, indicate respectively that we appear to have entered a warm IPO period (which may well last until ~2035); which indicates that we could reach +1.8C by 2034 (when considering the confidence range).

https://www.skepticalscience.com/SkS_Analogy_04_Ocean_Time_Lag.html

Extract: "Greenhouse gases (GHG) determine amount of warming, but oceans delay the warming.
…
This figure therefore shows the temperature anomaly starting in 1970, the year when the temperature increase due to greenhouse gases began to emerge from the background noise. This figure indicates 3 things: (1) the time lag between emitting greenhouse gases and when we see the principle effect is about 30 years, due mostly to the time required to heat the oceans, (2) the rate of temperature increase predicted by a climate sensitivity of 3°C tracks well with the observed rate of temperature increase, and (3) we have already locked in more than 1.5°C warming. As of 2017 we have reached 406 ppm CO2. At the current increase of 2 ppm CO2/yr., this implies that we will reach 440 ppm and lock in 2°C warming by 2035 … if we don’t act now."

The second reference is:

Henley, B. J and King, A. D. (2017) Trajectories toward the 1.5C Paris target: Modulation by the Interdecadal Pacific Oscillation, Geophysical Research Letters, doi:10.1002/2017GL073480

http://onlinelibrary.wiley.com/doi/10.1002/2017GL073480/abstract

Abstract: "Global temperature is rapidly approaching the 1.5°C Paris target. In the absence of external cooling influences, such as volcanic eruptions, temperature projections are centered on a breaching of the 1.5°C target, relative to 1850–1900, before 2029. The phase of the Interdecadal Pacific Oscillation (IPO) will regulate the rate at which mean temperature approaches the 1.5°C level. A transition to the positive phase of the IPO would lead to a projected exceedance of the target centered around 2026. If the Pacific Ocean remains in its negative decadal phase, the target will be reached around 5 years later, in 2031. Given the temporary slowdown in global warming between 2000 and 2014, and recent initialized decadal predictions suggestive of a turnaround in the IPO, a sustained period of rapid temperature rise might be underway. In that case, the world will reach the 1.5°C level of warming several years sooner than if the negative IPO phase persists."

Plain Language Summary
Global temperature is rapidly approaching the 1.5°C Paris target. In this study, we find that in the absence of external cooling influences, such as volcanic eruptions, the midpoint of the spread of temperature projections exceeds the 1.5°C target before 2029, based on temperatures relative to 1850–1900. We find that the phase of the Interdecadal Pacific Oscillation (IPO), a slow-moving natural oscillation in the climate system, will regulate the rate at which global temperature approaches the 1.5°C level. A transition to the positive phase of the IPO would lead to a projected exceedance of the target centered around 2026. If the Pacific Ocean remains in its negative phase, however, the projections are centered on reaching the target around 5 years later, in 2031. Given the temporary slowdown in global warming between 2000 and 2014, and recent climate model predictions suggestive of a turnaround in the IPO, a sustained period of rapid temperature rise might be underway. In that case, the world will reach the 1.5°C level of warming several years sooner than if the negative IPO phase persists.


See also the associated following article entitled: "Pacific Ocean shift could see 1.5C limit breached within a decade":

https://www.carbonbrief.org/pacific-ocean-shift-could-see-1point5-limit-breached-within-decade

[AbruptSLR]

Under the new Shared Socioeconomic Pathways, SSP, scenarios (see the first two summary images) RCP 8.5 roughly corresponds to the SSP5 baseline.  Thus I provide a link to a reference that describes SSP5, from which I took the third attached image of SSP5's assumed world population (peaking at about 8.5 billion circa 2060), GDP, Energy Demand and Food Demand.  The fourth attached image shows the UN's 2017 world population projections that indicate a 50/50 chance of reaching a world population of over 10 billion by 2060 (which SSP3 assumes).  Thus either we had better get off our current BAU pathway immediately, or the world population will become unsustainable long before 2060:

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."

For SSP population information see:

http://www.sciencedirect.com/science/article/pii/S0959378014001095

For a SSP Poster see:

https://unfccc.int/files/science/workstreams/research/application/pdf/part1_iiasa_rogelj_ssp_poster.pdf

For Current SSP information see the website entitled: "SSP Database (Shared Socioeconomic Pathways) - Version 1.1"

https://tntcat.iiasa.ac.at/SspDb/dsd?Action=htmlpage&page=about
&

See also:

2018 World Population Data Sheet With Focus on Changing Age Structures
https://www.prb.org/2018-world-population-data-sheet-with-focus-on-changing-age-structures/
The world population will reach 9.9 billion by 2050, up 2.3 billion or 29 percent from an estimated 7.6 billion people now, according to projections by Population Reference Bureau (PRB) included in the 2018 World Population Data Sheet.

[AbruptSLR]

The following reference provides direct terrestrial paleo-evidence from Antarctica supporting Hansen's ice-climate feedback mechanism during the Antarctic Cold Reversal (ACR; 14,600-12,700 years ago). including cooling of the surface of the Southern Ocean, slowing of the Overturning Current and accelerated ice mass loss from marine glaciers around Antarctica as warm CDW was directed towards the grounding line for these marine glaciers (see the attached image):

Fogwill et. al. (2017), "Antarctic Ice Sheet Discharge Driven by Atmospheric-Ocean Feedbacks at the Last Glacial Termination", Scientific Reports, 7, Article No. 39979, doi: 10.1038/srep39979.

http://www.nature.com/articles/srep39979

Abstract: "Reconstructing the dynamic response of the Antarctic ice sheets to warming during the Last Glacial Termination (LGT; 18,000–11,650 yrs ago) allows us to disentangle ice-climate feedbacks that are key to improving future projections. Whilst the sequence of events during this period is reasonably well known, relatively poor chronological control has precluded precise alignment of ice, atmospheric and marine records, making it difficult to assess relationships between Antarctic ice-sheet (AIS) dynamics, climate change and sea level. Here we present results from a highly-resolved ‘horizontal ice core’ from the Weddell Sea Embayment, which records millennial-scale AIS dynamics across this extensive region. Counterintuitively, we find AIS mass-loss across the full duration of the Antarctic Cold Reversal (ACR; 14,600–12,700 yrs ago), with stabilisation during the subsequent millennia of atmospheric warming. Earth-system and ice-sheet modelling suggests these contrasting trends were likely Antarctic-wide, sustained by feedbacks amplified by the delivery of Circumpolar Deep Water onto the continental shelf. Given the anti-phase relationship between inter-hemispheric climate trends across the LGT our findings demonstrate that Southern Ocean-AIS feedbacks were controlled by global atmospheric teleconnections. With increasing stratification of the Southern Ocean and intensification of mid-latitude westerly winds today, such teleconnections could amplify AIS mass loss and accelerate global sea-level rise."

Extract: "Supported by marine geological evidence of enhanced iceberg calving, and independent ice-sheet and Earth system modelling experiments, the Patriot Hills BIA provides the first direct terrestrial evidence that the Antarctic ice sheet was highly responsive to global ice-ocean-atmosphere feedbacks during the LGT. Modelling suggests this pattern could be Antarctic wide, sustained by ice-ocean feedbacks amplified by the delivery of CDW onto the Antarctic Continental Shelf. The counterintuitive finding of sustained ice-sheet mass loss across this sector of the AIS during a period of atmospheric cooling suggests that Southern Ocean AIS feedbacks were likely modulated by global atmospheric teleconnections during a period of asynchronous hemispheric climate change. Defining the details of these atmosphere-ocean-ice feedbacks is crucial to reducing uncertainty in sea level projections, and understanding the implications of observed high-latitude Southern Hemisphere environmental changes today, which may conspire to amplify future Antarctic ice mass loss."

[AbruptSLR]

The linked reference finds limited ice-stream bed erosion beneath the PIG.  To me this is clear evidence supporting the belief that most of the paleo-ice mass loss from the PIG occurred due to Marine Ice Cliff Instability (MICI), as indicated by Pollard, DeConto and Alley (2018):

Davies, D., Bingham, R. G., King, E. C., Smith, A. M., Brisbourne, A. M., Spagnolo, M., Graham, A. G. C., Hogg, A. E., and Vaughan, D. G.: How dynamic are ice-stream beds?, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-214, in review, 2017.

https://www.the-cryosphere-discuss.net/tc-2017-214/

Abstract. Projections of sea-level rise contributions from West Antarctica's dynamically thinning ice streams contain high uncertainty because some of the key processes involved are extremely challenging to observe. An especially poorly observed parameter is sub-decadal stability of ice-stream beds. Only two previous studies have made repeated geophysical measurements of ice-stream beds at the same locations in different years, but both studies were limited in spatial extent. Here, we present the results from repeat radar measurements of the bed of Pine Island Glacier, West Antarctica, conducted 3–6 years apart, along a cumulative ~ 60 km of profiles. Analysis of the correlation of bed picks between repeat surveys show that 90 % of the ice-stream bed displays no significant change despite the glacier increasing in speed by up to 40 % over the last decade. We attribute the negligible detection of morphological change at the bed of Pine Island Glacier to the ubiquitous presence of a deforming till layer, wherein sediment transport is in steady state, such that sediment is transported along the basal interface without inducing morphological change to the radar-sounded bed. Significant change was only detected in one 500 m section of the bed where a change in bed morphology occurs with a difference in vertical amplitude of 3–5 m. Given the precision of our measurements, the maximum possible erosion rate that could go undetected along our profiles is 500 mm a-1, far exceeding erosion rates reported for glacial settings from proglacial sediment yields, but substantially below subglacial erosion rates of 1000 mm a-1 previously reported from repeat geophysical surveys in West Antarctica.

