by David Perlman

Antarctica's massive coastal glaciers are quickly melting into the sea as the oceans around the continent grow warmer - and the pace of ice loss is speeding up.An international satellite network measuring the thickness of the glaciers as they shrink year by year has found that the glaciers have melted so rapidly during the past 10 years that the continent is losing almost as much ice as Greenland, according to researchers gathering the satellite data.

The team from Chile, England and the Netherlands is led by Eric Rignot, a radar engineer and glacier specialist at UC Irvine and NASA's Jet Propulsion Laboratory who has watched the shrinking glaciers and gathered data for the past 15 years from Canadian, Japanese and European polar-orbiting satellites.

Those satellites carry radar instruments that can measure the thickness of each glacier with remarkable accuracy, and they have now mapped more than 85 percent of the entire coastline of Antarctica, covering all the continent's major glaciers.

Unlike Greenland's coastal glaciers, where meltwater from the ice on the surface seeps down to the base of each glacier and lubricates it to speed its flow to the sea, the glaciers on Antarctica move down from the land as huge ice sheets and spread out over the ocean, where the thick glaciers are known as ice shelves.

For many years, scientists have watched some of these giant ice shelves breaking apart and crashing into the sea, and now more and more of them are melting as they move out over the ocean.

The cause: Antarctic waters like the Bellingshausen and Amundsen seas are warming, and as their water temperatures rise they melt the undersides of the ice sheets so the sheets become thinner and the seas intrude farther and farther inland - to melt still more of the ice, Rignot explained in a phone interview.

Although the effect of all this ice loss on global sea levels is still small - measured in a rise of only a few thousands of an inch each year so far from the melting in Antarctica - that increase has nearly doubled in the past 10 years, he estimated.

"We're concerned that the rate of glacier melting will double rapidly," Rignot said.

Ice loss is most pronounced in Antarctica's Pine Island Bay region, where three major glaciers are losing ice fast, and on the northern tip of the Antarctic Peninsula, Rignot and his colleagues reported.

Glaciers in those two regions alone lost about 212 billion tons of ice from 1996 to 2006 - an amount very similar to the total loss of ice on Greenland, Rignot and his team calculated.

The east coast of the Antarctic Peninsula is where two major ice shelves - called Larsen A and Larsen B - disintegrated in 1995 and 2002. Those immense events were among the most convincing early signals that global warming is real and dangerous.

The researchers calculated the increase in mass of the glaciers as snow has piled up on them, and compared those numbers with the losses due to melting into the sea. The calculations yield what Rignot and his colleagues term the "ice sheet mass balance," and the overall result is increasingly negative, they report.

"Large uncertainties remain in predicting Antarctica's future contribution to sea level rise," Rignot said.

"The ice sheets are responding faster to climate change than (anyone) anticipated," he said.

E-mail David Perlman at dperlman@sfchronicle.com.
© 2008 San Francisco Chronicle