Reading a scientific paper on the train this weekend, I found, to my amazement, that my hands were shaking. This has never happened to me before, but nor have I ever read anything like it. Published by a team led by James Hansen at NASA, it suggests that the grim reports issued by the Intergovernmental Panel on Climate Change could be absurdly optimistic.

The IPCC predicts that sea levels could rise by as much as 59 centimeters this century. Hansen's paper argues that the slow melting of ice sheets the panel expects doesn't fit the data. The geological record suggests that ice at the poles does not melt in a gradual and linear fashion, but flips suddenly from one state to another. When temperatures increased to 2-3 degrees Celsius above today's level 3.5 million years ago, sea levels rose not by 59 cm but by 25 meters. The ice responded immediately to changes in temperature.

We now have a pretty good idea of why ice sheets collapse. The buttresses that prevent them from sliding into the sea break up; meltwater trickles down to their base, causing them suddenly to slip; and pools of water form on the surface, making the ice darker so that it absorbs more heat. These processes are already taking place in Greenland and West Antarctica.