[AbruptSLR]

The linked reference presents new findings that the retreat of the Barents Sea Ice Sheet at the end of the last ice age resulted in the explosive release of methane from Arctic seafloor hydrates as overpressure from the ice sheet disappeared.  The researchers find that this serves as a good past analogy of what may likely happen in the near-term future if the WAIS were to collapse, and/or if marine terminating glaciers in Greenland were to retreat rapidly.  As methane has a GWP100 of about 35 such explosive releases of methane could have a significant impact on global warming this century.  Such short-term methane forcings would be superimposed on top of Hansen's ice-climate feedback mechanism.

K. Andreassen, A. Hubbard, M. Winsborrow, H. Patton, S. Vadakkepuliyambatta, A. Plaza-Faverola, E. Gudlaugsson, P. Serov, A. Deryabin, R. Mattingsdal, J. Mienert & S. Bünz (02 Jun 2017), "Massive blow-out craters formed by hydrate-controlled methane expulsion from the Arctic seafloor", Science, Vol. 356, Issue 6341, pp. 948-953
DOI: 10.1126/science.aal4500

http://science.sciencemag.org/content/356/6341/948

Abstract: "Widespread methane release from thawing Arctic gas hydrates is a major concern, yet the processes, sources, and fluxes involved remain unconstrained. We present geophysical data documenting a cluster of kilometer-wide craters and mounds from the Barents Sea floor associated with large-scale methane expulsion. Combined with ice sheet/gas hydrate modeling, our results indicate that during glaciation, natural gas migrated from underlying hydrocarbon reservoirs and was sequestered extensively as subglacial gas hydrates. Upon ice sheet retreat, methane from this hydrate reservoir concentrated in massive mounds before being abruptly released to form craters.  We propose that these processes were likely widespread across past glaciated petroleum provinces and that they also provide an analog for the potential future destabilization of subglacial gas hydrate reservoirs beneath contemporary ice sheets."

We should not forget that the only contemporary ice sheet comparable to the paleo marine ice sheet in the Barents Sea basis, that is at risk of being destabilized this century is the WAIS; which could make a significant contribution to methane emissions into the atmosphere give a sufficient abrupt collapse scenario.

[AbruptSLR]

Most people do not want to consider the 'Deep Uncertainty' associated with the potential collapse of the WAIS; which in my opinion puts pressure on ice sheet modelers to only evaluate lower bound dynamic ice collapse cases such as yellow curve marked '+ WAIS, DP16' in the first attached image from the first linked reference Bakker et al. (2017); which forced those authors to estimate the upper bound curve marked '+ WAIS, worst case' based on expert opinion (see the extract below from Bakker et al., 2017).  While I concur with Bakker et al. (2017)'s upper bound curve, I believe that it is difficult for non-experts to appreciate what 'Deep' uncertainties are consider by the upper bound curve that are not considered by the lower bound curve; therefore, I provide the following partial list of considerations:

1. If ECS is dominated by warming of the Tropical Pacific as indicated by the central panel of the second image (from Andrew Ringberg 2015) then the accumulating heat from the future Tropical Pacific will telecommunicate that heat (both through the atmosphere and through the ocean) to the WAIS much quicker than assumed by researchers with dynamic ice models such as Pollard, DeConto and Alley; which could lead to hydrofracturing and cliff failures decades earlier than their 2016, 2017 and 2018 model projections consider.

2. Pollard, DeConto and Alley's regional climate model does not adequately account for Hansen's ice-climate feedback mechanism, and also their model mesh resolution is not adequate to account for fine interactions.

3. Both the PIIS and the Thwaites Ice Shelf, are deteriorating earlier than assumed by any published dynamic ice model that I have seen.

4. No dynamic ice model that I have seen adequately accounts for the 3D nature of the likely ice cliff faces for the Thwaites Glacier.

Bakker, A. M. R., Wong, T. E., Ruckert, K. L., & Keller, K. (2017). Sea-level projections representing the deeply uncertain contribution of the West Antarctic ice sheet. Scientific Reports, 7(1). doi:10.1038/s41598-017-04134-5

https://www.nature.com/articles/s41598-017-04134-5
or
https://arxiv.org/abs/1609.07119

Abstract: "Future sea-level rise poses nontrivial risks for many coastal communities. Managing these risks often relies on consensus projections like those provided by the IPCC. Yet, there is a growing awareness that the surrounding uncertainties may be much larger than typically perceived. Recently published sea-level projections appear widely divergent and highly sensitive to non-trivial model choices and the West Antarctic Ice Sheet (WAIS) may be much less stable than previously believed, enabling a rapid disintegration. In response, some agencies have already announced to update their projections accordingly. Here, we present a set of probabilistic sea-level projections that approximate deeply uncertain WAIS contributions. The projections aim to inform robust decisions by clarifying the sensitivity to non-trivial or controversial assumptions. We show that the deeply uncertain WAIS contribution can dominate other uncertainties within decades. These deep uncertainties call for the development of robust adaptive strategies. These decision-making needs, in turn, require mission-oriented basic science, for example about potential signposts and the maximum rate of WAIS induced sea-level changes.

Extract: "It is important to note that we do not intend to assign an implicit probability distribution to these deeply uncertain projections. We simply want to characterize and communicate key aspects of the deeply uncertain WAIS contribution to sea-level rise."
&

Future Fate of the Polar Ice Sheets: Rob DeConto - March 30


See also the second linked reference Kopp et al. (2017) that provides additional discussion of 'Deep; uncertainties with regard to future ice mass loss from Antarctica:

Robert E. Kopp et al (2017), "Evolving understanding of Antarctic ice-sheet physics and ambiguity in probabilistic sea-level projections", arXiv:1704.05597v2

https://arxiv.org/pdf/1704.05597.pdf

Abstract: "Mechanisms such as ice-shelf hydrofracturing and ice-cliff collapse may rapidly increase discharge from marine-based ice sheets. Here, we link a probabilistic framework for sea-level projections to a small ensemble of Antarctic ice-sheet (AIS) simulations incorporating these physical processes to explore their influence on global-mean sea-level (GMSL) and relative sea-level (RSL). We compare the new projections to past results using expert assessment and structured expert elicitation about AIS changes. Under high greenhouse gas emissions (Representative Concentration Pathway [RCP] 8.5), median projected 21st century GMSL rise increases from 79 to 146 cm. Without protective measures, revised median RSL projections would by 2100 submerge land currently home to 153 million people, an increase of 44 million. The use of a physical model, rather than simple parameterizations assuming constant acceleration of ice loss, increases forcing sensitivity: overlap between the central 90% of simulations for 2100 for RCP 8.5 (93–243 cm) and RCP 2.6 (26–98 cm) is minimal. By 2300, the gap between median GMSL estimates for RCP 8.5 and RCP 2.6 reaches > 10 m, with median RSL projections for RCP 8.5 jeopardizing land now occupied by 950 million people (vs. 167 million for RCP 2.6). The minimal correlation between the contribution of AIS to GMSL by 2050 and that in 2100 and beyond implies current sea-level observations cannot exclude future extreme outcomes. The sensitivity of post-2050 projections to deeply uncertain physics highlights the need for robust decision and adaptive management frameworks."
&

Finally the third linked reference (Wong & Keller, 2017) provides a worked example of how the concept of 'Deep Uncertainty' can be applied to coastal flood risk projections for New Orleans (see the third attached image):

Tony E. Wong and Klaus Keller (2017), "Deep Uncertainty Surrounding Coastal Flood Risk Projections: A Case Study for New Orleans", Earth’s Future, 5, 1015–1026, https://doi.org/10.1002/2017EF000607

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017EF000607
or
https://arxiv.org/ftp/arxiv/papers/1705/1705.07722.pdf

Abstract: "Future sea-level rise drives severe risks for many coastal communities. Strategies to manage these risks hinge on a sound characterization of the uncertainties. For example, recent studies suggest that large fractions of the Antarctic ice sheet (AIS) may rapidly disintegrate in response to rising global temperatures, leading to potentially several meters of sea-level rise during the next few centuries. It is deeply uncertain, for example, whether such an AIS disintegration will be triggered, how much this would increase sea-level rise, whether extreme storm surges intensify in a warming climate, or which emissions pathway future societies will choose. Here, we assess the impacts of these deep uncertainties on projected flooding probabilities for a levee ring in New Orleans, Louisiana. We use 18 scenarios, presenting probabilistic projections within each one, to sample key deeply uncertain future projections of sea-level rise, radiative forcing pathways, storm surge characterization, and contributions from rapid AIS mass loss. The implications of these deep uncertainties for projected flood risk are thus characterized by a set of 18 probability distribution functions. We use a global sensitivity analysis to assess which mechanisms contribute to uncertainty in projected flood risk over the course of a 50-year design life. In line with previous work, we find that the uncertain storm surge drives the most substantial risk, followed by general AIS dynamics, in our simple model for future flood risk for New Orleans."

Edit, for those who do not know, or cannot remember how heat energy is telecommunicated from the Tropical Pacific to West Antarctica see the fourth image from Fogwill et al (2011), that shows that the atmospheric Rossby wavetrain pattern depends on the ENSO and SAM conditons.

