Read more: 鈥�Climate change: It鈥檚 even worse than we thought鈥�
The Summit weather station in Greenland sits more than 3000 metres up atop the country鈥檚 vast ice sheet. The temperature there on a typical summer鈥檚 day is a chilly -10 掳C. In July this year, however, the temperature rose above freezing. At one point, , leading to floods that washed away bridges. This was not a one-off event: that absorbs more heat and melts faster (see Losing its sparkle). Along the coastline, the floating tongues of glaciers are breaking up. As these 鈥渄ams鈥� disappear, the rivers of ice behind them are accelerating and thinning.
Until recently, we thought it would be centuries before Greenland lost a significant amount of ice. The Antarctic ice sheet was expected to grow, with increased snowfall compensating for melting around the edges. The 2007 IPCC report assumed that the two ice sheets would .
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Even then, many experts disputed this, and satellite measurements have since shown and climbing. , as well as , suggest that the planet will soon have warmed enough to melt Greenland鈥檚 ice sheet entirely 鈥� if it hasn鈥檛 already become warm enough. The question is how long the melting itself will take.
Most glaciologists now think that sea level will rise by at least a metre by 2100, and possibly by as much as 2 metres. That is enough to flood many low-lying cities or render them vulnerable to storm surges. of NASA鈥檚 Goddard Institute for Space Studies in New York is even more pessimistic. He argues that as the ice starts melting, positive feedbacks will kick in, accelerating the ice loss. Satellite observations do indeed show the rate of ice loss doubling every 10 years but, as Hansen himself points out, we cannot yet be certain that this is a long-term trend rather than a short-term blip.
Then there is the possibility of the west Antarctic ice sheet collapsing, as it has on in response to warming. Recent discoveries about the state of the ice and the nature of the underlying topography suggest that than previously expected.
Whether that happens or not, we should not be deceived by the small effects we are seeing now, says oceanographer of the Potsdam Institute. 鈥淪ea-level rise is slow to start, but in the longer run will turn out to be one of the gravest impacts and longest legacies of the global warming we are causing now.鈥�
