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
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.
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).
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.
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.
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…
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.
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.