[sidd]

Re: Davies on stability of PIG bed doi:10.5194/tc-12-1615-2018

"clear evidence supporting the belief that most of the paleo-ice mass loss from the PIG occurred due to Marine Ice Cliff Instability "

I don't see anything in this paper about MICI. Perhaps you mean a different paper ?
 
sidd

[AbruptSLR]

Re: Davies on stability of PIG bed doi:10.5194/tc-12-1615-2018

"clear evidence supporting the belief that most of the paleo-ice mass loss from the PIG occurred due to Marine Ice Cliff Instability "

I don't see anything in this paper about MICI. Perhaps you mean a different paper ?
 
sidd

While it is true that Davies et al (2017) do not discuss MICI, the 'clear evidence' that I was referring to is the rough nature of the stream bed shown in their radar images (e.g. see the first attached image); which to me indicates that in paleo times there has been a limited amount of dynamic basal shear wearing down such rough stream bed features.  To make my point clearer I provide the second image from Frank Pattyn (2018 Jul 16), that shows that even with a relatively level stream bed (as is currently the case for the PIG) MICI calving can occur without a large amount of basal sliding to wear down the stream bed roughness.

[AbruptSLR]

The linked reference discusses how the Southern Ocean may be an unreliable sink for carbon dioxide (see the attached image).  We will eventually learn whether the Southern Ocean continues as a sink for carbon dioxide if La Nina events become less frequent with continued global warming:

Jeff Tollefson (2016) "How much longer can Antarctica's hostile ocean delay global warming?", Nature, Vol 539, Issue 7629

http://www.nature.com/news/how-much-longer-can-antarctica-s-hostile-ocean-delay-global-warming-1.20978#graphic

Extract: "If that trend were to continue, atmospheric CO2 levels would rise even faster in the future. However, a study in Science last year found that the carbon sink started to strengthen in the early 2000s (see ‘The unreliable sink’)."

[AbruptSLR]

The linked video of a May 28, 2008 calving event of Ilulissat Glacier (also called Jakobshavn Glacier) gives some idea of the dynamic nature of cliff failures.  As noted below the height of the ice cliff face (above water) for Ilulissat/Jakobshavn is 300 to 400-ft; while the height of the ice cliff face for Thwaites could be many times this height if/when the grounding line retreats down the retrograde stream bed:

Title: "Chasing Ice"



Extract: " This rare footage has gone on record as the largest glacier calving event ever captured on film, by the 2016 Guiness Book of World Records.

On May 28, 2008, Adam LeWinter and Director Jeff Orlowski filmed a historic breakup at the Ilulissat Glacier in Western Greenland. The calving event lasted for 75 minutes and the glacier retreated a full mile across a calving face three miles wide. The height of the ice is about 3,000 feet, 300-400 feet above water and the rest below water."

P.S. While many have already seen this video, its dynamic nature never fails to impress me.

Edit: For those who are not familiar with the profile (elevations in kilometers) of the Thwaites Glacier, I provide the attached representative image.

[AbruptSLR]

Seawater percolation into below sea level layers of the firn in Antarctic ice shelves is not a good thing, and is more widespread than previously assumed (as indicated by the linked reference):

Cook, S., Galton-Fenzi, B. K., Ligtenberg, S. R. M., and Coleman, R.: Brief Communication: Widespread potential for seawater infiltration on Antarctic ice shelves, The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-146, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2018-146/

Abstract. Antarctica's future contribution to sea level change depends on the fate of its fringing ice shelves. One variable which may affect the rates of iceberg calving from ice shelves is the presence of liquid water, including the percolation of seawater into permeable firn layers. Here, we present evidence that most ice shelves around Antarctica have regions where permeable firn exists below sea level. The findings indicate that seawater infiltration onto ice shelves may be much more widespread in Antarctica than previously recognised. Our results present the most likely locations for seawater infiltration to occur, and may be used as an indicator of where future radar studies might be focussed.

[wdmn]

Thanks for all your work ASLR.

I find it strange that these top scientists in their field have not "adequately account[ed] for Hansen's ice-climate feedback mechanism."

How could they not get called out for such an oversight?

[Pmt111500]

Thanks for all your work ASLR.

I find it strange that these top scientists in their field have not "adequately account[ed] for Hansen's ice-climate feedback mechanism."

How could they not get called out for such an oversight?

Attributing the mechanism to Hansen is not correct though he's the most famous and vocal advocate of this. Can't recall who was the first to do a paper on this sensible and probably true mechanism, but it's been around for only maybe 10? years (at least there was some talk of this early in 2010s), so oversights before that cannot be 'called out'.

[wdmn]

The model projections in question are from 2016, 2017, and 2018.

[Pmt111500]

The model projections in question are from 2016, 2017, and 2018.

Ah ok, didn't read further up, then it might have been possible to insert the calculus describing the mechanism somehow to the models.
The fully coupled models are though very complex so they'd have to make sure runaways (quadratic or log-linear) on other variables wouldn't happen.

[Lennart van der Linde]

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.

[AbruptSLR]

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.

Lennart,

I didn't have the heart to mention the seemingly arbitrary limitation that you raised from the video about DeConto's dynamic ice projections, as the most telling quote from the video for me is where he states that his life has been ruined since his 2015 paper with Pollard & Alley about ice cliff failures and hydrofracturing.

We should all ask ourselves why any of us might think that consensus climate science deals well with 'Deep Uncertainty'.  I very much appreciate the work of DeConto, Pollard and Alley, but none of us should expect that their projections and confidence levels adequately address 'Deep Uncertainty'; but rather we should work to better appreciates the multiple caveats buried in their presentations, that particularly cause their finding to project delayed dates for initiating the potential collapse phase of particularly the WAIS.

W.r.t. delayed dates for initiating cliff failures for particularly both the PIG and Thwaites, I list the following three considerations of the many 'Deep Uncertainty' issues that I hope will be better addressed in AR6:

1. The first from Nature Geoscience 2012, DOI: 10.1038/NGEO1671, shows in good detail the areas of the Antarctic subject to the indicated number of days of surface ice melting in January 2005.  This image indicates that both the PIG/Thwaite drainage basins and the Ross Sea Embayment areas are subject to a substantial risk of episodic surface ice melting in the future (as per Pollard's concern).  Also the second image show episodic surface ice meltwater on PIIS circa January 2005, and I note that hydrofacturing of an entire ice shelf can occur in as little as a day, once appropriate crevassing and surface meltwater conditions have occurred.

2. The third image from Orsi et al. 2012 reports temperature measurements from a station at the WAIS Divide which indicates that for the 50-year period from 1957 to 2007 the surface temperature at the WAIS Divide increased at a mean rate of 0.231 C/decade; however for the 20-year period that mean rate of surface temperature was 0.804 C/decade.  This means that the WAIS is currently one of the fastest warming locations on Earth, which means that after 2040 surface melting of the ice sheet will be come more frequently a factor that affects both ice sheet  and ice shelf (as occurred in the Antarctic Peninsula for the Larsen B ice shelf) stability.  Futhermore, strong El Nino events can temporarily raise surface temperatures well above the decade trend line for periods of month, thus increasing the risk of high episodic ice mass loss during future strong El Nino events (& we appear to be entering a period of more frequent El Nino events).

3. The linked reference discusses state of the surface temperature at the West Antarctic Divide for the past ~ 40,000 years (fourth image bottom panel).  Findings indicate that current climate models are challenged to hind cast the observed findings and that models with low climate sensitivities can be eliminated from consideration.  Furthermore, they find that an Antarctic Amplification of 2 to 3 time GMSTA.  These findings do not bode well for the stability of the WAIS with continued global warming:

Kurt M. Cuffey, Gary D. Clow, Eric J. Steig, Christo Buizert, T. J. Fudge, Michelle Koutnik, Edwin D. Waddington, Richard B. Alley, and Jeffrey P. Severinghaus (2016), "Deglacial temperature history of West Antarctica", PNAS, vol. 113 no. 50, 14249–14254, doi: 10.1073/pnas.1609132113

http://www.pnas.org/content/113/50/14249

Abstract: "The most recent glacial to interglacial transition constitutes a remarkable natural experiment for learning how Earth’s climate responds to various forcings, including a rise in atmospheric CO2. This transition has left a direct thermal remnant in the polar ice sheets, where the exceptional purity and continual accumulation of ice permit analyses not possible in other settings. For Antarctica, the deglacial warming has previously been constrained only by the water isotopic composition in ice cores, without an absolute thermometric assessment of the isotopes’ sensitivity to temperature. To overcome this limitation, we measured temperatures in a deep borehole and analyzed them together with ice-core data to reconstruct the surface temperature history of West Antarctica. The deglacial warming was 11.3±1.8 ∘  11.3±1.8∘ C, approximately two to three times the global average, in agreement with theoretical expectations for Antarctic amplification of planetary temperature changes. Consistent with evidence from glacier retreat in Southern Hemisphere mountain ranges, the Antarctic warming was mostly completed by 15 kyBP, several millennia earlier than in the Northern Hemisphere. These results constrain the role of variable oceanic heat transport between hemispheres during deglaciation and quantitatively bound the direct influence of global climate forcings on Antarctic temperature. Although climate models perform well on average in this context, some recent syntheses of deglacial climate history have underestimated Antarctic warming and the models with lowest sensitivity can be discounted."

Extract: "Of greatest immediate interest, however, is our demonstration that the global deglacial temperature change was amplified by a factor of 2–3 in the Antarctic, that Antarctic warming was largely achieved by 15 ka in coherence with records from Southern Hemisphere mountain ranges, and that climate models of the deglaciation perform well on average, but that the ones with lowest sensitivity can be discounted. The early warming of the Southern Hemisphere, which our study helps to quantify, arose from combined effects of reduced northward oceanic heat transport, increased insolation, and increasing atmospheric CO2. Quantitative simulation of this phenomenon could provide an illuminating challenge for model studies."

