Sea level rise: It's worse than we thought

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Even before it was released, the report was outdated.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.

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