The complexity of these, and possibly other feedback mechanisms that have yet to be identified, makes predicting the future of the WAIS very difficult. Glaciological models do not yet capture many of these feedback processes, and yet researchers are increasingly finding that they are crucial in controlling how ice sheets behave. What we do know is that most ice streams along the northern edge of the WAIS have been thinning and accelerating, and that there is currently a net loss of mass from the Antarctic Ice Sheet (Zwally et al, 2006).

Other elements ofthe cryosphere are also already responding to global warming: Arctic sea ice extent has decreased dramatically over recent decades and the late-summer Arctic Ocean could be ice-free even before 2050; mountain glaciers are in retreat around the world and this retreat appears to be accelerating; springtime snow cover in the Northern Hemisphere has decreased by approximately 10 per cent in the last 50 years; and permafrost in the Arctic is melting over large areas.

It is clear that the polar regions are reacting to climate change faster than the rest of the planet, and the presence of multiple positive feedback mechanisms will likely maintain this disproportionate response. While we do not yet have the capability to simulate the reaction of the West Antarctic Ice Sheet to warming, we can expect that increasing greenhouse emissions are likely to continue, and probably will accelerate loss ofice from the region and, correspondingly, contribute to further increases in the rate of sea level rise.

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