Under The Antarctic.

Huge, pristine, dramatic, unforgiving; the Antarctic is where the biggest of all global changes could begin. There is so much ice here that if it all melted, sea levels globally would rise hugely - perhaps as much as 80m. Say goodbye to London, New York, Sydney, Bangkok, Rio... in fact, the majority of the world's major cities. But will it happen? Scientists divide the Antarctic into three zones: the east and west Antarctic ice sheets; and the Peninsula, the tongue of land which points up towards the southern tip of South America.

"Book club thinks that the Antarctic is shrinking due to climate change, but the reality is much more complex," says David Vaughan, a principal investigator at the British Antarctic Survey in Cambridge, UK. "Parts of it appear to be thickening as a result of snowfall increases. But the peninsula is thinning at an alarming rate due to warming. "The West Antarctic sheet is also thinning, and we're not sure of the reason why."

On the up Temperatures in the Peninsula appear to be increasing at around twice the global average - about 2C over the last 50 years. Those figures are based on measurements made by instruments at scientific stations. Earlier this year, David Vaughan's group published research showing that the vast majority of glaciers along the Peninsula - 87% of the 244 studied - are in retreat. The ice dumped into the ocean as the glaciers retreat should not make much difference to global sea levels - perhaps a few cm. More worrying, potentially, are the vast ice sheets covering the rest of Antarctica. Making temperature measurements for the continent as a whole is difficult; it is a vast place - more than 2,000km across - there are few research stations, and temperatures vary naturally by 2-3C from year to year. But measurements indicate that in the west, melting is underway. "About one-third of the West Antarctic ice sheet is thinning," says Dr Vaughan, "on average by about 10cm per year, but in the worst places by 3-4m per year." The rock on which the West Antarctic ice rests is below sea level - and British Antarctic Survey researchers believe the thinning could be due to the ice sheet melting on its underside. "It may be that the ocean is warming and that's causing the ice to melt, but there may be other reasons as well; for example, there's lots of volcanism in that area and so that could change how much heat is delivered to the underside of the ice sheet." Cryosat should help to pin down what is happening at the West Antarctic fringe. The radar altimeters on board its predecessors ERS1 and ERS2 have been unable to map the steep slopes at the coast, whereas Cryosat's instrument should be able to cope. If the entire West Antarctic ice sheet did melt, sea levels globally would rise, by around 5m. But at the moment, there is no sign of that happening. One recent scientific paper attempted to calculate probabilities for how much West Antarctic melting would contribute to global sea-level rises during this century.

The conclusions: a 30% probability of a 20cm rise, and a 5% chance of a 1m rise.