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Sunday, January 23, 2011

Must-read Hansen and Sato draft paper: We are at a climate tipping point that, once crossed, enables multi-meter sea level rise this century

Climate change is likely to be the predominant scientific, economic, political and moral issue of the 21st century
by Joseph Romm, Climate Progress, January 20, 2011

Right now, we’re headed towards an ice-free planet.  That takes us through the Eemian interglacial period of about 130,000 years ago when sea levels were 15 to 20 feet higher, when temperatures had been thought to be about 1 °C warmer than today.  Then we go back to the “early Pliocene, when sea level was about 25 m [82 feet] higher than today,” as NASA’s James Hansen and Makiko Sato explain in a new draft paper, “Paleoclimate Implications for Human-Made Climate Change.”

The question is how much warmer was it in the Eemian and early Pliocene than today — and how fast can the great ice sheets disintegrate?

We already know we’re at CO2 levels that risk catastrophe if they are sustained or exceeded for any extended period of time (see Science: CO2 levels haven’t been this high for 15 million years, when it was 5 to 10 °F warmer and seas were 75 to 120 feet higher).

Hansen and Sato go further, saying we’re actually at or very near the highest temperatures of the current Holocene interglacial — the last 12,000 years of relatively stable climate that has made modern civilization possible.


They argue that the Eemian was warmer than the Holocene maximum by “at most by about 1 °C, but probably by only several tenths of a degree Celsius.”  They make the remarkable finding that sea level rise will be highly nonlinear this century on our current business-as-usual [BAU] emissions that:
BAU scenarios result in global warming of the order of 3-6 °C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain.
While this conclusion takes them well outside of every other recent prediction of sea level rise (SLR), Hansen deserves to be listened to because he has been right longer than almost anyone else in the field (see “Right for three decades: 1981 Hansen study finds warming trend that could raise sea levels“).   Also, at least one recent study that attempts to integrate a linear historically-based analysis with a rapid response term finds we are headed towards SLR of “as much as 1.9 metres (6 ft., 3 in.) by 2100″ if we stay on BAU (see “Sea levels may rise 3 times faster than IPCC estimated, could hit 6 feet by 2100“).

Hansen and Sato make their case for a strong nonlinear SLR based on a “phase change feedback mechanism,” that, as we’ll see, appears consistent with the recent scientific literature and observations:
There is a simple explanation for why the Eemian and Holsteinian were only marginally warmer than the Holocene and yet had (both) poles several degrees Celsius warmer. Earth at peak Holocene temperature is poised such that additional warming instigates large amplifying high-latitude feedbacks. Mechanisms on the verge of being instigated include loss of Arctic sea ice, shrinkage of the Greenland ice sheet, loss of Antarctic ice shelves, and shrinkage of the Antarctic ice sheets. These are not runaway feedbacks, but together they strongly amplify the impacts in polar regions of a positive (warming) climate forcing. 
Augmentation of peak Holocene temperature by even 1 °C would be sufficient to trigger powerful amplifying polar feedbacks, leading to a planet at least as warm as in the Eemian and Holsteinian periods, making ice sheet disintegration and large sea level rise inevitable. 
Empirical evidence supporting these assertions abounds. Global temperature increased 0.5 °C in the past three decades (Hansen et al., 2010) to a level comparable to the prior Holocene maximum, or a few tenths of a degree higher. Satellite observations reveal rapid reduction of Arctic sea ice (Stroeve et al., 2007) and surface melt on a large growing portion of the Greenland ice sheet (Steffen et al., 2004; Tedesco et al., 2011). 
Arctic response to human-made climate forcing is more apparent than Antarctic change, because the response time is quicker due to the large proportion of land area and Greenland’s temperature, which allows a large expansion of the area with summer melting. 
However, we must expect ice sheet mass balance changes will occur simultaneously in both hemispheres. Why? Because ice sheets in both hemispheres were in near-equilibrium with Holocene temperatures. That is probably why both Greenland and Antarctica began to shed ice in the past decade or so, because global temperature is just rising above the Holocene level. 
Ice sheet disintegration in Antarctica depends on melting the underside of ice shelves as the ocean warms, a process well underway at the Pine Island glacier (Scott et al., 2009). The glacier’s grounding line has retreated inland by tens of kilometers (Jenkins et al., 2010), and thinning of the ice sheet has spread inland hundreds of kilometers (Wingham et al., 2009).
The article has a longer discussion of the ‘albedo flip’ underlying their conclusion:
Summer melting on lower reaches of the ice sheets and on ice shelves introduces the “albedo flip” mechanism (Hansen et al., 2007). This phase change of water causes a powerful local feedback, which, together with moderate global warming, can substantially increase the length of the melt season. Such increased summer melting has an immediate local temperature effect, and it also will affect sea level, on a time scale that is being debated, as discussed below. 
We suggest that the warmest interglacials in the past 450,000 years were warm enough to bring the “albedo flip” phenomenon into play, while interglacials in the earlier part of the 800,000 year ice core record were too cool for surface melt on the Greenland and Antarctic ice sheets and ice shelves to be important. Increased surface melting, loss of ice shelves, and reduction of summer and autumn sea ice around the Antarctic and Greenland continents during the warmest interglacials would have a year-round effect on temperature, because the increased area of open water has its largest impact on surface air temperature in the cool seasons. 
Further, we suggest that the stability of sea level during the Holocene is a consequence of the fact that global temperature remained just below the level required to initiate the “albedo flip” mechanism on Greenland and West Antarctica. 
One implication of this interpretation is that the world today is on the verge of a level of global warming for which the equilibrium surface air temperature response on the ice sheets will exceed the global mean temperature increase by much more than a factor of two.
Coincidentally, a new article in Nature Geoscience, “Radiative forcing and albedo feedback from the Northern Hemisphere cryosphere between 1979 and 2008,” appears to lend support to this thesis.  After “synthesizing a variety of remote sensing and field measurements,” the authors find “the albedo feedback from the Northern Hemisphere cryosphere” is “substantially larger than comparable estimates obtained from 18 climate models.”  The news release notes:
A new analysis of the Northern Hemisphere’s “albedo feedback” over a 30-year period concludes that the region’s loss of reflectivity due to snow and sea ice decline is more than double what state-of-the-art climate models estimate. 
The findings are important, researchers say, because they suggest that Arctic warming amplified by the loss of reflectivity could be even more significant than previously thought.
Also, the Hansen-Sato thesis seems consistent with a 2008 study in Geophysical Research Letters by leading tundra experts, “Accelerated Arctic land warming and permafrost degradation during rapid sea ice loss.” The lead author is David Lawrence of the National Center for Atmospheric Research (NCAR), who I have interviewed a number of times. The study’s ominous conclusion:
We find that simulated western Arctic land warming trends during rapid sea ice loss are 3.5 times greater than secular 21st century climate-change trends. The accelerated warming signal penetrates up to 1500 km inland…