Best,
ASLR

[AbruptSLR]

It is also unclear to me why anyone would think that mankind will get off a BAU pathway before 2040, when I believe that cliff failures and hydrofracturing is reasonably likely to initiate in the WAIS (e.g. see the linked article, and I note that the Keeling Curve shows that Mauna Loa CO2 concentrations are currently 2.93ppm above where they were a year ago):

Title: "Carbon emissions from advanced economies rise for 1st time in 5 years"

https://www.axios.com/carbon-emissions-advanced-economies-rise-globally-07b2f169-f184-42f0-b724-88026b843475.html

Extract: "The International Energy Agency said Tuesday that combined carbon dioxide emissions from the world's advanced economies are set to rise 0.5% in 2018, ending a 5-year declining trend."

[AbruptSLR]

The model projections in question are from 2016, 2017, and 2018.

Ah ok, didn't read further up, then it might have been possible to insert the calculus describing the mechanism somehow to the models.
The fully coupled models are though very complex so they'd have to make sure runaways (quadratic or log-linear) on other variables wouldn't happen.

To re-emphasize my point from Reply #219, Bronselaer et al (2018) used an AIS model that does not account for MICI nor hydro-fracturing (which are not likely to be significant before 2040, and thus there is no concern for runaways) to show that by 2040 upwelling of relatively warm circumpolar deep water, CDW, around Antarctic will shift the potential ice melting temperature difference upward from below 1,000 m depth to roughly 750 m depth, and will increasingly advect this warm CDW towards the grounding line (see the attached image) for key marine glaciers such as the Pine Island Glacier, PIG and the Thwaites Glacier. What is critical to note wrt Bronselaer et al (2018) is that by 2040 the temperature of the Southern Ocean would be thousands of years from full equilibrium, but for key AIS marine glaciers both the top and bottom ice surfaces exposed to air and water respectively will experience ice mass loss sufficient to trigger localized MICI and hydro-fracturing mechanisms [if they had been included in Bronselaer et al (2018)'s] model.  In summary, this shows that the ice-climate feedback mechanism is already significant w.r.t. to potentially triggering significant ice cliff failures in the WAIS circa 2040.

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

Abstract: "Meltwater from the Antarctic Ice Sheet is projected to cause up to one metre of sea-level rise by 2100 under the highest greenhouse gas concentration trajectory (RCP8.5) considered by the Intergovernmental Panel on Climate Change (IPCC). However, the effects of meltwater from the ice sheets and ice shelves of Antarctica are not included in the widely used CMIP5 climate models, which introduces bias into IPCC climate projections. Here we assess a large ensemble simulation of the CMIP5 model ‘GFDL ESM2M’ that accounts for RCP8.5-projected Antarctic Ice Sheet meltwater. We find that, relative to the standard RCP8.5 scenario, accounting for meltwater delays the exceedance of the maximum global-mean atmospheric warming targets of 1.5 and 2 degrees Celsius by more than a decade, enhances drying of the Southern Hemisphere and reduces drying of the Northern Hemisphere, increases the formation of Antarctic sea ice (consistent with recent observations of increasing Antarctic sea-ice area) and warms the subsurface ocean around the Antarctic coast. Moreover, the meltwater-induced subsurface ocean warming could lead to further ice-sheet and ice-shelf melting through a positive feedback mechanism, highlighting the importance of including meltwater effects in simulations of future climate."

Caption for the second attached image: "Fig. 5 | Mechanism for ocean warming. a, Hovmoller diagram of the meltwater-induced ocean temperature anomaly, averaged along the Antarctic coast, as a function of time. The black dotted line indicates the maximum warming in a given year. b, c, Schematic of the meltwater-induced Southern Ocean subsurface warming, shown as a zonal-mean cross-section. In the pre-industrial state (b), isopycnals (black lines) are tilted towards the ocean surface by westerly winds (black circles, directed out of the page), away from the continental shelf, with an upward heat flux transporting heat from the warm CDW (orange water) towards the cooler surface (blue water), as shown by the red arrow. In the perturbed state (c), meltwater from the Antarctic Ice Sheet freshens the surface (blue), depressing isopycnals (solid to dashed black lines) so that isopycnal mixing transports heat towards the continent rather than towards the ocean surface (red arrow), leading to coastal warming at depth around the shelf and cooling at the surface."

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In my last post (& in Reply #219) regarding Bronselaer et al (2018), I focused on the implications for the PIG and Thwaites Glacier of the ice-climate feedback raising the depth of the warm CDW up to about El-500m depth circa 2040 (or before).  So, in this post, I note that the lip of the entrance to the Filchner Trough has a sill depth of about El-650m, which could accommodate a large volume of warm CDW beneath the FRIS (Filchner Ronne Ice Shelf) circa 2040, see the first image.  The second image shows a large array of instrumentation currently monitoring the circulation patterns beneath the FRIS (see also the first linked reference); which is currently observing that seasonally modified CDW finds its way beneath the FRIS.  The third image show the locations of two subglacial basins with very smooth seafloors that can sustain rapid groundling line retreat (for both the Moller and Institute Ice Streams) once the warm CDW has melted the ice providing buttressing support at the lip of the basins.  The fourth image comes from the second reference and shows recently measure bathymetry beneath the FRIS in this critical area.

Hellmer, H. , Hattermann, T. , Ryan, S. , Janout, M. and Schröder, M. (2018): The Filchner Trough / Filchner Ice Shelf cavity system , 27th International Polar Conference, Rostock, Germany, 25 March 2018 - 29 March 2018 .

http://epic.awi.de/46889/

Abstract: "Since austral summer 2013/14 AWI maintains a mooring array on the eastern slope of Filchner Trough at 76°S to monitor any flow of warm waters of open ocean origin towards the Filchner Ice Shelf (FIS) cavity. During the austral summers 2015/16 and 2016/17, seven oceanographic moorings were deployed beneath FIS through hot-water-drilled access holes to investigate and monitor the processes controlling the supply of ocean heat to the ice shelf base. This data, transferred to AWI via satellite link, shows that two ‘regimes’ exist beneath FIS. Dense High Salinity Shelf Water (HSSW), formed in front of the Ronne Ice Shelf, dominates the southern cavity and exits as Ice Shelf Water (ISW) the cavity along the western flank of the Filchner Trough. Less dense HSSW with a local origin in front of FIS enters the cavity on the eastern side of the Filchner Trough during parts of the year but seems to be trapped at depth, interacting laterally with derivatives of the Ronne-sourced HSSW. No evidence exists that it penetrates to the deep southern FIS grounding line. At 76°S, the flow of warm waters towards FIS is seasonal, limited to late summer/early winter, being replaced by ISW for the rest of the year. The link of the two sub-ice shelf circulation regimes to different regions of dense water formation on the continental shelf, and its sensitivity to the inflow of warm waters need to be investigated further to reduce the uncertainty of estimates on the FIS mass balance for today and the future."
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S. H. R. Rosier et al. ( 08 June 2018), "A New Bathymetry for the Southeastern Filchner‐Ronne Ice Shelf: Implications for Modern Oceanographic Processes and Glacial History", JGR Oceans, https://doi.org/10.1029/2018JC013982

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018JC013982

Abstract
The Filchner‐Ronne Ice Shelf, the ocean cavity beneath it, and the Weddell Sea that bounds it, form an important part of the global climate system by modulating ice discharge from the Antarctic Ice Sheet and producing cold dense water masses that feed the global thermohaline circulation. A prerequisite for modeling the ice sheet and oceanographic processes within the cavity is an accurate knowledge of the sub‐ice sheet bedrock elevation, but beneath the ice shelf where airborne radar cannot penetrate, bathymetric data are sparse. This paper presents new seismic point measurements of cavity geometry from a particularly poorly sampled region south of Berkner Island that connects the Filchner and Ronne ice shelves. An updated bathymetric grid formed by combining the new data with existing data sets reveals several new features. In particular, a sill running between Berkner Island and the mainland could alter ocean circulation within the cavity and change our understanding of paleo‐ice stream flow in the region. Also revealed are deep troughs near the grounding lines of Foundation and Support Force ice streams, which provide access for seawater with melting potential. Running an ocean tidal model with the new bathymetry reveals large differences in tidal current velocities, both within the new gridded region and further afield, potentially affecting sub‐ice shelf melt rates.

Plain Language Summary
The Filchner‐Ronne Ice Shelf in Antarctica is the largest body of floating ice in the world and plays an important role in the global climate system through its interactions with the ocean and Antarctic Ice Sheet. Due to its thickness and remoteness, the shape of the large ocean cavity beneath this floating ice shelf is poorly understood, yet this information is crucial for ice and ocean models of the region. In this study we present recent measurements of the thickness of this ocean cavity and, in combination with previous measurements, produce a new map of the area. A number of new features are revealed in this map, and we discuss the implications for ocean currents within the cavity, as well as past and future ice sheet flow.

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Seawater percolation into below sea level layers of the firn in Antarctic ice shelves is not a good thing, and is more widespread than previously assumed (as indicated by the linked reference):
...

For those who do not understand why meltwater in the firn of Antarctic ice shelves is a concern I provide the attached image with a title from the article which reads: "Conceptual illustration of firn air depletion and its consequences for ice-shelf hydrology and stability. (a) An ice shelf covered by a firn layer containing sufficient air. The inset shows meltwater being stored in the pore space of the firn. (b) An ice shelf with a depleted firn layer. Due to the absence of pore space, meltwater forms ponds that drain into fractures. Alternatively, water is routed to the fractures efficiently as shown in the leftmost fractures. Courtesy: Journal of Glaciology and Kuipers et al":

"Firn air depletion as a precursor of Antarctic ice-shelf Collapse" Peter Kuipers Munneke, Stefan R.M. Ligtenberg, Michiel van den Broeke, David G. Vaughan, Journal of Glaciology, 60 (220), (2014). DOI: 10.3189/2014JoG13J183.

