Ice Apocalypse - Multiple Meters Sea Level Rise

[AbruptSLR]

This linked review indicates that CMIP6 will include some feedback mechanism associate with ice mass loss from simulated interactive ice sheets; however, none of these simulations will include ice-cliff or hydrofracturing mechanisms:

Heinze, C., Eyring, V., Friedlingstein, P., Jones, C., Balkanski, Y., Collins, W., Fichefet, T., Gao, S., Hall, A., Ivanova, D., Knorr, W., Knutti, R., Löw, A., Ponater, M., Schultz, M. G., Schulz, M., Siebesma, P., Teixeira, J., Tselioudis, G., and Vancoppenolle, M.: Climate feedbacks in the Earth system and prospects for their evaluation, Earth Syst. Dynam. Discuss., https://doi.org/10.5194/esd-2018-84, in review, 2018.

https://www.earth-syst-dynam-discuss.net/esd-2018-84/

Abstract. Earth system models (ESMs) are key tools for providing climate projections under different scenarios of human-induced forcing. ESMs include a large number of additional processes and feedbacks such as biogeochemical cycles that traditional physical climate models do not consider. Yet, some processes such as cloud dynamics and ecosystem functional response still have fairly high uncertainties. In this article, we present an overview of climate feedbacks for Earth system components currently included in state-of-the-art ESMs and discuss the challenges to evaluate and quantify them. Uncertainties in feedback quantification arise from the interdependencies of biogeochemical matter fluxes and physical properties, the spatial and temporal heterogeneity of processes, and the lack of long-term continuous observational data to constrain them. We present an outlook for promising approaches that can help quantifying and constraining the large number of feedbacks in ESMs in the future. The target group for this article includes generalists with a background in natural sciences and an interest in climate change as well as experts working in interdisciplinary climate research (researchers, lecturers, and students). This study updates and significantly expands upon the last comprehensive overview of climate feedbacks in ESMs, which was produced 15 years ago (NRC, 2003).

Extract: "ESMs are being continuously expanded to include additional processes. For example, the ESMs which form part of the Coupled Model Intercomparison Project Phase 6 (CMIP6, Eyring et al. (2016a)) will for the first time include interactive ice sheets (Nowicki et al., 2016) and several models will have interactive chemistry and aerosols (Collins et al., 2017)."

[AbruptSLR]

In the linked article the Bulletin of the Atomic Scientists supports the position that consensus climate science is under representing the risk of abrupt climate change (and even it does not clearly identify the ice-climate feedback risks):

Title: "Climate report understates threat"

https://thebulletin.org/2018/10/climate-report-understates-threat/

Extract: "The UN’s Intergovernmental Panel on Climate Change’s Special Report on Global Warming of 1.5 degrees Celsius, released on Monday, is a major advance over previous efforts to alert world leaders and citizens to the growing climate risk. But the report, dire as it is, misses a key point: Self-reinforcing feedbacks and tipping points—the wildcards of the climate system—could cause the climate to destabilize even further. The report also fails to discuss the five percent risk that even existing levels of climate pollution, if continued unchecked, could lead to runaway warming—the so-called “fat tail” risk. These omissions may mislead world leaders into thinking they have more time to address the climate crisis, when in fact immediate actions are needed. To put it bluntly, there is a significant risk of self-reinforcing climate feedback loops pushing the planet into chaos beyond human control."

[AbruptSLR]

While the linked article has no new information about how unstable Thwaites is, it does indicate just how concerned the global community is about this glacier, & the associated research may provide some more information in a year or two:

Title: "Just how unstable is the massive Thwaites glacier? Scientists are about to find out."

https://www.pri.org/stories/2018-05-01/just-how-unstable-massive-thwaites-glacier-scientists-are-about-find-out

Extract: "US and British scientific agencies announced their biggest joint Antarctic research effort in more than a generation on Monday.

The focus is Thwaites Glacier, which is roughly the size of Florida and sits on the western edge of Antarctica.
…
"Thwaites is the 800-pound gorilla, just because of how big and wide and deep into West Antarctica this particular outlet glacier goes,” says Chris Shuman, a University of Maryland Baltimore County professor working at the NASA's Goddard Space Flight Center and who is unaffiliated with the project. “So understanding it will greatly improve our ability to understand the response of the entire West Antarctic Ice Sheet.""

[AbruptSLR]

The linked article provides a brief summary about the stability of Antarctic ice shelves, but you have to read between the lines to gain an understanding of the stability of the Thwaites Ice Shelf:

Title: "SOTC: Ice Shelves"

https://nsidc.org/cryosphere/sotc/iceshelves.html

Extract: "The Thwaites and Pine Island Glaciers flow into Pine Island Bay, and drain the West Antarctic Ice Sheet. These glaciers would only need to retreat a short distance before leading to accelerated retreat. Such a process may have happened before (Wise et al. 2017)."

[AbruptSLR]

The linked reference provides more insights about the instability of Thwaites; however, I note that projections from models of marine glaciers typically still err on the side of least drama:

Yu, H., Rignot, E., Seroussi, H., and Morlighem, M.: Retreat of Thwaites Glacier, West Antarctica, over the next 100 years using various ice flow models, ice shelf melt scenarios and basal friction laws, The Cryosphere Discuss., https://doi.org/10.5194/tc-2018-104, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2018-104/

Abstract. Thwaites Glacier (TG), West Antarctica, experiences rapid, potentially irreversible grounding line retreat and mass loss in response to enhanced ice shelf melting. Several numerical models of TG have been developed recently, showing a large spread in the evolution of the glacier in the coming decades to a century. It is, however, not clear how different parameterizations of basal friction and ice shelf melt or different approximations in ice stress balance affect projections.Here, we simulate the evolution of TG using different ice shelf melt, basal friction laws and ice sheet models of varying levels of complexity to quantify the effect of these model configurations on the results. We find that the grounding line retreat and its sensitivity to ocean forcing is enhanced when a full-Stokes model is used, ice shelf melt is applied on partially floating elements, and a Budd friction is used. Initial conditions also impact the model results. Yet, all simulations suggest a rapid, sustained retreat along the same preferred pathway. The highest retreat rate occurs on the eastern side of the glacier and the lowest rate on a subglacial ridge on the western side. All the simulations indicate that TG will undergo an accelerated retreat once it retreats past the western ridge. Combining the results, we find the uncertainty is small in the first 30 years, with a cumulative contribution to sea level rise of 5mm, similar to the current rate. After 30 years, the mass loss depends on the model configurations, with a 300% difference over the next 100 years, ranging from 14 to 42mm.

Edit: Also, I note that none of the models used in this study included either ice-cliff, or hydrofracturing, failure mechanisms.

