As ice sheet models are not likely to be able to reasonably replicate the key mechanisms that will likely contribute to a possible collapse of the WAIS this century, until (at the earliest) possibly the end of the DOE's Accelerated Climate Model Energy, ACME, program in 2024; paleo-evidence remains prominent in guiding our climate change policy. In this regard, Hansen et al 2015 have performed an excellent service by providing extensive paleo discussion, much of which is directed towards the North Atlantic and Greenland, which I will let other discuss; as in this post I focus primarily on the late Eemain (late MIS 5e) event driven by Antarctic ice mass loss, when sea level rose to about 9m above present beginning at: "119 ky b2k and peak sea level at 118.1 +/- 1.4 ky b2k" (see the first image from O'Leary et al 2013) and the following extract from page 20066 of Hansen et al 2015:
Extract: "O’Leary et al. (2013) provide a new perspective on Eemian sea level change using over 100 well-dated U-series coral reefs at 28 sites along the 1400 km west coast of Australia and incorporating GIA corrections on regional sea level. In agreement with Hearty et al. (2007), their analyses suggest that sea level was relatively stable at 3–4m in most of the Eemian, followed by a rapid (<1000 yr) late-Eemian sea level rise to about +9 m. U-series dating of the corals has the sea level rise begin at 119 ky b2k and peak sea level at 118.1_1.4 ky b2k. This dating of peak sea level is consistent with the estimate of Hearty and Neumann (2001) of _118 ky b2k as the time of rapid climate changes and extreme storminess.
End-Eemian sea level rise would seem to be a paradox, because orbital forcing then favored growth of Northern Hemisphere ice sheets. We will find evidence, however, that the sea level rise and increased storminess are consistent, and likely related to events in the Southern Ocean."
The second image shows that in the 118 +/- 5 ky b2k time frame the spring and summer insolation anom. north of 60oN was negative while south of 60oS it was positive; which indicates that this abrupt SLR event is primarily associate with a partial collapse of the AIS as indicated by the following extract from page 20074 to 20075 of Hansen et al 2015:
Extract: "Late Eemian sea level rise is seemingly a paradox, because glacial-interglacial sea level change is mainly a result of the growth and decay of Northern Hemisphere ice sheets. Northern warm-season insolation anomalies were negative and declining in the latter part of the Eemian (Fig. 3a), so Northern Hemisphere ice sheets should have been growing. We suggest that the explanation for a mid-Eemian sea level minimum is a substantial late-Eemian collapse of the Antarctic ice sheet facilitated by the positive warm-season insolation anomaly on Antarctica and the Southern Ocean during the late Eemian (Fig. 3b).
Persuasive presentation of this interpretation requires analysis of relevant climate mechanisms with a global model as well as a detailed discussion of paleoclimate data. We will show that these analyses in turn help to explain ongoing climate change today, with implications for continuing climate change this century."
The following extract from page 20107-20108 of Hansen et al 2015, cites still more paleo evidence that the SLR surge at the late-Eemian was driven by AIS ice mass loss, particularly in the WAIS, and calls for "… higher resolution models with more realistic sea ice distribution and seasonal change than our present model produces
Extract: "We suggest that the Southern Hemisphere was the source for brief late-Eemian sea level rise. The positive warm-season insolation anomaly on the Southern Ocean and AMOC slowdown due to C26 added to Southern Ocean heat, causing ice shelf melt, ice sheet discharge, and sea level rise. Rapid Antarctica ice loss would cool the Southern Ocean and increase sea ice cover, which may have left telltale evidence in ice cores. Indeed, Masson-Delmotte et al. (2011) suggest that abrupt changes of _18O in the EDML and TALDICE ice cores (those most proximal to the coast) indicate a change in moisture origin, likely due to increased sea ice. Further analysis of Antarctic data for the late Eemian might help pinpoint the melting and help assess vulnerability of Antarctic ice sheets to ocean warming, but this likely will require higher resolution models with more realistic sea ice distribution and seasonal change than our present model produces."
The third image provides an example of the type of global relative SLR pattern that occurs due to the collapse of the WAIS, which must be applied both to correctly interpret paleo sea level data and also future sea level projections.