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I estimate that by 2040 that the calving face of RIS should have retreated at least 35km (using the current rate of retreat), and I postulate that this retreat of the ice shelf face will cause the Ross Gyre current (see the first & second attached images) to follow the retreating face until the gyre current is directed at the shallow trough leading to the base of the Byrd Glacier (see the third image).  Also, by 2040 I believe that that the Antarctic Bottom Water (AABW) production in this region will be sufficiently reduced (by both changing atmospheric patterns and the strengthening of the CDW volume & temperature) so that the cold AABW entrained in the Ross Gyre will not be able to adequately dilute the CDW entained in the Ross Gyre (assuming also that the ice melt from the Getz Ice Shelf has slowed sufficiently to reduce the dilution of the CDW crossing the continental shelf with meltwater).  Therefore, I postulate that a warm tongue of CDW may make its way to the grounding line for the Byrd Glacier, which will help to trigger a positive feedback cycle (including a "saline pump" advection action working with the Byrd Glacier) that will alternate the circulation patterns in the Ross Sea Embayment so as to direct the warm CDW tongue water from the Byrd Glacier southward along the grounding lines of the Siple Coast ice streams (note that there is a pre-existing shallow trough leading from the Byrd Glacier to the Siple Coast;  As the RIS is already thinner than the FRIS, and the CDW will be warmer by then, I postulate that a large portion of the RIS will be subject to a melt-pond collapse mechanism between 2050 & 2060; which, will reduce the buttressing of the RIS on the Siple Coast ice streams.

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My Reply #279 provides images of episodic surface ice melting event in the WAIS during January 2005; and just to reinforce the consideration that such episodic surface melting events will likely increase in frequency, extent and duration with continued global warming I provide the attached image showing the episodic surface ice melting event from January 2016 (which was more intense than the 2005 event).  Such episodic event increase the risk of associated hydrofracture calving events for key WAIS ice shelves, within the next couple of decades.

Edit, I decided to add the second image that show changes in yearly surface temperature around Antarctica between 1981 and 2007, that clearly indicates the trend that the surface temperature of the WAIS is warming more rapidly than the EAIS.

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I provide the attached image showing current annual Antarctic snowfall in order to emphasize that:

1. Current snowfall on for the FRIS and the RIS (Ross Ice Shelf) is relatively low, and thus it is not likely that snow cover on these two ice shelfs would reduce the risk of hydrofracturing circa 2040.
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2. Current snowfall on the Byrd Subglacial Basin catchment is relatively high; which may result in steeper surface gradients on the Amundsen Sea Embayment, ASE, marine glaciers circa 2040; which in turn would increase the likelihood of associated ice cliff failures in this area in that timeframe.

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I provide the attached image of DeConto et al. (2016) Figure 4c in order to make two different points:

1. The ice-climate feedback as described by Hansen (and others) is a very complex feedback mechanism which many/most researchers fail to fully replicate in their models, include those by Pollard and DeConto, even though the attached image has a red dotted curve that indicates that they think that they have accounted for this feedback.  However, they clearly have only accounted for a small portion of the full feedback mechanism as they do not show any significant impact from this feedback mechanism until circa 2100, while Bronselaer et al (2018) clearly demonstrate that this feedback is already moderately changing the nature of the CDW today and that by 2040 the ice-climate feedback on the CDW will be significant enough possibly induce ice cliff failures in some key ASE marine glacier.
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2. This image shows the Totten Glacier (T) beginning to collapse circa 2080, however, the following two linked references indicate that wind induced upwelling of warm CDW is already accelerating ice mass loss from the Totten Ice Shelf (and grounding line).  Therefore, if Pollard and DeConto's model(s) were refined to account for the ice-climate feedback identified by Bronselaer et al. (2018) then their projections of ice mass loss from Totten may well be accelerated.

P.S.: A full modeling of the ice-climate feedback mechanism would need to account for many different sub-mechanisms including: a. ENSO interactions, b. Ocean Overturning Current interactions; c. Arctic & Antarctic sea ice interactions; d. the Bipolar Seesaw mechanism; e. associated Cloud feedback interactions; e. associated changes in Arctic and Antarctic snow/rain fall patterns; f. associated changings in CDW circulation patterns; g. associated changes in both atmospheric and oceanic telecommunication patterns; h. associated regional changes in both SSTA and land surface temperatures; etc.

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The linked UN article about the IPCC's Special Report: Global Warming of 1.5C; indicates that GMSTA cannot be kept below 1.5C without action on short-lived climate pollutants (SLCPs) like methane; however, the attached image shows the Mauna Loa atmospheric methane concentration thru Dec 4, 2018; which indicates that there is currently no slowdown in the current trend in methane emissions.  Therefore, it is difficult for me to see how GMSTA is going to say well below Pliocene levels [see Pollard, DeConto & Alley (2018) & Reply #220] in the coming decades:

Title: "Keeping warming to 1.5˚C impossible without reducing Short-lived Climate Pollutants"

https://www.unenvironment.org/news-and-stories/press-release/keeping-warming-15c-impossible-without-reducing-short-lived-climate

Extract: "Our best chance to avoid runaway warming is to act right away to reduce highly potent but short-lived climate pollutants (SLCPs) like methane, tropospheric ozone, hydrofluorocarbons (HFCs) and black carbon. This must go hand-in-hand with deep and persistent cuts in long-lived greenhouse gases like carbon dioxide (CO2).

Fast and immediate action on SLCPs can avoid over half a degree of warming by 2050. It will also avoid over 50% of the predicted warming in the Arctic by 2050, thereby significantly decreasing the chances of triggering dangerous climate tipping points, like the irreversible release of carbon dioxide and methane from thawing Arctic permafrost.

Drew Shindell, Professor of Climate Sciences at Duke University and a lead author on the IPCC report, said no scenario exists where the world can get to 1.5 degrees without reducing these highly potent, but short-lived climate forcers alongside CO2."

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Per the linked reference, '… carbon dioxide outgassing from rivers is estimated to be equivalent to one-fifth of combined emissions from fossil fuel combustion and cement production …"; however, Allen & Pavelsky (2018) estimate that the surface area of rivers and streams has been underestimated by about 44%.  This is not good news especially when considering methane emissions from lakes, reservoirs, rivers and streams.

George H. Allen &Tamlin M. Pavelsky (10 Aug 2018), "Global extent of rivers and streams", Science, Vol. 361, Issue 6402, pp. 585-588, DOI: 10.1126/science.aat0636

http://science.sciencemag.org/content/361/6402/585

Extract: "Expanding the role of rivers
The surfaces of rivers and streams are interfaces for a host of chemical exchanges with the atmosphere and biosphere. For instance, carbon dioxide outgassing from rivers is estimated to be equivalent to one-fifth of combined emissions from fossil fuel combustion and cement production. Allen and Pavelsky used satellite imagery to estimate the surface area of rivers and streams (see the Perspective by Palmer and Ruhi). The stunning map that they generated results in an upward revision, by about one-third, to the total surface area of rivers and streams on Earth."
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Abstract: "The turbulent surfaces of rivers and streams are natural hotspots of biogeochemical exchange with the atmosphere. At the global scale, the total river-atmosphere flux of trace gasses such as carbon dioxide depends on the proportion of Earth’s surface that is covered by the fluvial network, yet the total surface area of rivers and streams is poorly constrained. We used a global database of planform river hydromorphology and a statistical approach to show that global river and stream surface area at mean annual discharge is 773,000 ± 79,000 square kilometers (0.58 ± 0.06%) of Earth’s nonglaciated land surface, an area 44 ± 15% larger than previous spatial estimates. We found that rivers and streams likely play a greater role in controlling land-atmosphere fluxes than is currently represented in global carbon budgets."

Also, the following reference indicates the carbon emissions from the soil are increasing with global warming:

Bond-Lamberty, B., Bailey, V. L., Chen, M., Gough, C. M., & Vargas, R. (2018). Globally rising soil heterotrophic respiration over recent decades. Nature, 560(7716), 80–83. doi:10.1038/s41586-018-0358-x; https://doi.org/10.1038/s41586-018-0358-x

https://www.nature.com/articles/s41586-018-0358-x

Abstract: "Global soils store at least twice as much carbon as Earth’s atmosphere. The global soil-to-atmosphere (or total soil respiration, RS) carbon dioxide (CO2) flux is increasing, but the degree to which climate change will stimulate carbon losses from soils as a result of heterotrophic respiration (RH) remains highly uncertain5–8. Here we use an updated global soil respiration database to show that the observed soil surface RH:RS ratio increased significantly, from 0.54 to 0.63, between 1990 and 2014 (P = 0.009). Three additional lines of evidence provide support for this finding. By analysing two separate global gross primary production datasets, we find that the ratios of both RH and RS to gross primary production have increased over time. Similarly, significant increases in RH are observed against the longest available solar-induced chlorophyll fluorescence global dataset, as well as gross primary production computed by an ensemble of global land models. We also show that the ratio of night-time net ecosystem exchange to gross primary production is rising across the FLUXNET2015 dataset. All trends are robust to sampling variability in ecosystem type, disturbance, methodology, CO2 fertilization effects and mean climate. Taken together, our findings provide observational evidence that global RH is rising, probably in response to environmental changes, consistent with metaanalyses and long-term experiments. This suggests that climate-driven losses of soil carbon are currently occurring across many ecosystems, with a detectable and sustained trend emerging at the global scale."

