Antarctic ice loss speeding up

13 January 2008Eric Hand

A comprehensive study of Antarctica’s ice confirms that the polar cap is shrinking. In 2006 alone, Antarctica lost nearly 200 billion tonnes of ice, researchers say — the equivalent of a global sea level rise of more than half a millimetre. That’s 75% more than losses in 1996, they add.

The study follows on a 2006 report that also concluded the rate of ice loss from glaciers melting and sliding away is greater than the gain from snow (see "Antarctica is shrinking"). That report concluded that from 2002 to 2005, Antarctica lost an average of 152 cubic kilometres (139 billion tonnes) a year.

“It reinforces the finding that the Antarctic is losing mass — which is still not a well-accepted result,” says Eric Rignot, an ice sheet expert at the University of California at Irvine and head of the team that reports the new result today in Nature Geoscience 1. “Doing it with an independent technique is very important,” he adds. Both groups used satellite data, but based on different techniques.

The finding lends support to growing concerns that the accelerated melting of ice caps could raise sea levels by more than two metres within hundreds of years — processes, and a worst-case scenario, that the Intergovernmental Panel on Climate Change (IPCC) has not included in its most recent predictions. The science, the IPCC said in 2007, was too uncertain.

Andrew Shepherd, who studies global ice sheet dynamics at the University of Edinburgh in Scotland, says Rignot's findings are in good agreement with his2 and other recent studies. Plus, he says, it really pins down how much ice is being lost along the coast.

Thick around the middle

Climate scientists are still debating what the overall effect of climate change will be on the world's two biggest ice caps: in Greenland and Antarctica. Global warming is accompanied by regional increases in precipitation that could thicken ice sheets, even as the edges melt away. Previous studies have shown that east Antarctica is getting thicker (see "East Antarctica puts on weight"). The 2001 IPCC report actually predicted that the Antarctic ice cap was likely to grow in the 21st century.

There are three main ways in which scientists have tried to estimate the thinning of the ice sheets, each with their strengths and weaknesses. Satellite altimetry, which simply measures the height of the ground, is useful since long-term data sets exist. But it is difficult to measure overall ice mass from this, because it’s impossible to know how much of the observed elevation is comprised of dense packed ice and how much is fluffy snow.

Gravity measurements have been a rich source of data since 2002, when the GRACE satellites were launched. These can be used to work out the mass of land or ice, but the measurements are complicated by glacial rebound — the way that rock beneath the ice tends to move upwards over time.

Satellite radar interferometry is very precise at measuring glacial velocities, from which researchers can calculate the loss of ice around the edges of the Antarctic. But there is no dedicated interferometry satellite, which makes the data hard to get.

Rignot used radar interferometry data from 85% of the coastline for three points in time: 1996, 2000 and 2006. He used a pre-existing climate model to calculate inputs in the interior, which was the most uncertain part of the calculation. He says the net losses in ice mass are very similar to losses he has calculated for Greenland (see "Glacial pace picks up").

Speeding up

Isabella Velicogna, now a colleague of Rignot's at Irvine, led the 2006 work that also showed Antarctic ice loss. Her study was based on gravity measurements and had more data points, but over a shorter time period. She says her and Rignot's results are in good agreement. “We’re both seeing a trend that is significant.”

That trend — a 75% increase in losses since 1996 — is frightening Rignot. Once the glaciers are well lubricated by water, it's hard to slow them down, even if global warming were to be arrested, he says. He estimates that the worst case scenario — a complete emptying of ice basins — would result in about a metre of sea level rise each from Greenland and Antarctica, as well as half a metre from remaining alpine glaciers.

Shepherd cautions that Rignot's three-point trend shouldn't be projected decades into the future, because there's reason to believe that the increasing amount of cold meltwater near the coast might slow further losses.

Rignot agrees that the trend might not continue: the triggers and feedbacks of glacial flows are poorly understood, he notes. But he still thinks the IPCC has been overly cautious in not reporting the possibilities of ice loss, which he says could occur in the next century. “Each time I look at some new data, I am astonished.”

References

http://www.nature.com/news/2008/080113/full/news.2008.438.html