[AbruptSLR]

As background information (for those who do not already know) the linked 2015 reference and associated image indicates that ice volume loss from Antarctic ice shelves has accelerated since 2003, and makes a contribution to the freshening of the surface waters of the Southern Ocean (and thus contributing to the ice-climate feedback mechanism), without making big change in sea level:

Fernando S. Paolo et al. (17 Apr 2015), "Volume loss from Antarctic ice shelves is accelerating", Science, Vol. 348, Issue 6232, pp. 327-331, DOI: 10.1126/science.aaa0940

http://science.sciencemag.org/content/348/6232/327

Abstract: "The floating ice shelves surrounding the Antarctic Ice Sheet restrain the grounded ice-sheet flow. Thinning of an ice shelf reduces this effect, leading to an increase in ice discharge to the ocean. Using 18 years of continuous satellite radar altimeter observations, we have computed decadal-scale changes in ice-shelf thickness around the Antarctic continent. Overall, average ice-shelf volume change accelerated from negligible loss at 25 ± 64 cubic kilometers per year for 1994–2003 to rapid loss of 310 ± 74 cubic kilometers per year for 2003–2012. West Antarctic losses increased by ~70% in the past decade, and earlier volume gain by East Antarctic ice shelves ceased. In the Amundsen and Bellingshausen regions, some ice shelves have lost up to 18% of their thickness in less than two decades."

See also:
Title: "Antarctic ice shelves rapidly thinning"

https://phys.org/news/2015-03-antarctic-ice-shelves-rapidly-thinning.html

Caption: "Eighteen years of change in thickness and volume of Antarctic ice shelves. Rates of thickness change (m/decade) are color-coded from -25 (thinning) to +10 (thickening). Circles represent percentage of thickness lost (red) or gained (blue) in 18 years. The central circle demarcates the area not surveyed by the satellites (south of 81.5ºS). Original data were interpolated for mapping purposes. Background is the Landsat Image Mosaic of Antarctica (LIMA). Credit: Scripps Institution of Oceanography, UC San Diego"

[AbruptSLR]

The linked reference indicates that the projected increase (with continued global warming) of more frequent strong El Nino events combined with the projected increase in positive SAM, will significantly increase ice mass loss from the ASE, which will increase the risk of a collapse of the WAIS:

Deb, P., A. Orr, D. H. Bromwich, J. P. Nicolas, J. Turner, and J. S. Hosking, 2018: Summer drivers of atmospheric variability affecting ice shelf thinning in the Amundsen Sea Embayment, West Antarctica. Geophy. Res. Lett., 45. doi: 10.1029/2018GL077092.

http://polarmet.osu.edu/PMG_publications/deb_bromwich_grl_2018.pdf

Abstract:  "Satellite data and a 35-year hindcast of the Amundsen Sea Embayment summer climate using the Weather Research and Forecasting model are used to understand how regional and large-scale atmospheric variability affects thinning of ice shelves in this sector of West Antarctica by melting from above and below (linked to intrusions of warm water caused by anomalous westerlies over the continental shelf edge). El Niño episodes are associated with an increase in surface melt but do not have a statistically significant impact on westerly winds over the continental shelf edge. The location of the Amundsen Sea Low and the polarity of the Southern Annular Mode (SAM) have negligible impact on surface melting, although a positive SAM and eastward shift of the Amundsen Sea Low cause anomalous westerlies over the continental shelf edge. The projected future increase in El Niño episodes and positive SAM could therefore increase the risk of disintegration of West Antarctic ice shelves."

Extract: "Our study suggests that ASE ice shelves could experience an intensification of melt in the future from both above and below as a result of both regional and large-scale atmospheric changes, potentially increasing the risk of their disintegration, which in turn could potentially trigger a collapse of the West Antarctic ice sheet (DeConto & Pollard, 2016). To better understand this threat will require further detailed investigation of the impacts of ENSO, the polarity of the SAM, and the depth/location of the ASL on ASE ice shelves. Also necessary is improving the reliability of future projections, such as ENSO and its teleconnections, as well as the response of the SAM to recovery of the Antarctic ozone hole and increased greenhouse gas emissions (Polvani, Waugh, et al., 2011)."

[AbruptSLR]

One key problem associated with reducing the 'Deep Uncertainty' associate with ice-climate feedback mechanisms is the problems of correct attribution, especially when consensus climate change studies such as the first linked report (for decision makers) essentially ignore this consideration, even though Hansen's book "Storms of my Grandchildren" clearly warns about the consequence of this important feedback:

Title: "Fingerprints Everywhere 2018"

http://www.climatesignals.org/fingerprints-everywhere-2018
http://www.climatesignals.org/sites/default/files/Fingerprints-Everywhere.pdf

Indeed, the second linked reference highlights the difficulties of climate change attribution; which makes it even harder for ice-climate feedback mechanisms to gain traction in the attribution game:

Judith L. Lean (22 February 2018), "Observation‐based detection and attribution of 21st century climate change", WIREs Climate Change, https://doi.org/10.1002/wcc.511

https://onlinelibrary.wiley.com/doi/abs/10.1002/wcc.511

Abstract: "Climate change detection and attribution have proven unexpectedly challenging during the 21st century. Earth’s global surface temperature increased less rapidly from 2000 to 2015 than during the last half of the 20th century, even though greenhouse gas concentrations continued to increase. A probable explanation is the mitigation of anthropogenic warming by La Niña cooling and declining solar irradiance. Physical climate models overestimated recent global warming because they did not generate the observed phase of La Niña cooling and may also have underestimated cooling by declining solar irradiance. Ongoing scientific investigations continue to seek alternative explanations to account for the divergence of simulated and observed climate change in the early 21st century, which IPCC termed a “global warming hiatus.” Amplified by media commentary, the suggestions by these studies that “missing” mechanisms may be influencing climate exacerbates confusion among policy makers, the public and other stakeholders about the causes and reality of modern climate change.

Understanding and communicating the causes of climate change in the next 20 years may be equally challenging. Predictions of the modulation of projected anthropogenic warming by natural processes have limited skill. The rapid warming at the end of 2015, for example, is not a resumption of anthropogenic warming but rather an amplification of ongoing warming by El Niño. Furthermore, emerging feedbacks and tipping points precipitated by, for example, melting summer Arctic sea ice may alter Earth’s global temperature in ways that even the most sophisticated physical climate models do not yet replicate."

For example, when non-experts think of the impacts of ice loss on the climate they typically think of the ice-albedo feedback and the potential rapid loss of Arctic Sea Ice; however, the third linked reference indicates the attribution problems associated with this feedback due to the influences of both natural GHG emissions and of anthropogenic aerosol forcing (which have been suppressing Arctic Sea Ice losses for decades until the recent reduction in anthropogenic aerosol emissions).  Furthermore, ice-albedo feedback can be one of the many different mechanisms contributing to ice-climate feedback, which further complicates attribution.

