FOR a few minutes David Holland forgets about his work and screams like a kid on a roller coaster. The small helicopter he's riding in is slaloming between towering cliffs of ice - the sheer sides of gigantic icebergs that had calved off Greenland's Jakobshavn glacier. "It was like in a James Bond movie," Holland says afterwards. "It's the most exciting thing I have ever done."

Jakobshavn has doubled its speed in the past 15 years, draining increasing amounts of ice from the Greenland ice sheet into the ocean, and Holland, an oceanographer at New York University, has been trying to find out why. Scientists like him are more than a little astonished at the rate at which our planet's frozen frontiers seem to be responding to global warming. The crucial question, though, is what will happen over the next few decades and centuries.

That's because the fate of the planet's ice, from relatively small ice caps in places like the Canadian Arctic, the Andes and the Himalayas, to the immense ice sheets of Greenland and Antarctica, will largely determine the speed and extent of sea level rise. At stake are the lives and livelihoods of hundreds of millions of people, not to mention millions of square kilometres of cities and coastal land, and trillions of dollars in economic terms.

In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC) forecast a sea level rise of between 19 and 59 centimetres by 2100, but this excluded "future rapid dynamical changes in ice flow". Crudely speaking, these estimates assume ice sheets are a bit like vast ice cubes sitting on a flat surface, which will stay in place as they slowly melt. But what if some ice sheets are more like ice cubes sitting on an upside-down bowl, which could suddenly slide off into the sea as conditions get slippery? "Larger rises cannot be excluded but understanding of these effects is too limited to assess their likelihood," the IPCC report stated.

Even before it was released, the report was outdated. Researchers now know far more. And while we still don't understand the dynamics of ice sheets and glaciers well enough to make precise predictions, we are narrowing down the possibilities. The good news is that some of the scarier scenarios, such as a sudden collapse of the Greenland ice sheet, now appear less likely. The bad news is that there is a growing consensus that the IPCC estimates are wildly optimistic.

The oceans are already rising. Global average sea level rose about 17 centimetres in the 20th century, and the rate of rise is increasing. The biggest uncertainty for those trying to predict future changes is how humanity will behave. Will we start to curb our emissions of greenhouse gases sometime soon, or will we continue to pump ever more into the atmosphere?

Even if all emissions stopped today, sea level would continue to rise. "The current rate of rise would continue for centuries if temperatures are constant, and that would add about 30 centimetres per century to global sea level," says Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany. "If we burn all fossil fuels, we are likely to end up with many metres of sea level rise in the long run, very likely more than 10 metres in my view."

This might sound dramatic, but we know sea level has swung from 120 metres lower than today during ice ages to more than 70 metres higher during hot periods. There is no doubt at all that if the planet warms, the sea will rise. The key questions are, by how much and how soon?

To pin down the possibilities, researchers have to look at what will happen to all the different contributors to sea level under various emissions scenarios. The single biggest contributor to sea level rise over the past century has been the melting of glaciers and ice caps outside of Greenland and Antarctica, from Alaska to the Himalayas. According to one recent estimate, the continued loss of this ice will add another 10 to 20 centimetres to sea level by 2100. It cannot get much worse than this: even if all this ice melted, sea level would only rise by about 33 centimetres.

Expanding waters

The second biggest contributor has been thermal expansion of the oceans. Its future contribution is relatively simple to predict, as we know exactly how much water expands for a given increase in temperature. A study published earlier this year found that even if all emissions stopped once carbon dioxide levels hit 450 parts per million (ppm) - an unrealistically optimistic scenario - thermal expansion alone would cause sea level to rise by 20 centimetres by 2100, and by another 10 centimetres by 3000. At the other extreme, if emissions peak at 1200 ppm, thermal expansion alone would lead to a 0.5-metre rise by 2100, and another 1.4 metres by 3000 (see "How high, how soon?").