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The first image is from Hansen et al (2016), showing the ocean stratification and precipitation portions of the ice-climate feedback mechanism.  I repost this image to remind readers that this mechanism proposes: a cooling of the sea surface temperature, SST, an increase in sea ice extent, SIE, and a reduction of sea surface salinity, SSS, not only due to ice melt but also due to precipitation (and its difference with evaporation).  The first two linked references confirm these trends (see the second and third images); however, as most consensus climate models ignore the ice-climate feedback mechanism, they both discuss possible alternate mechanisms.  Nevertheless, since ~1980, this SST, SIE and SSS data is fully consistent with the ice-climate mechanism, and I also note that the associated change in SST tends to mask the full strength of climate sensitivity when measured by GMSTA:

Zhang, L., Delworth, T. L., Cooke, W., & Yang, X. (2018). Natural variability of Southern Ocean convection as a driver of observed climate trends. Nature Climate Change. doi:10.1038/s41558-018-0350-3, https://doi.org/10.1038/s41558-018-0350-3

https://www.nature.com/articles/s41558-018-0350-3

Abstract: "Observed Southern Ocean surface cooling and sea-ice expansion over the past several decades are inconsistent with many historical simulations from climate models. Here we show that natural multidecadal variability involving Southern Ocean convection may have contributed strongly to the observed temperature and sea-ice trends. These observed trends are consistent with a particular phase of natural variability of the Southern Ocean as derived from climate model simulations. Ensembles of simulations are conducted starting from differing phases of this variability. The observed spatial pattern of trends is reproduced in simulations that start from an active phase of Southern Ocean convection. Simulations starting from a neutral phase do not reproduce the observed changes, similarly to the multimodel mean results of CMIP5 models. The long timescales associated with this natural variability show potential for skillful decadal prediction."

Extract: "However, we cannot conclude that internally generated SO deep convection is the only driver, even in recent observations. The SO deep-convection change could work together with various other mechanisms identified in earlier studies3–16, such as wind-driven ice transport and cold/warm-temperature advection, and anthropogenic surface freshening due to an amplified hydrological cycle and ice-sheet melting. As mentioned above, the surface wind trend favours warm SST and decreasing sea ice over the Antarctic Peninsula through warm advection and over the Amundsen– Bellingshausen seas through enhanced vertical mixing caused by anomalous negative wind stress curl. Our model also shows that the long-lasting westerly winds over the SO induce upwelling and a spin-up of the AABW cell, which in turn generates the warm SST.  The surface freshwater changes due to shifted storm tracks and melting ice sheet in future may slow down the SO MOC15, which also cannot be excluded. It is also possible that melting of land-based ice sheets, a process usually not included in climate models, could cause surface freshening and the subsequent suppressed convection and SST cooling."
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Purich, A., England, M. H., Cai, W., Sullivan, A., & Durack, P. J. (2018). Impacts of Broad-Scale Surface Freshening of the Southern Ocean in a Coupled Climate Model. Journal of Climate, 31(7), 2613–2632. doi:10.1175/jcli-d-17-0092.1, https://doi.org/10.1175/JCLI-D-17-0092.1

https://journals.ametsoc.org/doi/10.1175/JCLI-D-17-0092.1

Abstract: "The Southern Ocean surface has freshened in recent decades, increasing water column stability and reducing upwelling of warmer subsurface waters. The majority of CMIP5 models underestimate or fail to capture this historical surface freshening, yet little is known about the impact of this model bias on regional ocean circulation and hydrography. Here experiments are performed using a global coupled climate model with additional freshwater applied to the Southern Ocean to assess the influence of recent surface freshening. The simulations explore the impact of persistent and long-term broad-scale freshening as a result of processes including precipitation minus evaporation changes. Thus, unlike previous studies, the freshening is applied as far north as 55°S, beyond the Antarctic ice margin. It is found that imposing a large-scale surface freshening causes a surface cooling and sea ice increase under preindustrial conditions, because of a reduction in ocean convection and weakened entrainment of warm subsurface waters into the surface ocean. This is consistent with intermodel relationships between CMIP5 models and the simulations, suggesting that models with larger surface freshening also exhibit stronger surface cooling and increased sea ice. Additional experiments are conducted with surface salinity restoration applied to capture observed regional salinity trends. Remarkably, without any mechanical wind trend forcing, these simulations accurately represent the spatial pattern of observed surface temperature and sea ice trends around Antarctica. This study highlights the importance of accurately simulating changes in Southern Ocean salinity to capture changes in ocean circulation, sea surface temperature, and sea ice."

Caption for the third attached image: "FIG. 1. Trends over 1950–2000. (a) Observed SSS trends from Durack and Wijffels (2010); (b) zonal-mean SSS trends for observations (Durack and Wijffels 2010) and CMIP5 models; (c) zonal-mean precipitation trends for observations (ERA-Interim, COREv2, GPCPv2.2, CMAP standard, CMAP enhanced) and CMIP5 models; and (d) zonal-mean P–E trends for observations (ERA-Interim, COREv2) and CMIP5 models. In (a), stippling indicates significance at the 95% level. In (b) observations are shown in dashed black. In (c)–(d) observations are shown in colors, and vary in the time periods they cover (refer to the data and methods section). In (b)–(d) CMIP5 models are shown in grey, and the ACCESS1.0 CMIP5 run is shown in dark blue."

Note: Sea Surface Salinity, SSS, can be measured remotely in units of PSU, see the extract for the linked website:

http://www.salinityremotesensing.ifremer.fr/sea-surface-salinity/definition-and-units

Extract: "Ocean salinity is generally defined as the salt concentration (e.g., Sodium and Chloride) in sea water. It is measured in unit of PSU (Practical Salinity Unit), which is a unit based on the properties of sea water conductivity. It is equivalent to per thousand or (o/00) or to g/kg."

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The first attached image, indicates a flattening of the influence of increasing values of ECS on GMSTA; thus implying that increases in atmospheric GHG concentrations may be more impactful on future global warming.  However, the oceans and land vegetation currently sequester about one half of all current anthropogenic emissions; thus if these carbon sinks are compromised with future global warming then mankind's ability to limit future increases in atmospheric GHG concentrations would also be compromised.  In this frame of mind, the first linked reference is entitled "Doubling Down on Our Faustian Bargain" and it indicates that temporary radiative forcing masking factors (such as: both anthropogenic & natural aerosols, and temporary increases in CO₂ absorption by plants) have allowed mankind to accumulate large accumulations of carbon in the atmosphere, land and ocean; that could actively contribute to future radiative forcing once the temporary masking factors have been eliminated. 

The second, third & fourth linked references cite research on forests, as an illustration of how sensitive such carbon sinks can be to future climate disruption (such as :wet-dry cycles, pests, fires, etc) especially as our current rate of increase of radiative forcing is much higher than at any time since the PETM; and thus vegetation (both on land & in the ocean) will not have adequate time to adapt to such rapidly changing climate conditions:


James Hansen, Pushker Kharecha, Makiko Sato (2013), "Doubling Down on Our Faustian Bargain", Environmental Research Letters.

http://iopscience.iop.org/article/10.1088/1748-9326/8/1/011006/meta
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http://www.columbia.edu/~jeh1/mailings/2013/20130329_FaustianBargain.pdf

Abstract: "Rahmstorf et al 's (2012) conclusion that observed climate change is comparable to projections, and in some cases exceeds projections, allows further inferences if we can quantify changing climate forcings and compare those with projections. The largest climate forcing is caused by well-mixed long-lived greenhouse gases. Here we illustrate trends of these gases and their climate forcings, and we discuss implications. We focus on quantities that are accurately measured, and we include comparison with fixed scenarios, which helps reduce common misimpressions about how climate forcings are changing.
Annual fossil fuel CO2 emissions have shot up in the past decade at about 3% yr-1, double the rate of the prior three decades (figure 1). The growth rate falls above the range of the IPCC (2001) 'Marker' scenarios, although emissions are still within the entire range considered by the IPCC SRES (2000). The surge in emissions is due to increased coal use (blue curve in figure 1), which now accounts for more than 40% of fossil fuel CO2 emissions."

The second linked article is entitled: "Forests 'held their breath' during global warming hiatus, research shows".  This illustrates Hansen's Faustian Bargain.

https://phys.org/news/2017-01-forests-held-global-hiatus.html

Extract: "The study shows that, during extended period of slower warming, worldwide forests 'breathe in' carbon dioxide through photosynthesis, but reduced the rate at which they 'breathe out'—or release the gas back to the atmosphere."


The third linked reference indicates that forests play a more important role in keeping the planet cool than was previously appreciated.  Thus if one assumes that they are entitled to make self-serving assumptions one could assume that decision makers will not only preserve forests but will expand them in the future.  However, the reality is that we are currently losing forests at a rate appropriate for a BAU scenario; and which could accelerate in the future.  Thus if we keep losing forest, our AR5 projections may err on the side of least drama:

Ryan M. Bright et al. Local temperature response to land cover and management change driven by non-radiative processes, Nature Climate Change (2017). DOI: 10.1038/nclimate3250

http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate3250.html

Abstract: "Following a land cover and land management change (LCMC), local surface temperature responds to both a change in available energy and a change in the way energy is redistributed by various non-radiative mechanisms. However, the extent to which non-radiative mechanisms contribute to the local direct temperature response for different types of LCMC across the world remains uncertain. Here, we combine extensive records of remote sensing and in situ observation to show that non-radiative mechanisms dominate the local response in most regions for eight of nine common LCMC perturbations. We find that forest cover gains lead to an annual cooling in all regions south of the upper conterminous United States, northern Europe, and Siberia—reinforcing the attractiveness of re-/afforestation as a local mitigation and adaptation measure in these regions. Our results affirm the importance of accounting for non-radiative mechanisms when evaluating local land-based mitigation or adaptation policies."

