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.

Sea change

Ice in the heart of Antarctica is retreating and causing sea level rise, scientists have shown for the first time.

The new research shows that the largest glacier in the West Antarctic Ice Sheet is now losing far more ice than is being replenished by snow.

"In terms of ice discharge, this is nothing like anyone has seen before - it's a huge amount of ice," says Andrew Shepherd at the Centre for Polar Observation and Modelling, University College London.

"People are concerned about retreating glaciers because they contribute to sea level rise", unlike sea ice break-up, Shepherd told New Scientist. "The vast majority of fresh water is locked in Antarctica and this is the first time we have seen an Antarctic glacier retreating. The concerns for sea level rise are real in that respect."

Shepherd notes that modelling studies which raise sea temperatures have produced similar patterns of thinning to that now observed. "They are simple models but it tells us that changes at the margins of the ice sheet can be transmitted inland."

But David Vaughan, a glaciologist at the British Antarctic Survey, says it is not yet clear whether the changes seen in the Antarctic ice sheet are related to human-induced climate change or the long-term re-equilibriation from the last ice age, which peaked 18,000 years ago.

"My best guess remains that over the next 100 to 200 years, overall, the ice in Antarctica will get a little thicker due to increased precipitation," he says.

"The continent would then be the only thing actually slowing sea level rise down. But there does remain a small chance of a rapid change in ice loss, with hazardous consequences for sea level rise."

The WAIS contains enough water to raise global sea level by a catastrophic five metres. Its largest glacier, the Pine Island Glacier, is now losing four billion tonnes of ice to the ocean, according to Shepherd's team's work. The glacier retreated by five kilometres between 1992 and 1999, and thinned by 10 metres.

If the entire glacier was lost at the current rate, it would take 600 years and raise sea level by 0.6 centimetres. But Shepherd points out the glacier could not be lost without a knock-on effect in its drainage basin. This contains enough ice to raise sea level by 50 centimetres.

And, says Shepherd, no-one yet knows whether the glacier's retreat is a process which will accelerate or stabilise.

Shepherd and his colleagues monitored the glacier using satellite altimetry to measure changes in elevation and satellite interferometry to measure ice velocity.

The data is a precious addition to knowledge of a virtually inaccessible area, says Vaughan. "You can count the number of people who have set foot there on one hand."

He says the more data collected by scientists in these areas, the more certain the predictions they make will be. But it will not be easy: "We can go through space to Europa, but getting people to Pine Island Glacier is beyond most national research organisations."

POLLUTION