B. L. Mueller et al (2018), "Attribution of Arctic Sea Ice Decline from 1953 to 2012 to Influences from Natural, Greenhouse Gas, and Anthropogenic Aerosol Forcing", Journal of Climate,
https://doi.org/10.1175/JCLI-D-17-0552.1

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-17-0552.1

Abstract: "The paper presents results from a climate change detection and attribution study on the decline of Arctic sea ice extent in September for the 1953–2012 period. For this period three independently derived observational datasets and simulations from multiple climate models are available to attribute observed changes in the sea ice extent to known climate forcings. Here we direct our attention to the combined cooling effect from other anthropogenic forcing agents (mainly aerosols), which has potentially masked a fraction of greenhouse gas–induced Arctic sea ice decline. The presented detection and attribution framework consists of a regression model, namely, regularized optimal fingerprinting, where observations are regressed onto model-simulated climate response patterns (i.e., fingerprints). We show that fingerprints from greenhouse gas, natural, and other anthropogenic forcings are detected in the three observed records of Arctic sea ice extent. Beyond that, our findings indicate that for the 1953–2012 period roughly 23% of the greenhouse gas–induced negative sea ice trend has been offset by a weak positive sea ice trend attributable to other anthropogenic forcing. We show that our detection and attribution results remain robust in the presence of emerging nonstationary internal climate variability acting upon sea ice using a perfect model experiment and data from two large ensembles of climate simulations."

[Gray-Wolf]

https://onlinelibrary.wiley.com/doi/abs/10.1002/wcc.511

Abstract: "Climate change detection and attribution have proven unexpectedly challenging during the 21st century. Earth’s global surface temperature increased less rapidly from 2000 to 2015 than during the last half of the 20th century, even though greenhouse gas concentrations continued to increase. A probable explanation is the mitigation of anthropogenic warming by La Niña cooling and declining solar irradiance.

I think it could be argued that the period highlighted was blighted by the flip side of the global warming coin, that of global dimming? The fact the the Pacific was ground zero for the incredible increase in Chinese 'dimming' lead to imbalance between the tropical Pacific/tropical Atlantic.
In its turn this lead to the increase in upper level winds over the Caribbean as the basins struggled to find parity and this began to lower Atlantic hurricane numbers via the aggressive shear environment any forming storms encountered.
It also lead to the uptick in the trades over the ENSO regions leading to the huge warm pool at the western extremes of that area.

A combination of China's scrabble to clean up its urban pollution and so save the population ( nobody wants to only be able to see the sun via TV screens in the city centres!) and the flipping of the Pacific 'Naturals' in 2014 helped us suddenly balance out the Basins and so allow warming to return to above the rates of warming measured over the last spurt of warming post our 'dimmed period ( 1980's/90's ).

I know it is early doors yet but I think we will find the years up to 2025 warming at rates well above those of the late 20th century both via the clearing of the airs over the Pacific but also the impacts of open water Arctic ( and recently Antarctic?) able to capture energy that used to be bounced harmlessly back into space and introduce it into the climate system?

[AbruptSLR]

To provide additional background information on why consensus climate science and decision makers have not yet been adequately portraying the true risks associated with ice-climate feedback mechanism, I provide the first image from Roe and Armour 2011, showing how a positive feedback mechanism pdf results in a skewed pdf for the contribution of that feedback mechanism to GMSTA.  Reticent climate scientists and policy makers tend to ignore the long-tail of the skewed GMSTA pdf (which is a measure of climate sensitivity) by limiting the confidence range under consideration.

Next, the second image, from Roe and Baker 2007, shows the relationship of fat-tailed climate sensitivity (a measure of GMSTA) pdf to a combination of positive feedback mechanisms (such as is the case for the ice-climate feedback) and different feedback parameters; which determine the fatness of the climate sensitivity tail that is being ignored by consensus climate scientists and policy makers.

Next, the third image shows the influence of different time durations on the pdf for climate sensitivity due to an assumed instantaneous doubling of atmospheric CO2, from Roe and Bauman 2011.  Policy makers assume that they are entitled to use the short-term climate sensitivity pdf for calculating the remaining carbon budget, but then the decline to correct their calculations for this duration effect when they delay their climate action by decades.

Lastly, for this post, the fourth image show that the BAU forcing pathway that we are currently following, resulting in a rate of forcing that is approximately 100 times faster than during the PETM.  Unfortunately, ECS will increase from our present value with such a high rate of forcing, but policy makers choice to ignore this consideration.

[AbruptSLR]

While I have cited Pattyn et al. (2018)'s finding previously, I put it again as an illustration of how climate scientists (including Rob DeConto) tend to bury this risk of abrupt ice-climate global response by watering that projected response down by various means including:
1. Mixing consideration of responses from ice-sheet models with and without ice-cliff failures and hydrofracting and with and without interaction with global circulation models and by not even mentioning responses such as those reported by Hansen et al. (2016) that consider ice-climate feedback for assumed ice mass loss scenarios.
2. Using IPCC values for climate sensitivity, which blend together many different means of estimating climate sensitivity, which tends to undervalue the relatively high values of ECS estimated by the best climate models.
3. Focusing of low emission forcing scenarios that tend to indicate that the response of ice sheets that have moved past their tipping points will take at least a millennia to develop; thus de-emphasizing the risk risks of our current BAU pathway which could result in ice sheet responses that develop within a century once they have passed their tipping points.
4. De-emphasizing the self-reinforcing and cascading risks of the hundreds of positive feedback mechanisms (including ice-climate mechanisms) with continued warming.

Pattyn, et al. (2018), "The Greenland and Antarctic ice sheets under 1.5 °C global warming", Nature Climate Change, DOI: 10.1038/s41558-018-0305-8 , https://www.nature.com/articles/s41558-018-0305-8

https://www.nature.com/articles/s41558-018-0305-8

Abstract: "Even if anthropogenic warming were constrained to less than 2 °C above pre-industrial, the Greenland and Antarctic ice sheets will continue to lose mass this century, with rates similar to those observed over the past decade. However, nonlinear responses cannot be excluded, which may lead to larger rates of mass loss. Furthermore, large uncertainties in future projections still remain, pertaining to knowledge gaps in atmospheric (Greenland) and oceanic (Antarctica) forcing. On millennial timescales, both ice sheets have tipping points at or slightly above the 1.5–2.0 °C threshold; for Greenland, this may lead to irreversible mass loss due to the surface mass balance–elevation feedback, whereas for Antarctica, this could result in a collapse of major drainage basins due to ice-shelf weakening."

Extract: "Evidence from the observed Larsen B collapse, and rapid front retreat of Jakobshavn Isbrae in Greenland, suggests that hydrofracturing could lead to the rapid collapse of ice shelves and potentially produce high ice cliffs with vertical exposure above 90 m rendering the cliffs mechanically unsustainable, possibly resulting in what has been termed marine ice cliff instability"

See also:

Title: "Modest warming risks 'irreversible' ice sheet loss, study warns"

https://phys.org/news/2018-11-modest-irreversible-ice-sheet-loss.html

[AbruptSLR]

...
I think it could be argued that the period highlighted was blighted by the flip side of the global warming coin, that of global dimming?
...

Gray-Wolf,

Thanks for your comments.  Certainly the variable and chaotic nature of both natural and anthropogenic forcing adds to the 'Deep Uncertainty' that tends to obscure the very real risks of abrupt ice-climate feedback this century.  The truth is that mankind is headed into uncharted waters within the next couple of decades, and we are not appropriately facing the warning provided by researchers such as James Hansen, & others, all of whom have to partially mute their warning in order to even be published [witness the difficulties that were encountered getting Hansen et al (2016) published and how infrequently it has been cited by consensus climate science].

