I note that for many years/decades many scientists have misinterpreted paleo-data, causing them to underestimate climate sensitivity, and we are currently forcing global warming at a rate over ten times that during the PETM; which is a clear example of human stupidity. Smart scientists like Hansen, DeConto and Pollard have used paleo-data to better calibrate their climate models to indicate the risk of abrupt climate change due to ice-climate interaction. The linked references below provide additional paleo-evidence of both high ice-climate feedback sensitivity and of high ECS values:
1) The first two linked articles appear in the May 28 2014 online version of Nature, about new paleo-evidence about how quickly the AIS can contribute to rapid SLR (including during Meltwater Pulse 1A):
Trevor Williams, (2014), "Climate science: How Antarctic ice retreats", Nature, doi:10.1038/nature13345http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13345.html
Summary: "New records of iceberg-rafted debris from the Scotia Sea reveal episodic retreat of the Antarctic Ice Sheet since the peak of the last glacial period, in step with changes in climate and global sea level."
M. E. Weber, P. U. Clark, G. Kuhn, A. Timmermann, D. Sprenk, R. Gladstone, X. Zhang, G. Lohmann, L. Menviel, M. O. Chikamoto, T. Friedrich & C. Ohlwein, (2014), "Millennial-scale variability in Antarctic ice-sheet discharge during the last deglaciation", Nature, (2014), doi:10.1038/nature13397http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13397.html
Extract: "Antarctica's melting glaciers launched so many icebergs into the ocean 14,600 years ago that sea level rose 6.5 feet (2 meters) in just 100 years, a new study reports. The results are the first direct evidence for dramatic melting in Antarctica's past — the same as predictions for its future.
"The Antarctic Ice Sheet had been considered to be fairly stable and kind of boring in how it retreated," said study co-author Peter Clark, a climate scientist at Oregon State University. "This shows the ice sheet is much more dynamic and episodic, and contributes to rapid sea-level rise.""
2) The following extract from the third linked article about the Weber et al (2014) paper, not only reinforces the importance of AIS SLR contribution to Meltwater Pulse 1A, but more importantly that the fresh melt water causes a stratification of ocean water with a cool surface and warmer deep waters that creates a positive feedback mechanism that accelerates the rate of grounding-line retreat of Antarctic marine glaciers, particularly like those in the ASE; which supports Hansen et al (2016)'s ice-climate feedback mechanism http://www.abc.net.au/science/articles/2014/05/29/4014978.htm
Extract: "Feedback system
Recent studies have shown that a significant amount of warming occurs directly from the ocean transferring heat to the ice shelves from underneath and causing melt.
"Our models indicate that when you add the fresh water, you initiate a positive feedback through subsurface ocean warming," says Menviel.
Fresh water from the Antarctic ice sheet melts into the Southern Ocean causing stratification of ocean water into separate layers, resulting in cool water on the surface, and warmer water deeper down which further erodes the icesheet.
"So what starts as a small melting can be amplified leading to more rapid melting than just through changes in atmospheric temperature," says Menviel."
3) The following fourth link leads to the University of Alaska Fairbank's website focused on Lake Elgygytgn research, and the extract following the link is from an article Posted on February 4th, 2014 by Laura Nielsen on "Inter-hemispheric climate coupling". The extract emphasizes that in the paleo-past the Antarctic generally responded more quickly to orbital induced solar insolation variations, and that repeatedly paleo-collapses of the WAIS resulted in subsequent Arctic amplification, due both to changes in ocean currents, and to increases in sea level pushing more warm Pacific water through the Bering St. into the Arctic Ocean. If the WAIS collapses this century, we may soon see a marked increase in Arctic amplification:http://frontierscientists.com/tag/lake-elgygytgyn/
Extract: "Antarctica and the Arctic
Climate at the North and South pole are connected. Sediment records from Antarctica show that the West Antarctic ice sheet melted at various times in history. Following many of those events, the Arctic warmed. These recurring intervals of paired warming show that climate in the two hemispheres is linked – it’s called inter-hemispheric climate coupling.
“When the West Antarctic ice sheet pulls back we see a corresponding warmth in the high lattitudes again, probably affecting the size of the Greenland ice sheet with major implications for changes in sea level,” says Julie Brigham-Grette. “Our results mesh with what glaciologists are seeing today. Seven of the 12 major ice shelves around the Antarctic are melting or are gone. We suspect the tipping point for the gradual de-glaciation of Greenland and the Arctic may be lower than glaciologists once thought.”
Earth is a complicated place. We can’t explain past warming using only orbital dynamics or levels of Carbon Dioxide. Scientists affiliated with the project outlined some past events that might explain the rapid warming the sediment records show occurred in both Antarctica and the Arctic around similar times.
When you imagine Antarctica, the picture includes large ice shelves that hang off the rocky edge of the ice-covered continent. Normally that ice keeps nearby ocean water very cold. The cold water travels along currents toward the north Pacific where it wells up to the surface. Ocean circulation can be affected, though. If Antarctic ice sheets disintegrate or melt away, they no longer enforce cold water currents that journey to the Arctic. Instead, surface ocean waters in the Arctic become warmer.
When Antarctica’s ice sheets disintegrate the ocean gains more water and sea levels rise globally. The Bering Strait usually restricts how much warm surface water approaches the Arctic from the south, but higher sea levels would mean warm surface water didn’t have to squeeze through such a narrow space, letting more warm water past the Bering Strait into the Arctic Ocean.
Either way, a warmer ocean means higher temperatures and more rainfall for the Arctic, which impacts paleoclimatology and sea ice history. Grasping the climate connections between the hemispheres gives us insight into our near future."
4) The fifth linked reference could not make it more clear that paleo-evidence from inter-glacial periods indicates that ECS is meaningfully higher than 3C and that climate models are commonly under predicting the magnitude of coming climate change.
Dana L. Royer (2016), "Climate Sensitivity in the Geologic Past", Annual Review of Earth and Planetary Sciences, Vol. 44 http://www.annualreviews.org/doi/abs/10.1146/annurev-earth-100815-024150?src=recsys
5) While the sixth linked (open access) reference has many appropriate qualifying statements and disclaimers, it notes that the AR5 paleo estimates of ECS were linear approximations that change when non-linear issues are considered. In particular the find for the specific ECS, S[CO2,LI], during the Pleistocence (ie the most recent 2 million years) that:
"During Pleistocene intermediate glaciated climates and interglacial periods, S[CO2,LI] is on average ~ 45 % larger than during Pleistocene full glacial conditions."
Therefore, researchers such as James Hansen who relied on paleo findings that during recent full glacial periods ECS was about 3.0C, did not know that during interglacial periods this value would be 45% larger, or 4.35C.
Köhler, P., de Boer, B., von der Heydt, A. S., Stap, L. B., and van de Wal, R. S. W. (2015), "On the state dependency of the equilibrium climate sensitivity during the last 5 million years", Clim. Past, 11, 1801-1823, doi:10.5194/cp-11-1801-2015.http://www.clim-past.net/11/1801/2015/cp-11-1801-2015.htmlhttp://www.clim-past.net/11/1801/2015/cp-11-1801-2015.pdf