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Saturday, December 29, 2012

Hansen & Sato: Update of Greenland Ice Sheet Mass Loss: Exponential?

Update of Greenland Ice Sheet Mass Loss: Exponential?
[3 guesses, first 2 don't count]

by James Hansen and Makiko Sato, December 26, 2012

Shepherd et al. (2012) provide an update of the mass loss by the Greenland ice sheet (and the Antarctic ice sheet). They compare several analysis methods, achieving a reasonably well-defined consensus. The data is 2-3 years more current than data we employed recently (Hansen & Sato, 2012), so a new look at the data seems warranted.

A crucial question is how rapidly the Greenland (or Antarctic) ice sheet can disintegrate in response to global warming. Earth's history makes it clear that burning all fossil fuels would cause eventual sea level rise of tens of meters, thus practically wiping out thousands of cities located on global coast lines. However, there seems to be little political or public interest in what happens next century and beyond, so reports of the IPCC (Intergovernmental Panel on Climate Change) focus on sea level change by 2100, i.e., during the next 87 years.

IPCC (2007) suggested a most likely sea level rise of a few tens of centimeters by 2100. Several subsequent papers suggest that sea level rise of ~1 meter is likely by 2100. However, those studies, one way or another, include linearity assumptions, so 1 meter can certainly not be taken as an upper limit on sea level rise (see discussion and references in the appendix below, excerpted from our recent paper). Sea level rise in the past century was nearly linear with global temperature, but that is expected behavior because the main contributions to sea level rise last century were thermal expansion of ocean water and melting mountain glaciers.

In contrast, the future sea level rise of greatest concern is that from the Greenland and Antarctic ice sheets, which has the potential to reach many meters. Hansen (2005) argues that, if business-as-usual increase of greenhouse gases continue throughout this century, the climate forcing will be so large that non-linear ice sheet disintegration should be expected and multi-meter sea level rise not only possible but likely.

Hansen (2007) suggests that the position reflected in IPCC documents may be influenced by a "scientific reticence." In such case the consensus movement of sea level rise estimates from a few tens of centimeters to ~1 meter conceivably is analogous to the reticence that the physics community demonstrated in its tentative steps to improve upon estimates of the electron charge made by the famous Millikan.[1]

Perceived authority[2] in the case of ice sheets stems from ice sheet models used to simulate paleoclimate sea level change. However, paleoclimate ice sheet changes were initiated by weak climate forcings changing slowly over thousands of years, not by a forcing as large or rapid as human-made forcing this century. Moreover, in a paper submitted for publication (Hansen et al., 2013) we present evidence that even paleoclimate data do not support the degree of lethargy and hysteresis that exists in such ice sheet models.

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