I strongly suspect that mankind will not be willing to accept the risks of abrupt ice-climate feedback until it is both manifest and unstoppable (at least for the next few centuries).

Best,
ASLR

[AbruptSLR]

I have previously posted (e.g.: see various posts from 163 thru 178) about the risks that potentially abrupt ice mass loss from Antarctica (in the next century) could have on the Earth's geomagnetic field; and the linked article emphasizes that the biggest risks to modern society is posed by the possibility that the field strength could degrade to zero within the next century, even if it takes a millennia to flip:

Title: "Earth's magnetic poles could start to flip. What happens then?"

https://phys.org/news/2018-12-earth-magnetic-poles-flip.html

Extract: " As Earth's magnetic shield fails, so do its satellites. First, our communications satellites in the highest orbits go down. Next, astronauts in low-Earth orbit can no longer phone home. And finally, cosmic rays start to bombard every human on Earth.

This is a possibility that we may start to face not in the next million years, not in the next thousand, but in the next hundred. If Earth's magnetic field were to decay significantly, it could collapse altogether and flip polarity – changing magnetic north to south and vice versa. The consequences of this process could be dire for our planet.

Most worryingly, we may be headed right for this scenario.

'The geomagnetic field has been decaying for the last 3,000 years,' said Dr. Nicolas Thouveny from the European Centre for Research and Teaching of Environmental Geosciences (CEREGE) in Aix-en-Provence, France. 'If it continues to fall down at this rate, in less than one millennium we will be in a critical (period).'
…
In the Atlantic Ocean between South America and Africa, there is a vast region of Earth's magnetic field that is about three times weaker than the field strength at the poles.

This is called the South Atlantic Anomaly (SAA), and it's the focus of the CoreSat project being led by Professor Chris Finlay from the Technical University of Denmark (DTU) near Copenhagen. Using data from multiple satellites, including the European Space Agency's (ESA) three Swarm satellites launched in 2013, this project is trying to figure out what is causing the SAA.
'This is a region where we see that satellites consistently (experience) electronic failures,' said Prof. Finlay. 'And we don't understand where this weak field region is coming from, what's producing it, and how it might change in the future.'
…
'The decrease in geomagnetic field is much more important and dramatic than the reversal,' said Dr. Thouveny. 'It is very important to understand if the present field will decay to zero in the next century, because we will have to prepare.' "

[sidd]

Re: ITCZ, question for abruptSLR

in the ACME modelling efforts, have they fixed the double ITCZ problem that most GCMs have ?

sidd

[AbruptSLR]

Re: ITCZ, question for abruptSLR

in the ACME modelling efforts, have they fixed the double ITCZ problem that most GCMs have ?

sidd

That last time I looked it was still there, but who knows what changes that they are continually making as they transform ACME into E3SM.

Edit: You can monitor the progress being made on the E3SM model at:

https://esgf-node.llnl.gov/projects/e3sm/
&
https://e3sm.org/about/events/all-hands-presentations/

[AbruptSLR]

There is a reasonable likelihood that the projected acceleration in Greenland ice mass loss in the circa 2040 (see the first linked reference and associate first image) could trigger an acceleration of ice mass loss from key marine glaciers in the AIS (particularly in the WAIS):

Calov, R., Beyer, S., Greve, R., Beckmann, J., Willeit, M., Kleiner, T., Rückamp, M., Humbert, A., and Ganopolski, A.: Simulation of the future sea level contribution of Greenland with a new glacial system model, The Cryosphere, 12, 3097-3121, https://doi.org/10.5194/tc-12-3097-2018, 2018.

https://www.the-cryosphere.net/12/3097/2018/tc-12-3097-2018.pdf

Abstract
We introduce the coupled model of the Greenland glacial system IGLOO 1.0, including the polythermal ice sheet model SICOPOLIS (version 3.3) with hybrid dynamics, the model of basal hydrology HYDRO and a parameterization of submarine melt for marine-terminated outlet glaciers. The aim of this glacial system model is to gain a better understanding of the processes important for the future contribution of the Greenland ice sheet to sea level rise under future climate change scenarios. The ice sheet is initialized via a relaxation towards observed surface elevation, imposing the palaeo-surface temperature over the last glacial cycle. As a present-day reference, we use the 1961–1990 standard climatology derived from simulations of the regional atmosphere model MAR with ERA reanalysis boundary conditions. For the palaeo-part of the spin-up, we add the temperature anomaly derived from the GRIP ice core to the years 1961–1990 average surface temperature field. For our projections, we apply surface temperature and surface mass balance anomalies derived from RCP 4.5 and RCP 8.5 scenarios created by MAR with boundary conditions from simulations with three CMIP5 models. The hybrid ice sheet model is fully coupled with the model of basal hydrology. With this model and the MAR scenarios, we perform simulations to estimate the contribution of the Greenland ice sheet to future sea level rise until the end of the 21st and 23rd centuries. Further on, the impact of elevation–surface mass balance feedback, introduced via the MAR data, on future sea level rise is inspected. In our projections, we found the Greenland ice sheet to contribute between 1.9 and 13.0 cm to global sea level rise until the year 2100 and between 3.5 and 76.4 cm until the year 2300, including our simulated additional sea level rise due to elevation–surface mass balance feedback. Translated into additional sea level rise, the strength of this feedback in the year 2100 varies from 0.4 to 1.7 cm, and in the year 2300 it ranges from 1.7 to 21.8 cm. Additionally, taking the Helheim and Store glaciers as examples, we investigate the role of ocean warming and surface runoff change for the melting of outlet glaciers. It shows that ocean temperature and subglacial discharge are about equally important for the melting of the examined outlet glaciers.

Caption for the first image: "Figure 10. Contribution of the Greenland ice sheet to future sea level rise under MAR forcing for different scenarios. Sea level rise is referenced to the year 2000. Beyond 2100, the forcings of the projections are from prolongations of the original MAR data (see main text for details). This is indicated by the vertical grey line at the year 2100 in panels (b) and (d). RCP 4.5 projections: (a) years 2000–2100 and (b) years 2000–2300. RCP 8.5 projections: (c) years 2000–2100 and (d) years 2000–2300. The different CMIP5 general circulation models utilized by MAR are indicated by colours. Different line characteristics specify optimal simulations with (solid) and without (long dashed) elevation correction for the SMB. The grey curves in panels (a) to (d) indicate a control simulation with solely the implied SMB (iSMB) as forcing. All simulations are with hybrid ice dynamics and HYDRO basal hydrology.
&

Furthermore, meltwater in the Greenland Ice Sheet deep percolation zone could have a significant impact on mass loss from Greenland by 2040.  The second linked reference discusses a new field method for better monitoring the accumulation of meltwater in this zone.  Obviously, ice mass loss from Greenland impacts ice mass loss from Antarctica via the bipolar seesaw mechanism:

Heilig, A., Eisen, O., MacFerrin, M., Tedesco, M., and Fettweis, X.: Seasonal monitoring of melt and accumulation within the deep percolation zone of the Greenland Ice Sheet and comparison with simulations of regional climate modeling, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-277, in review, 2018.