The fourth reference (see also the second attached image) indicates a two-fold increase of carbon cycle sensitivity to tropical temperature variations:

Wang, X., Piao, S., Ciais, P., Friedlingstein, P., Myneni, R.B., Cox, P., Heimann, M., Miller, J., Peng, S.P., Wang, T., Yang, H. and Chen, A., (2014), "A two-fold increase of carbon cycle sensitivity to tropical temperature variations", Nature, 2014; DOI: 10.1038/nature12915.

http://www.nature.com/nature/journal/v506/n7487/full/nature12915.html#extended-data

http://sites.bu.edu/cliveg/files/2014/01/wang-nature-2014.pdf

Abstract: "Earth system models project that the tropical land carbon sink will decrease in size in response to an increase in warming and drought during this century, probably causing a positive climate feedback. But available data are too limited at present to test the predicted changes in the tropical carbon balance in response to climate change. Long-term atmospheric carbon dioxide data provide a global record that integrates the interannual variability of the global carbon balance. Multiple lines of evidence demonstrate that most of this variability originates in the terrestrial biosphere. In particular, the year-to-year variations in the atmospheric carbon dioxide growth rate (CGR) are thought to be the result of fluctuations in the carbon fluxes of tropical land areas. Recently, the response of CGR to tropical climate interannual variability was used to put a constraint on the sensitivity of tropical land carbon to climate change. Here we use the long-term CGR record from Mauna Loa and the South Pole to show that the sensitivity of CGR to tropical temperature interannual variability has increased by a factor of 1.9 ± 0.3 in the past five decades. We find that this sensitivity was greater when tropical land regions experienced drier conditions. This suggests that the sensitivity of CGR to interannual temperature variations is regulated by moisture conditions, even though the direct correlation between CGR and tropical precipitation is weak. We also find that present terrestrial carbon cycle models do not capture the observed enhancement in CGR sensitivity in the past five decades. More realistic model predictions of future carbon cycle and climate feedbacks require a better understanding of the processes driving the response of tropical ecosystems to drought and warming."

Caption for the second attached image: " Figure 1 | Change in detrended anomalies in CGR and tropical MAT, in dCGR/dMAT and in ªintCGR over the past five decades. a, Change in detrended CGR anomalies at Mauna Loa Observatory (black) and in detrended tropical MAT anomalies (red) derived from the CRU data set16. Tropical MAT is calculated as the spatial average over vegetated tropical lands (23uN to 23u S).  The highest correlations between detrended CGR and detrended tropicalMAT are obtained when no time lags are applied (R50.53, P,0.01). b, Change in dCGR/dMAT during the past five decades. c, Change in cintCGR during the past five decades. In b and c, different colours showdCGR/dMATor cint CGR estimated with moving time windows of different lengths (20 yr and 25 yr). Years on the horizontal axis indicate the central year of the moving time window used to derive dCGR/dMAT or cintCGR (for example, 1970 represents period 1960–1979 in the 20-yr time window). The shaded areas show the confidence interval of dCGR/dMATand cintCGR, as appropriate, derived using 20-yr or 25-yr moving windows in 500 bootstrap estimates."

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Just a quick note to remind readers that positive feedback mechanisms reinforce each other, which increases ECS with continued warming.  For example, the linked reference provides evidence that Arctic Amplification over the past 30-years has caused shrubs to grow taller throughout the tundra; which in turn causes more Arctic Amplification via both decreased albedo, and insulation of the ground that leads to accelerated permafrost degradation, etc.

Title: 'Taller plants moving into Arctic because of climate change"

https://www.sciencedaily.com/releases/2018/09/180926140816.htm

Extract: "While the Arctic is usually thought of as a vast, desolate landscape of ice, it is in fact home to hundreds of species of low-lying shrubs, grasses and other plants that play a critical role in carbon cycling and energy balance.

Now, Arctic experts have discovered that the effects of climate change are behind an increase in plant height across the tundra over the past 30 years.
…
"Taller plants trap more snow, which insulates the underlying soil and prevents it from freezing as quickly in winter.

"An increase in taller plants could speed up the thawing of this frozen carbon bank, and lead to an increase in the release of greenhouse gases.

"We found that the increase in height didn't happen in just a few sites, it was nearly everywhere across the tundra."

See also:

Bjorkman et al. (2018), "Plant functional trait change across a warming tundra biome", Nature, DOI: 10.1038/s41586-018-0563-7

https://www.nature.com/articles/s41586-018-0563-7

Abstract: "The tundra is warming more rapidly than any other biome on Earth, and the potential ramifications are far-reaching because of global feedback effects between vegetation and climate. A better understanding of how environmental factors shape plant structure and function is crucial for predicting the consequences of environmental change for ecosystem functioning. Here we explore the biome-wide relationships between temperature, moisture and seven key plant functional traits both across space and over three decades of warming at 117 tundra locations. Spatial temperature–trait relationships were generally strong but soil moisture had a marked influence on the strength and direction of these relationships, highlighting the potentially important influence of changes in water availability on future trait shifts in tundra plant communities. Community height increased with warming across all sites over the past three decades, but other traits lagged far behind predicted rates of change. Our findings highlight the challenge of using space-for-time substitution to predict the functional consequences of future warming and suggest that functions that are tied closely to plant height will experience the most rapid change. They also reveal the strength with which environmental factors shape biotic communities at the coldest extremes of the planet and will help to improve projections of functional changes in tundra ecosystems with climate warming."

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As almost no CMIP5 models included ice-climate feedback mechanisms, the linked Wikipedia article on climate change feedbacks does not even mention it.  That appears to be the consensus way to deal with 'Deep Uncertainty', i.e. 'Out of sight, out of mind':

Title: "Climate change feedback"

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

Edit:
Also, the linked Wikipedia article on runaway climate change does not explicitly discuss ice-climate feedback mechanisms; but at least it cites Hansen et al (2013):

Title: " Runaway climate change"

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

Extract: "Hansen et al. 2013 suggests that the Earth could become in large parts uninhabitable and note that this may not even require burning of all fossil fuels, because of higher climate sensitivity (3–4 °C or 5.4–7.2 °F) based on a 550 ppm scenario."

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The linked reference presents findings that can be used to help calibrate Hansen's ice-climate feedback mechanism:

Pepijn Bakker & Matthias Prange (03 August 2018), "Response of the Intertropical Convergence Zone to Antarctic Ice Sheet Melt", Geophysical Research Letters, https://doi.org/10.1029/2018GL078659

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL078659

"Abstract
Past cooling events in the Northern Hemisphere have been shown to impact the location of the intertropical convergence zone (ITCZ) and therewith induce a southward shift of tropical precipitation. Here we use high resolution coupled ocean‐atmosphere simulations to show that reasonable past melt rates of the Antarctic Ice Sheet can similarly have led to shifts of the ITCZ, albeit in opposite direction, through large‐scale surface air temperature changes over the Southern Ocean. Through sensitivity experiments employing slightly negative to large positive meltwater fluxes, we deduce that meridional shifts of the Hadley cell and therewith the ITCZ are, to a first order, a linear response to Southern Hemisphere high‐latitude surface air temperature changes and Antarctic Ice Sheet melt rates. This highlights the possibility to use past episodes of anomalous melt rates to better constrain a possible future response of low latitude precipitation to continued global warming and a shrinking Antarctic Ice Sheet.

Plain Language Summary
Changes in high‐latitude climate can impact the tropical regions through so‐called atmospheric and oceanic teleconnections. Research has mostly focused on past southward shifts in the band of heavy tropical precipitation, called the intertropical convergence zone (ITCZ), linked to large‐scale cooling in the Northern Hemisphere resulting from large‐scale continental ice sheet buildup or a slowdown of the large‐scale Atlantic meridional ocean circulation. Here we use high resolution climate simulations to show that melting of the Antarctic Ice Sheet can similarly lead to northward shifts of the ITCZ and the displacement of the accompanying rain belt. Future melt rates of the Antarctic Ice Sheet are highly uncertain, but our work shows that it might have a nonnegligible impact on the tropical climate. Moreover, we find that because of the apparent linearity of the system under consideration, studying episodes of past changes in the size of the Antarctic Ice Sheet can help us constrain the possible changes in the low latitude hydroclimate."

[sidd]

Wikipedia is open editing, unless there is a lock on the article, so i would suggest correcting errors. I have  done so myself.

ITCZ shifts north in response to antarctic ice melt seems to be a robust response. I have seen this now in a buncha papers. I wonder if Held has ever blogged about it.

sidd
 

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In some of my past posts I have identified the relatively poor past capability of consensus scientists (e.g.: AR4 & AR5) and of extant policy makers (e.g.: the Paris Accord) to deal effectively with 'Deep Uncertainty', as a major obstacle to first acknowledging and then addressing the risks that the numerous ice-climate feedback mechanisms may follow one of several possible pathways to abrupt climate change and abrupt sea level rise.  Therefore, I provide the first following link to the website for the 'Society for Decision Making Under Deep Uncertainty' (DMDU); which, provides numerous publications related to: Robust Decision Making in the face of Deep Uncertainty in Climate Change, including the four linked articles:

http://www.deepuncertainty.org/category/recent-publications/

1. For interactive monitoring for adaptive response systems, see:
http://www.deepuncertainty.org/2018/09/03/designing-a-monitoring-system-to-detect-signals-to-adapt-to-uncertain-climate-change/

Extract: "Adaptive plans aim to anticipate uncertain future changes by combining low-regret short-term actions with long-term options to adapt, if necessary. Monitoring and timely detection of relevant changes, and critical transitions or tipping points is crucial to ensure successful and timely implementation and reassessment of the plan."