The following extract from page 20113 to 20114 of Hansen et al 2015 and the fourth image discuss/show both observed and projected trends in (a) the Southern Merdional Overturning Circulation (in Sverdrups) and (b) the Southern Hemisphere mean sea ice area anomaly. This comparison shows both the importance of the recent freshwater influx into the Southern Ocean both on the SMOC and the SH ice area anom. that support Hansen et al 2015 main warning that current GCM projections are underestimating the possibly profound importance of ASLR on both further planetary energy imbalance and on future storminess.
Extract: "The Weddell and Ross Sea regions have large freshwater flux that is mainly icebergs. In contrast, the large Amundsen- Bellingshausen fresh water flux is mainly basal melt. This distinctive spatial variation may help account for observed sea ice increasing in the Weddell and Ross Seas, while decreasing in the Amundsen and Bellingshausen Seas. Note also that the Weddell Sea and Ross Sea sectors are respectively the regions where the EDML and TALDICE Antarctic ice cores are suggestive of expanding sea ice (Masson-Delmotte et al., 2011) at end-Eemian time."
The Hansen et al 2015 conclusion on page 20121 includes the following extract emphasis that the observed impacts of the freshening of the Southern Ocean are occurring even faster than their own model projects (which indicates that their findings may be conservative in a scientific sense):
Extract: "The Eemian, less than 2 oC warmer than pre-industrial Earth, itself provides a clear indication of the danger, even though the orbital drive for Eemian warming differed from today’s human-made climate forcing. Ongoing changes in the Southern Ocean, while global warming is less than 1 oC, provide a strong warning, as observed changes tend to confirm the mechanisms amplifying change. Predicted effects, such as cooling of the surface ocean around Antarctica, are occurring even faster than modeled."
Furthermore, I not that in the following Pollard, DeConto & Alley (2015) reference the authors show that without the inclusion of their postulated hydrofracturing and ice cliff failure mechanism, it is impossible to replicate the observed paleo SLR evidence from the Paleocene (including the Eemian)
Pollard, D., R.M. DeConto and R.B. Alley (2015) "Potential Antarctic Ice Sheet retreat driven by hydrofracturing and ice cliff failure", Earth Plan. Sci. Lett., 412, 112-121
Finally, I re-post the following reference by Alley et al (2015) that examines paleo-evidence (focused on WAIS behavior) to concluded that: " Although sea-level histories and physical understanding allow the possibility that ice-sheet response could be quite fast, no strong constraints are yet available on the worst-case scenario. Recent work also suggests that the Greenland and East Antarctic Ice Sheets share some of the same vulnerabilities to shrinkage from marine influence."
Alley, R.B., S. Anandakrishnan. K. Christianson, H.J. Horgan, A. Muto, B.R. Parizek, D. Pollard and R.T. Walker (2015) "Oceanic forcing of ice-sheet retreat: West Antarctica and more", Ann. Rev. Earth Plan. Sci., 43, 7.1-7.25.http://www.annualreviews.org/doi/abs/10.1146/annurev-earth-060614-105344?journalCode=earth
Abstract: "Ocean-ice interactions have exerted primary control on the Antarctic Ice Sheet and parts of the Greenland Ice Sheet, and will continue to do so in the near future, especially through melting of ice shelves and calving cliffs. Retreat in response to increasing marine melting typically exhibits threshold behavior, with little change for forcing below the threshold but a rapid, possibly delayed shift to a reduced state once the threshold is exceeded. For Thwaites Glacier, West Antarctica, the threshold may already have been exceeded, although rapid change may be delayed by centuries, and the reduced state will likely involve loss of most of the West Antarctic Ice Sheet, causing >3 m of sea-level rise. Because of shortcomings in physical understanding and available data, uncertainty persists about this threshold and the subsequent rate of change. Although sea-level histories and physical understanding allow the possibility that ice-sheet response could be quite fast, no strong constraints are yet available on the worst-case scenario. Recent work also suggests that the Greenland and East Antarctic Ice Sheets share some of the same vulnerabilities to shrinkage from marine influence."