https://www.the-cryosphere-discuss.net/tc-2017-277/

Abstract. Increasing melt over the Greenland ice sheet (GrIS) recorded over the past years has resulted in significant changes of the percolation regime of the ice sheet. It remains unclear whether Greenland's percolation zone will act as meltwater buffer in the near future through gradually filling all pore space or if near-surface refreezing causes the formation of impermeable layers, which provoke lateral runoff. Homogeneous ice layers within perennial firn, as well as near-surface ice layers of several meter thickness are observable in firn cores. Because firn coring is a destructive method, deriving stratigraphic changes in firn and allocation of summer melt events is challenging. To overcome this deficit and provide continuous data for model evaluations on snow and firn density, temporal changes in liquid water content and depths of water infiltration, we installed an upward-looking radar system (upGPR) 3.4 m below the snow surface in May 2016 close to Camp Raven (66.4779° N/46.2856° W) at 2120 m a.s.l. The radar is capable to monitor quasi-continuously changes in snow and firn stratigraphy, which occur above the antennas. For summer 2016, we observed four major melt events, which routed liquid water into various depths beneath the surface. The last event in mid-August resulted in the deepest percolation down to about 2.3 m beneath the surface. Comparisons with simulations from the regional climate model MAR are in very good agreement in terms of seasonal changes in accumulation and timing of onset of melt. However, neither bulk density of near-surface layers nor the amounts of liquid water and percolation depths predicted by MAR correspond with upGPR data. Radar data and records of a nearby thermistor string, in contrast, matched very well, for both, timing and depth of temperature changes and observed water percolations. All four melt events transferred a cumulative mass of 56 kg/m2 into firn beneath the summer surface of 2015. We find that continuous observations of liquid water content, percolation depths and rates for the seasonal mass fluxes are sufficiently accurate to provide valuable information for validation of model approaches and help to develop a better understanding of liquid water retention and percolation in perennial firn.
&

It is imaginable that an acceleration of ice mass loss from the GIS circa 2040 might possibly trigger an eruption of Mt Takahe in the Byrd Subglacial Basin, BSB (see the second attached image), which could accelerate ice mass loss in Antarctica just as happen about 17.7 kya as the linked article entitled: "Massive Antarctic volcanic eruptions linked to abrupt Southern hemisphere climate changes", cites that abrupt climate change was associated with a series of halogen rich eruptions from Mt Takahe in the BSB.  Who knows what lies in mankind's future should a collapse of the WAIS this century should trigger similar volcanic eruptions in West Antarctica:

https://phys.org/news/2017-09-massive-antarctic-volcanic-eruptions-linked.html

Extract: ""Detailed chemical measurements in Antarctic ice cores show that massive, halogen-rich eruptions from the West Antarctic Mt. Takahe volcano coincided exactly with the onset of the most rapid, widespread climate change in the Southern Hemisphere during the end of the last ice age and the start of increasing global greenhouse gas concentrations," according to McConnell, who leads DRI's ultra-trace chemical ice core analytical laboratory.

Climate changes that began ~17,700 years ago included a sudden poleward shift in westerly winds encircling Antarctica with corresponding changes in sea ice extent, ocean circulation, and ventilation of the deep ocean. Evidence of these changes is found in many parts of the Southern Hemisphere and in different paleoclimate archives, but what prompted these changes has remained largely unexplained.

"We know that rapid climate change at this time was primed by changes in solar insolation and the Northern Hemisphere ice sheets," explained McConnell. "Glacial and interglacial cycles are driven by the sun and Earth orbital parameters that impact solar insolation (intensity of the sun's rays) as well as by changes in the continental ice sheets and greenhouse gas concentrations."
"We postulate that these halogen-rich eruptions created a stratospheric ozone hole over Antarctica that, analogous to the modern ozone hole, led to large-scale changes in atmospheric circulation and hydroclimate throughout the Southern Hemisphere," he added. "Although the climate system already was primed for the switch, we argue that these changes initiated the shift from a largely glacial to a largely interglacial climate state. The probability that this was just a coincidence is negligible."

See also the subject reference:

Joseph R. McConnell el al., "Synchronous volcanic eruptions and abrupt climate change ∼17.7 ka plausibly linked by stratospheric ozone depletion," PNAS (2017). https://doi.org/10.1073/pnas.1705595114 

www.pnas.org/cgi/doi/10.1073/pnas.1705595114

Extract: "Glacial-state greenhouse gas concentrations and Southern Hemisphere climate conditions persisted until ∼17.7 ka, when a nearly synchronous acceleration in deglaciation was recorded in paleoclimate proxies in large parts of the Southern Hemisphere, with many changes ascribed to a sudden poleward shift in the Southern Hemisphere westerlies and subsequent climate impacts. We used high-resolution chemical measurements in the West Antarctic Ice Sheet Divide, Byrd, and other ice cores to document a unique, ∼192-y series of halogen-rich volcanic eruptions exactly at the start of accelerated deglaciation, with tephra identifying the nearby Mount Takahe volcano as the source. Extensive fallout from these massive eruptions has been found >2,800 km from Mount Takahe. Sulfur isotope anomalies and marked decreases in ice core bromine consistent with increased surface UV radiation indicate that the eruptions led to stratospheric ozone depletion. Rather than a highly improbable coincidence, circulation and climate changes extending from the Antarctic Peninsula to the subtropics—similar to those associated with modern stratospheric ozone depletion over Antarctica—plausibly link the Mount Takahe eruptions to the onset of accelerated Southern Hemisphere deglaciation ∼17.7 ka."

Extract: "Previous studies (e.g., ref. 42) suggested that rising insolation initiated melting of Northern Hemisphere (NH) ice sheets at 19 ka, which triggered a reduction in the strength of the Atlantic overturning circulation, and, through the bipolar seesaw, resulted in SH warming and CO2 release from the Southern Ocean, although the exact mechanisms driving the CO2 release are still debated. We postulate that the ∼192-y series of halogen-rich eruptions of Mount Takahe and the subsequent ozone hole (26) initiated a series of events analogous to the modern ozone hole that acted to accelerate deglaciation at 17.7 ka. First, stratospheric ozone depletion changed SH atmospheric circulation, resulting in a rapid increase and poleward shift in the westerlies (35) (SI Appendix, Fig. S7). Second, consequent widespread perturbations in SH hydrometeorology, including increased austral summer subtropical precipitation between ∼15°S and ∼35°S (Figs. 1F and 5), led to enhanced CH4 wetland emissions (43)."

