Some of the most startling news since the 2001 TAR can be found in Chapter 4 of the IPCC Fourth Assessment Report: Changes in Snow, Ice, and Frozen Ground. In particular, there is a section, toward the second half of the chapter, on recent observations of the response of great ice sheets in Greenland and Antarctica to our warming climate. There you will read about all sorts of new melting tricks that ice sheets are showing us. Seismometers on the ice record more icequakes than there used to be, and more in summers than winters. Floating ice shelves are breaking up catastrophically, allowing the ice flow from the land to the sea to accelerate. The ice sheets are melting faster than the Third Assessment Report assumed we would see until the year 2100, and the melting appears to be accelerating.

We are concerned about the fate of the great ice sheets for the reason of sea level rise. The ice sheets have the potential to raise sea level by about 70 meters, enough to completely and catastrophically change the map of the Earth. The ice sheets are not melting too much today, but the question is how quickly and strongly they will respond to warming in the future.

Forecasts such as for ice sheet melting are based on computer models, which try to simulate the underlying mechanisms controlling what they are trying to simulate.