2. For robustness in many-objective climate action, see:
http://www.deepuncertainty.org/2018/08/31/including-robustness-considerations-in-the-search-phase-of-many-objective-robust-decision-making-2018/

Extract: "Many-Objective Robust Decision Making (MORDM) is a prominent model-based approach for dealing with deep uncertainty. MORDM has four phases: a systems analytical problem formulation, a search phase to generate candidate solutions, a trade-off analysis where different strategies are compared across many objectives, and a scenario discovery phase to identify the vulnerabilities."

3. For an example of robust management of resources, see
http://www.deepuncertainty.org/2018/08/07/real%e2%80%90options-water-supply-planning-multistage-scenario-trees-for-adaptive-and-flexible-capacity-expansion-under-probabilistic-climate-change-uncertainty-2018/

Extract: "Planning water supply infrastructure includes identifying interventions that cost‐effectively secure an acceptably reliable water supply. Climate change is a source of uncertainty for water supply developments as its impact on source yields is uncertain. Adaptability to changing future conditions is increasingly viewed as a valuable design principle of strategic water planning. Because present decisions impact a system’s ability to adapt to future needs, flexibility in activating, delaying and replacing engineering projects should be considered in least‐cost water supply intervention scheduling. This is a principle of Real Option Analysis (ROA) which this paper applies to least‐cost capacity expansion scheduling via multistage stochastic mathematical programming."

4. For discussion of adaptive pathways to manage sea level rise, see:
http://www.deepuncertainty.org/2018/08/07/strategic-adaptation-pathway-planning-to-manage-sea-level-rise-and-changing-coastal-flood-risk-2018/

Extract: "Communities around the world are already committed to future sea-level rise. Long-term adaptation planning to manage associated coastal flood impacts is, however, challenged by uncertainty and contested stakeholder priorities. This study provides a proof of concept for a combined robust decision making (RDM) and dynamic adaptive policy pathways (DAPP) approach in coastal flood risk management. The concept uses model-based support and largely open source tools to help local government plan coastal adaptation pathways. Key steps in the method are illustrated using a hypothetical case study in Australia. The study shows how scenario discovery can provide multi-dimensional descriptions of adaptation tipping points which may inform the development of technical signpost indicators. Transient scenarios uncovered limitations in seemingly robust adaptation policies, where historical path dependencies may constrain the rate of adaptation and the extent to which future coastal flood impacts can be successfully managed. Lived values have the potential to offer insights about non-material social trade-offs that residents may need to accept for the benefit of reduced flood risk, and could form a basis for defining socially-oriented signpost indicators. However, the nuances and subjectivity of lived values means that ongoing engagement with residents is essential as part of a combined RDM and DAPP approach to preserve the communities’ way of life. The learnings from this hypothetical case study suggest that testing in a real world participatory setting could be valuable in further developing a combined RDM and DAPP approach to plan adaptation pathways and manage future coastal flood risk."

With regard to the last linked article's comment that '... historical path dependencies may constrain the rate of adaptation ...'; I have encountered this situation in Louisiana and I note that our global modern world is built on many historical dependencies that will likely severely limit mankind's ability to respond appropriately to abrupt climate change beginning circa 2040.

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We should all recognize that the timing and impact of hydrofracturing can be accelerated from that assumed by Pollard and DeConto by such mechanisms as the formation of basal channels in ice shelves where: "These channels also result in ice surface deformation, which diverts supraglacial rivers into the transverse fractures."  Thus, it may take less atmospheric warming and less associated ice shelf surface melting to cause hydrofracturing to collapse Antarctic ice shelves than Pollard and DeConto are assuming (which might support the projection of abrupt sea level rise some years before 2040:

Christine F. Dow et al. (13 Jun 2018), "Basal channels drive active surface hydrology and transverse ice shelf fracture", Science Advances, Vol. 4, no. 6, eaao7212, DOI: 10.1126/sciadv.aao7212

http://advances.sciencemag.org/content/4/6/eaao7212

Abstract: "Ice shelves control sea-level rise through frictional resistance, which slows the seaward flow of grounded glacial ice. Evidence from around Antarctica indicates that ice shelves are thinning and weakening, primarily driven by warm ocean water entering into the shelf cavities. We have identified a mechanism for ice shelf destabilization where basal channels underneath the shelves cause ice thinning that drives fracture perpendicular to flow. These channels also result in ice surface deformation, which diverts supraglacial rivers into the transverse fractures. We report direct evidence that a major 2016 calving event at Nansen Ice Shelf in the Ross Sea was the result of fracture driven by such channelized thinning and demonstrate that similar basal channel–driven transverse fractures occur elsewhere in Greenland and Antarctica. In the event of increased basal and surface melt resulting from rising ocean and air temperatures, ice shelves will become increasingly vulnerable to these tandem effects of basal channel destabilization."

Caption for the attached image: " (A) Schematic of an ice shelf basal channel and a coincident ice surface depression that funnels meltwater, resulting in river formation and incision. The ice shelf is shown in gray. (B to D) Surface (red) and basal (blue) ice cross-sectional profiles from radar along the flight lines in Fig. 2A for (B) October 2011 and (C and D) December 2014. The green arrows indicate the location of the parallel surface rivers identified from Landsat imagery. The black arrows indicate the extent of the basal channel. The data gap in (D) is due to the ice shelf rift. m asl, meters above sea level."

[AbruptSLR]

...

ITCZ shifts north in response to antarctic ice melt seems to be a robust response. I have seen this now in a buncha papers. ...

For those who do not know (aside from the unsettled question of associated cloud feedbacks) when the ITCZ (see the first image showing the location of the ITCZs) shifts poleward (and the oceanic overturning circulation slows) more radiative solar energy is absorbed by the equatorial ocean water; which increases the moist static energy (MSE) resulting from increased evaporation from that warmer equatorial ocean water.  This energy is telecommunicated through the atmosphere to the appropriate pole by Rossby Waves (see the second image showing a typical atmospheric bridge from the Equatorial Pacific to the Arctic); which contributes to accelerated polar amplification.  This is true whether the shift in the ITCZ is due to increases in GHGs or due to increased ice-climate feedback mechanisms.

Furthermore, I note that most consensus climate scenarios aimed at keeping GMSTA below 2C involve the use of geoengineering (see the linked article below); typically to allow GMSTA to temporarily peak near 2C and then to drop-off due to the application of geoengineering.  However, it is important to note that the application of such geoengineering scenarios are at best ineffective at (and at worse contribute to) stopping abrupt climate change driven by ice-climate feedback mechanisms that once initiated (say by temporarily allowing GMSTA to approach 2C) are driven by gravity induced ice mass loss from key marine (& marine-terminating) glaciers.

Russotto, R. D. and Ackerman, T. P.: Energy transport, polar amplification, and ITCZ shifts in the GeoMIP G1 ensemble, Atmos. Chem. Phys., 18, 2287-2305, https://doi.org/10.5194/acp-18-2287-2018, 2018.

https://www.atmos-chem-phys.net/18/2287/2018/

Abstract. The polar amplification of warming and the ability of the Intertropical Convergence Zone (ITCZ) to shift to the north or south are two very important problems in climate science. Examining these behaviors in global climate models (GCMs) running solar geoengineering experiments is helpful not only for predicting the effects of solar geoengineering but also for understanding how these processes work under increased carbon dioxide (CO2). Both polar amplification and ITCZ shifts are closely related to the meridional transport of moist static energy (MSE) by the atmosphere. This study examines changes in MSE transport in 10 fully coupled GCMs in experiment G1 of the Geoengineering Model Intercomparison Project (GeoMIP), in which the solar constant is reduced to compensate for the radiative forcing from abruptly quadrupled CO2 concentrations. In G1, poleward MSE transport decreases relative to preindustrial conditions in all models, in contrast to the Coupled Model Intercomparison Project phase 5 (CMIP5) abrupt4xCO2 experiment, in which poleward MSE transport increases. We show that since poleward energy transport decreases rather than increases, and local feedbacks cannot change the sign of an initial temperature change, the residual polar amplification in the G1 experiment must be due to the net positive forcing in the polar regions and net negative forcing in the tropics, which arise from the different spatial patterns of the simultaneously imposed solar and CO2 forcings. However, the reduction in poleward energy transport likely plays a role in limiting the polar warming in G1. An attribution study with a moist energy balance model shows that cloud feedbacks are the largest source of uncertainty regarding changes in poleward energy transport in midlatitudes in G1, as well as for changes in cross-equatorial energy transport, which are anticorrelated with ITCZ shifts.

Edit: see also the third image to help understand how ECS increases as the Hadley Cell expands.

[AbruptSLR]

...
Furthermore, I note that most consensus climate scenarios aimed at keeping GMSTA below 2C involve the use of geoengineering ...

Even if green geoengineering pathways such as those discussed in the linked article (& associated image) were to be fully implemented (which is doubtful), they may well not be sufficient to stop an abrupt collapse of the WAIS beginning circa 2040.

Title: "Explainer: Why some US Democrats want a 'Green New Deal' to tackle climate change"

https://www.carbonbrief.org/explainer-why-some-us-democrats-want-a-green-new-deal-to-tackle-climate-change

Extract: "A growing number of Democrats in the US Congress are hoping to create a new set of policies which would trigger a rapid decarbonisation of the US economy. They have labelled the plan as the “green new deal”."