Caption for the second image: "Spatial extent of the glaciochemical anomaly. Evidence of the ∼192-y anomaly has been found >2,800 km from Mount Takahe in ice core (circles) chemical records (SI Appendix, Fig. S3) as well as radar surveys from much of West Antarctica. Also shown are area volcanoes (triangles). September/October horizontal wind vectors at 600 hPa based on 1981–2010 National Centers for Environmental Prediction reanalysis fields show transport patterns consistent with observations."
&

Furthermore, via the bipolar seesaw mechanism and a rapid freshening of the Southern Ocean (due to the initial collapse of the WAIS after 2040) could be rapidly telecommunicated back to the North Hemisphere [see Turney et al (2017)] where it might trigger rapid ice mass loss from the Northeast Greenland Ice Stream (NEGIS) as occurred from 7.8 to 1.2 kya [see Larsen et al (2018) and the third attached image]:

Turney, et al. (2017), "Rapid global ocean-atmosphere response to Southern Ocean freshening during the last glacial", Nature Communications 8, Article No. 520, https://doi.org/10.1038/s41467-017-00577-6

https://www.nature.com/articles/s41467-017-00577-6

Extract: "An ensemble of transient meltwater simulations show that Antarctic-sourced salinity anomalies can generate climate changes that are propagated globally via an atmospheric Rossby wave train."
&

Nicolaj K. Larsen et al. (14 May 2018), "Instability of the Northeast Greenland Ice Stream over the last 45,000 years", Nature Communications, Volume 9, Article number: 1872, doi:10.1038/s41467-018-04312-7

http://www.nature.com/articles/s41467-018-04312-7

Abstract: "The sensitivity of the Northeast Greenland Ice Stream (NEGIS) to prolonged warm periods is largely unknown and geological records documenting such long-term changes are needed to place current observations in perspective. Here we use cosmogenic surface exposure and radiocarbon ages to determine the magnitude of NEGIS margin fluctuations over the last 45 kyr (thousand years). We find that the NEGIS experienced slow early Holocene ice-margin retreat of 30–40 m a−1, likely as a result of the buttressing effect of sea-ice or shelf-ice. The NEGIS was ~20–70 km behind its present ice-extent ~41–26 ka and ~7.8–1.2 ka; both periods of high orbital precession index and/or summer temperatures within the projected warming for the end of this century. We show that the NEGIS was smaller than present for approximately half of the last ~45 kyr and is susceptible to subtle changes in climate, which has implications for future stability of this ice stream."


See also: "Antarctica: What Would Happen if All the Volcanoes Buried Beneath the Ice Erupted?"

http://www.newsweek.com/antarctica-subglacial-volcanoes-eruption-risk-sea-ice-659537

[AbruptSLR]

Re: ITCZ, question for abruptSLR

in the ACME modelling efforts, have they fixed the double ITCZ problem that most GCMs have ?

sidd

That last time I looked it was still there, but who knows what changes that they are continually making as they transform ACME into E3SM.

Edit: You can monitor the progress being made on the E3SM model at:

https://esgf-node.llnl.gov/projects/e3sm/
&
https://e3sm.org/about/events/all-hands-presentations/

I note that the CMIP6 model output is starting to come out and can be accessed at the following links:

https://esgf-node.llnl.gov/projects/cmip6/
&
https://esgf-node.llnl.gov/search/cmip6/

If David Pollard has the time and money available, maybe he should take the output (maybe from the E3SM output) from one of these models with interactive ice sheets (and which thus theoretically should include the influence of associated freshwater hosing) circa 2040 and use it as input to their AIS model with ice-cliff failure mechanisms and hydrofracturing and see what happens by 2100.  Such runs may inspire CMIP7 models to also incorporate ice-cliff failure mechanisms and hydrofracturing.

[AbruptSLR]

With West Antarctic ice shelves progressively becoming more fragile with continuing warming; and with storm activity trending higher (see the attached image); who knows what future damage both storm surge and long period gravity waves will do to these ice shelves (see the following two references):

ZHAO CHEN et al. (2018), "Ocean-excited plate waves in the Ross and Pine Island Glacier ice shelves", Journal of Glaciology, Volume 64, Issue 247, pp. 730-744 https://doi.org/10.1017/jog.2018.66

https://www.cambridge.org/core/journals/journal-of-glaciology/article/oceanexcited-plate-waves-in-the-ross-and-pine-island-glacier-ice-shelves/B7339CE9FD25772A9157259076623C5E

Abstract: "Ice shelves play an important role in buttressing land ice from reaching the sea, thus restraining the rate of grounded ice loss. Long-period gravity-wave impacts excite vibrations in ice shelves that can expand pre-existing fractures and trigger iceberg calving. To investigate the spatial amplitude variability and propagation characteristics of these vibrations, a 34-station broadband seismic array was deployed on the Ross Ice Shelf (RIS) from November 2014 to November 2016. Two types of ice-shelf plate waves were identified with beamforming: flexural-gravity waves and extensional Lamb waves. Below 20 mHz, flexural-gravity waves dominate coherent signals across the array and propagate landward from the ice front at close to shallow-water gravity-wave speeds (~70 m s−1). In the 20–100 mHz band, extensional Lamb waves dominate and propagate at phase speeds ~3 km s−1. Flexural-gravity and extensional Lamb waves were also observed by a 5-station broadband seismic array deployed on the Pine Island Glacier (PIG) ice shelf from January 2012 to December 2013, with flexural wave energy, also detected at the PIG in the 20–100 mHz band. Considering the ubiquitous presence of storm activity in the Southern Ocean and the similar observations at both the RIS and the PIG ice shelves, it is likely that most, if not all, West Antarctic ice shelves are subjected to similar gravity-wave excitation."

See also:

P. D. Bromirski et al. (16 June 2017), "Tsunami and infragravity waves impacting Antarctic ice shelves", JGR Oceans, https://doi.org/10.1002/2017JC012913

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2017JC012913

Abstract
The responses of the Ross Ice Shelf (RIS) to the 16 September 2015 8.3 (Mw) Chilean earthquake tsunami (>75 s period) and to oceanic infragravity (IG) waves (50–300 s period) were recorded by a broadband seismic array deployed on the RIS from November 2014 to November 2016. Here we show that tsunami and IG‐generated signals within the RIS propagate at gravity wave speeds (∼70 m/s) as water‐ice coupled flexural‐gravity waves. IG band signals show measureable attenuation away from the shelf front. The response of the RIS to Chilean tsunami arrivals is compared with modeled tsunami forcing to assess ice shelf flexural‐gravity wave excitation by very long period (VLP; >300 s) gravity waves. Displacements across the RIS are affected by gravity wave incident direction, bathymetry under and north of the shelf, and water layer and ice shelf thicknesses. Horizontal displacements are typically about 10 times larger than vertical displacements, producing dynamical extensional motions that may facilitate expansion of existing fractures. VLP excitation is continuously observed throughout the year, with horizontal displacements highest during the austral winter with amplitudes exceeding 20 cm. Because VLP flexural‐gravity waves exhibit no discernable attenuation, this energy must propagate to the grounding zone. Both IG and VLP band flexural‐gravity waves excite mechanical perturbations of the RIS that likely promote tabular iceberg calving, consequently affecting ice shelf evolution. Understanding these ocean‐excited mechanical interactions is important to determine their effect on ice shelf stability to reduce uncertainty in the magnitude and rate of global sea level rise.

