Vast and remote

CLIMATE CHANGE

Predicted global sea level rise swells

Global sea level rise in the 21st century could be significantly higher than previously estimated, according to the most comprehensive glacier dataset ever compiled.

The missing factor is the melting of the world's largest temperate glaciers in Alaska and Canada, say Mark Meier and Mark Dyurgerov at the University of Colorado at Boulder. New data from the University of Alaska show this has been underplayed in earlier calculations, they say.

The Intergovernmental Panel on Climate Change reported in 2001 that the expected rise in sea level by 2100 due to glacier melting alone was between 1 and 23 centimetres. The estimate represents the consensus of many of the world's scientists.

Meier and Dyurgerov's new range is much higher, at between 20 and 46 cm, and they say it could be even greater. Combined with the IPCC's estimate for sea level rise caused by other processes, such as ocean warming, of 11 to 43 cm, the total 21st century rise could be as much as 89 cm.

" These estimates in sea-level rise may seem small, but a 30 cm rise in sea level will typically cause a retreat of shoreline of 30 metres. This would have substantial social and economic impacts, " Meier says. But he admits: " We are still very data poor. This is the best we can do at the moment."

David Vaughan, at the British Antarctic Survey, says he will not be at all surprised if Meier and Dyurgerov are right about the Alaskan Glaciers. But we should not be too harsh on the IPCC prediction, he says: " The IPCC group made an reasonable estimate based on what they knew at the time. It wasn't set in stone, it was a best-estimate. And as we get new data that estimate will change, up and probably down."

Vast and remote

Meier claims that the IPCC prediction fails to reflect the true likely sea level rise for three reasons. Firstly, their inventory of glaciers included little data from the large glaciers in Alaska and Canada and does not account for a recent increase in wastage. The Malaspina and Bering glaciers are both over 2000 square miles in area - the largest non-polar glaciers in the world.

The lack of data has largely been due to the vast size of the glaciers, their remoteness and the high rain and snow fall which makes using helicopters very difficult. However, new laser altimeter data taken from light aircraft is starting to reveal a clearer picture, Meier says.

Secondly Meier says the IPCC have not accounted for the fact that when precipitation increases over a glacier, it appears to shed more melt water for a given temperature rise. He claims this observation is an " empirical fact" and that precipitation over the Alaskan glaciers has recently been found to be four times higher that previously thought.

Finally, he says, the IPCC underestimates the likely future contribution of ice melting in Greenland and Antarctica, where temperatures are still currently low enough to prevent large-scale melting.

The new data from the University of Alaska shows that the long term contribution to sea level rise from the wastage of the Alaskan and Canadian glaciers is 0.12 millimetres per year, but that this has more than doubled to 0.32 mm in the last decade. The present rate of wastage in some glaciers is greater that it has been for 5000 years, says Meier.

The new research was presented at the American Association for the Advancement of Science's 2002 annual meeting in Boston.