Back to Hansen-Sato.  They have extended discussion of “linear versus non-linear ice sheet disintegration” and conclude:
The asymmetry of glacial-interglacial climate cycles, with rapid warming and sea level rise in the warming phase and a slower descent into ice ages, suggests that amplifying feedbacks can make the “wet” ice sheet disintegration process relatively rapid (Hansen et al., 2007). But how rapid? 
Paleoclimate records include cases in which sea level rose several meters per century, even though known natural positive forcings are much smaller than the human-made forcing. This implies that ice sheet disintegration can be a highly nonlinear process. 
We suggest that a nonlinear process spurred by an increasing forcing and amplifying feedbacks is better characterized by the doubling time for the rate of mass disintegration, rather than a linear rate of mass change. If the doubling time is as short as a decade, multi-meter sea level rise could occur this century. Observations of mass loss from Greenland and Antarctica are too brief for significant conclusions, but they are not inconsistent with a doubling time of a decade or less. The picture will become clearer as the measurement record lengthens. 
What constraints or negative feedbacks might limit nonlinear growth of ice sheet mass loss? An ice sheet sitting primarily on land above sea level, such as most of Greenland, may be limited by the speed at which it can deliver ice to the ocean via outlet glaciers. But much of the West Antarctic ice sheet, resting on bedrock below sea level, is not so constrained.
And so they end their paper with this prediction and warning:
IPCC BAU (business-as-usual) scenarios assume that greenhouse gas emissions will continue to increase, with the nations of the world burning most of the fossil fuels including unconventional fossil fuels such as tar sands.
An alternative extreme, one that places a substantial rising price on carbon emissions, would have CO2 emissions beginning to decrease within less than a decade, as the world moves on energy systems beyond fossil fuels, leaving most of the remaining coal and unconventional fossil fuels in the ground. In this extreme scenario, let’s call it fossil fuel phase-out (FFPO), CO2 would rise above 400 ppm but begin a long decline by mid-century (Hansen et al., 2008). 
The European Union 2 °C scenario, call it EU2C, falls in between these two extremes. 
BAU scenarios result in global warming of the order of 3-6 °C. It is this scenario for which we assert that multi-meter sea level rise on the century time scale are not only possible, but almost dead certain. Such a huge rapidly increasing climate forcing dwarfs anything in the paleoclimate record. Antarctic ice shelves would disappear, and the lower reaches of the Antarctic ice sheets would experience summer melt comparable to that on Greenland today. 
The other extreme scenario, FFPO, does not eliminate the possibility of multi-meter sea level rise, but it leaves the time scale for ice sheet disintegration very uncertain, possibly very long. If the time scale is several centuries, then it may be possible to avoid large sea level rise by decreasing emissions fast enough to cause atmospheric greenhouse gases to decline in amount. 
What about the intermediate scenario, EU2C? We have presented evidence in this paper that prior interglacial periods were less than 1 °C warmer than the Holocene maximum. If we are correct in that conclusion, the EU2C scenario implies a sea level rise of many meters. It is difficult to predict a time scale for the sea level rise, but it would be dangerous and foolish to take such a global warming scenario as a goal.
If Hansen and Sato are right, we will know within a decade or two.  Unfortunately, continuing to do nothing while we wait to find out all but ensures we cross the tipping point and enter the realm of worst-case scenarios.  Further delay is beyond immoral.


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