[AbruptSLR]

With sufficient global warming who knows what extinctions are in the Earth's future (in the next few centuries):

J.L. Penn el al. (2018), "Temperature-dependent hypoxia explains biogeography and severity of end-Permian marine mass extinction", Science, Vol. 362, Issue 6419, eaat1327, DOI: 10.1126/science.aat1327

http://science.sciencemag.org/content/362/6419/eaat1327

Abstract: "Rapid climate change at the end of the Permian Period (~252 million years ago) is the hypothesized trigger for the largest mass extinction in Earth’s history. We present model simulations of the Permian/Triassic climate transition that reproduce the ocean warming and oxygen (O2) loss indicated by the geologic record. The effect of these changes on animal survival is evaluated using the Metabolic Index (Φ), a measure of scope for aerobic activity governed by organismal traits sampled in diverse modern species. Modeled loss of aerobic habitat predicts lower extinction intensity in the tropics, a pattern confirmed with a spatially explicit analysis of the marine fossil record. The combined physiological stresses of ocean warming and O2 loss can account for more than half the magnitude of the “Great Dying.”"
See also:

Title: "Biggest mass extinction caused by global warming leaving ocean animals gasping for breath"

https://phys.org/news/2018-12-biggest-mass-extinction-global-ocean.html

Extract: ""Under a business-as-usual emissions scenarios, by 2100 warming in the upper ocean will have approached 20 percent of warming in the late Permian, and by the year 2300 it will reach between 35 and 50 percent," Penn said. "This study highlights the potential for a mass extinction arising from a similar mechanism under anthropogenic climate change." "

[sidd]

Re: ITCZ

thanks for recalling the pointer to ACME/ESM.

Re: GIS

I think is is becoming clear that melt will overwhelm firn retention and will run to the ocean. Watching melt lakes climb as ELA climbs is disquieting. I see that meltlakes are close to the top of 67N saddle now. What is worse is the huge latent heat the water sheds to ice upon any refreeze. That raises ice temperature and Glen's law exponent kills you with warmer ice speeding out the big tidewater outlets. 79N is still remarkably quiescent in spite of substantial melt lakes over NEGIS.

But perhaps this discussion should move to one of the Greenland threads.

sidd

[AbruptSLR]

...
 79N is still remarkably quiescent in spite of substantial melt lakes over NEGIS.

But perhaps this discussion should move to one of the Greenland threads.

sidd

In my opinion a possible Ice Apocalypse (named by prokaryotes) includes all aspects of the multiple ice-climate feedback mechanisms, including those dealing with Greenland including: its bipolar interaction with Antarctica, its slowing of the oceans overturning current and its feedback with Arctic Amplification (etc.)

[AbruptSLR]

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

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

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

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

[AbruptSLR]

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

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

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

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

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

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

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

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

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

Extract: "…  we monitored the surface features of the ice rumple using Landsat 7 ETM+ images from 2003–2014 (Figure 7). The images in this time series showed a gradual dissipation of the ice rumple, strongly indicating continuous thinning of the Thwaites ice shelf. Furthermore, the disappearance of surface features (e.g., crevasses and surface gradient) from 2013 onwards suggests that the ice shelf has been ungrounded, removing the pressure point that had been maintaining the ice rumple. Nonetheless, the ice shelf might have been in contact with the pinning point even after the disappearance of surface features, as intermittent ice contact to the pinning point could be possible due to ice shelf thickness fluctuations [29].
…
According to the optimization result, the ice thickness was 711.64 ± 14.25 m and 683.76 ± 12.48 m in 2011 and 2012, respectively. This is quite different from the known ice thickness of the Thwaites ice shelf near the grounding line (~1 km), despite the low vertical resolution of the radar sounder used for such measurements [7]. This indicates a thickness decrease of 36.17 ± 17.27 m during that one-year period. However, as the center of the pressure source in 2012 was located 341.96 m upstream and 210.10 m to the west from that in 2011, it is difficult to substantiate the thickness and thickness change with the obtained datasets. Our results for the surface depression and thinning of the ice rumple in the Thwaites ice shelf were much higher than that previously reported. It is rather surprising to observe such high (>10 m) surface depressions in an ice shelf in such a short time; Rignot et al. [30] and Paolo et al. [14] reported that the thinning rate of the Thwaites ice shelf was 6.13 and 2.80 m/year, respectively. …
From 2011–2013, our deformation maps showed the recent fading of a small ice rumple in the surface of the Thwaites ice shelf, West Antarctica. The pinning point was located nearly 5 km offshore from the previously estimated grounding line in 2011, and appeared sometime between 1996 and 2011 when the grounding line of Thwaites Glacier retreated. The deformation pattern we found, along with a time series of Landsat 7 ETM+ imagery, showed that the ice was still in contact with the basal topography as late as 2013 but is likely to have since been unpinned. We then used the deformation maps with the simple viscoelastic deformation model (widely used in volcanic studies) to interpret the surface changes in terms of pressure changes at the bottom of the ice shelf by applying an idealized spherical pressure source. The estimated numbers were reasonable and the ice shelf thickness at the center of the spherical pressure source was also estimated using the depth and radius. The surface depression and thinning of this ice rumple were found to be much higher than those of previously reported levels for the broader region, …"

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

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

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

[AbruptSLR]

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

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

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


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

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

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

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

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

[AbruptSLR]

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

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

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

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

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

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

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

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

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

[sidd]

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

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

sidd

[SteveMDFP]

I thought this was a rather good popular press article about Thwaites, who studies it and how, and potential risks for civilization:

THE RACE TO UNDERSTAND ANTARCTICA’S MOST TERRIFYING GLACIER
https://www.wired.com/story/antarctica-thwaites-glacier-breaking-point/

It actually explains matters such as the implications of a retrograde glacier bed geometry, ice-cliff failure, and the proposal to construct an artificial sill, among other concepts that have been raised here.

[AbruptSLR]

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

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

sidd

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

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

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

[AbruptSLR]

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

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

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

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

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

[AbruptSLR]

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

Title: "The International Thwaites Glacier Collaboration"

https://thwaitesglacier.org/projects

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

[AbruptSLR]

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

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

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

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

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

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

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

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

[gerontocrat]

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

There is a consistent narrative -

AGW is causing land-based ice to melt faster, adding massive additional amounts of freshwater to the ocean surface.

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

BUT since then the opposite. Antarctic sea ice extent is in decline, not just at max and min but during the melt season. Temporary aberration? Or is something  extra going on?

[AbruptSLR]

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

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

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

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

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

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

[AbruptSLR]

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

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

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

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

[AbruptSLR]

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

BUT since then the opposite. Antarctic sea ice extent is in decline, not just at max and min but during the melt season. Temporary aberration? Or is something  extra going on?

The ice-climate feedback mechanism is not dependent on the presences of Antarctic sea ice, as the linked reference and associated image show that this feedback mechanism works with low salinity surface water (and/or sea ice) along the Antarctic coastline:

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

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

[wdmn]

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

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

Abstract
Past Earth system states offer possible model systems for the warming world of the coming decades. These include the climate states of the Early Eocene (ca. 50 Ma), the Mid-Pliocene (3.3–3.0 Ma), the Last Interglacial (129–116 ka), the Mid-Holocene (6 ka), preindustrial (ca. 1850 CE), and the 20th century. Here, we quantitatively assess the similarity of future projected climate states to these six geohistorical benchmarks using simulations from the Hadley Centre Coupled Model Version 3 (HadCM3), the Goddard Institute for Space Studies Model E2-R (GISS), and the Community Climate System Model, Versions 3 and 4 (CCSM) Earth system models. Under the Representative Concentration Pathway 8.5 (RCP8.5) emission scenario, by 2030 CE, future climates most closely resemble Mid-Pliocene climates, and by 2150 CE, they most closely resemble Eocene climates. Under RCP4.5, climate stabilizes at Pliocene-like conditions by 2040 CE. Pliocene-like and Eocene-like climates emerge first in continental interiors and then expand outward. Geologically novel climates are uncommon in RCP4.5 (<1%) but reach 8.7% of the globe under RCP8.5, characterized by high temperatures and precipitation. Hence, RCP4.5 is roughly equivalent to stabilizing at Pliocene-like climates, while unmitigated emission trajectories, such as RCP8.5, are similar to reversing millions of years of long-term cooling on the scale of a few human generations. Both the emergence of geologically novel climates and the rapid reversion to Eocene-like climates may be outside the range of evolutionary adaptive capacity.

graph source: Bob Kopp @bobkopp

[wdmn]

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

East Antarctica has the potential to reshape coastlines around the world through sea level rise, but scientists have long considered it more stable than its neighbor, West Antarctica. Now, new detailed NASA maps of ice velocity and elevation show that a group of glaciers spanning one-eighth of East Antarctica’s coast have begun to lose ice over the past decade, hinting at widespread changes in the ocean.

[AbruptSLR]

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

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

wdmn,

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

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

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

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

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

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

Best,
ASLR

[AbruptSLR]

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

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

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

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

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

Website title: "Polar Oceanography"

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

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

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

[AbruptSLR]

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

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

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

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

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

[AbruptSLR]

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

wdmn,

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Best,
ASLR

[AbruptSLR]

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

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

2. The location of the Totten Basin.

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

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

[AbruptSLR]

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

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

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

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

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

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

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

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

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

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

[wdmn]

Many thanks ASLR. I have to read your posts very slowly and several times to begin to understand. The amount of work you put into them is greatly appreciated.

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

[AbruptSLR]

...

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

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

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

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

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

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

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

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

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

[RoxTheGeologist]

Excellent graphs, thanks ASLR.

[AbruptSLR]

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

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

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

[AbruptSLR]

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

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

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

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

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

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

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

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

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

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

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

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

[AbruptSLR]

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

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

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

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

[AbruptSLR]

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

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

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

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

Extract: "During the early Pliocene Epoch, 3–5 Ma, the climate of the Arctic region of Earth changed abruptly from ice-free to ice-capped. The climate forcing factors then (solar constant, orbital parameters, CO2 concentration and locations of the continents) were all very similar to today. Therefore, it is difficult to explain why the early Pliocene climate was so different from that of today. That problem is known as the Pliocene Paradox, (Cronin (2010); Fedorov et al. (2006, 2010)). This paper presents a plausible explanation of the Pliocene paradox.
…
As stated above, a key feature of this family of mathematical models is that they incorporate physical principles that are nonlinear. As is well known, nonlinear equations can have multiple solutions, unlike linear equations which can have only one unique solution (if well-posed). In our mathematical models, the same set of equations can have two or more co-existing solutions, for example an ice-capped solution (like today’s climate) and an ice-free solution (like the Cretaceous climate), even with the same values of the forcing parameters. The determination of which solution is actually realized by the planet at a given time is dependent on past history. Changes in forcing parameters may drive the system abruptly from one stable state to another, at so-called “tipping points”. In this paper, these tipping points are investigated mathematically, and are shown to be bifurcation points, which can be investigated using mathematical bifurcation theory. Bifurcation theory tells us that the existence of bifurcation points is preserved (but the numerical values may change) under small deformations of the model equations. Thus, even though this conceptual model may not give us precise quantitative information about climate changes, qualitatively there is good reason to believe that the existence of the bifurcation points in the model will be preserved in similar more refined models and in the real world.
…
The change from ice-free to ice-covered in the Arctic occurred abruptly, during the Pliocene Epoch, 5.3 to 2.6 Ma. It has been a longstanding challenge for paleoclimatologists to explain this dramatic change in the climate.

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

Another interesting paradox concerning Polar glaciation is the fact that, although both poles have transitioned abruptly from ice-free to ice-covered, they did so at very different geological times. The climate forcing conditions of Earth are highly symmetric between the two hemispheres and for most of the history of Earth the climates of the two poles have been very similar. However, there was an anomalous period of about 30 million years, from the Eocene-Oligocene boundary (34 Ma) to the early Pliocene (4 Ma), when the Antarctic was largely ice-covered but the Arctic was ice-free.
…
Thus, the EBM presented here, as illustrated in Figure 7, provides a plausible explanation for the Pliocene paradox. The slowly-acting physical forcings of decreasing CO2 concentration and decreasing ocean heat transport FO were amplified by the mechanisms of ice-albedo feedback and water vapour feedback, both of which act very strongly when the temperature crosses the freezing point of water. For millions of years before the Pliocene, while the Arctic temperature remained well above freezing, the climate changed very little. However, once the freezing temperature was reached, the Arctic climate changed abruptly via a saddlenode bifurcation as in Figure 7 b), to a new frozen state. This simple mechanism suffices to explain the Pliocene paradox. No more complicated explanations are necessary.
…
Several other explanations have been proposed for the Pliocene paradox. There is convincing evidence that, at the beginning of the Pliocene, there was a permanent El Niño condition in the tropical Pacific ocean, see Cronin (2010); Fedorov et al. (2006, 2010). (However, some have disputed this finding, see Watanabe et al. (2011).) It has been suggested that a permanent El Niño condition could explain the warm early Pliocene, and that the onset of the El Niño – La Niña Southern Oscillation (ENSO) was the cause of sudden cooling of the Arctic during the Pliocene. Today, it is known that ENSO can influence weather patterns as far away as the Arctic.

Another suggestion is that Hadley cell feedback contributed to the abrupt cooling of the Arctic during the Pliocene. Recent work shows that an increase in pole-to-equator temperature gradient causes the Hadley cells to contract towards the equator, while increasing in circulation velocity, see Lewis and Langford (2008); Langford and Lewis (2009). This would cause a decrease in equator to pole atmospheric heat transport, which would in turn accelerate Arctic cooling; this is called Hadley cell feedback.  Further work on modelling this mechanism is in progress. It is conjectured here that Hadley cell feedback may in fact have caused the end of a permanent El Niño condition in the Pliocene, as follows. It is known that the La Niña phase of ENSO is forced in part by the Trade Winds blowing East to West across the tropical Pacific Ocean. The Trade Winds are the surface component of the Hadley circulation. Therefore, acceleration of the Hadley circulation would strengthen the Trade Winds, enhancing the conditions for La Niña and ending the permanent El Niño. Further work on this conjecture also is in progress.
…
In the Tropics, many of the values of the forcing parameters are different from their values in the Arctic and Antarctic, see Table 2. The geological record shows little change in the tropical climate over the past 100 million years, other than a little cooling. Even when Arctic climate changed dramatically in the Pliocene, the Tropical climate changed very little.

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


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

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

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

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

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