Tropics may speed glacial melt

U. WASHINGTON (US) — Accelerated melting of two outlet glaciers in the Antarctic is likely due in part to an increase in sea-surface temperatures in the tropical Pacific Ocean.

Sea-level pressure north of the Amundsen Sea that’s higher than normal sets up westerly winds that push surface water away from the glaciers and allow warmer deep water to rise to the surface under the edges of the glaciers, says Eric Steig, a University of Washington professor of Earth and space sciences.

“This part of Antarctica is affected by what’s happening on the rest of the planet, in particular the tropical Pacific,” he says.

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The research involves the Pine Island and Thwaites glaciers on the West Antarctic Ice Sheet, two of the five largest glaciers in Antarctica. Those two glaciers are important because they drain a large portion of the ice sheet.

As they melt from below, they also gain speed, draining the ice sheet faster and contributing to sea level rise. Eventually that could lead to global sea level rise of as much as six feet, though that would take hundreds to thousands of years, Steig says.

NASA scientists recently documented that a section of the Pine Island Glacier the size of New York City had begun breaking off into a huge iceberg.

Steig notes that such an event is normal and scientists were fortunate to be on hand to capture a video recording. Neither that event nor the new findings clearly link thinning Antarctic ice to human causes.

But Steig’s research shows that unusual winds in this area are linked to changes far away, in the tropical Pacific Ocean. Warmer-than-usual sea-surface temperatures, especially in the central tropics, lead to changes in atmospheric circulation that influence conditions near the Antarctic coastline.

Recent decades have been exceptionally warm in the tropics, he says, and to whatever extent unusual conditions in the tropical Pacific can be attributed to human activities, unusual conditions in Antarctica also can be attributed to those causes.

He notes that sea-surface temperatures in the tropical Pacific last showed significant warming in the 1940s, and the impact in the Amundsen Sea area then was probably comparable to what has been observed recently.

That suggests that the 1940s tropical warming could have started the changes in the Amundsen Sea ice shelves that are being observed now, he says.

Steig presented his findings on December 6, 2011 at the fall meeting of the American Geophysical Union in San Francisco. He emphasized that natural variations in tropical sea-surface temperatures associated with the El Niño Southern Oscillation play a significant role. The 1990s were notably different from all other decades in the tropics, with two major El Niño events offset by only minor La Niña events.

“The point is that if you want to predict what’s going to happen in the next fifty, one hundred, one thousand years in Antarctica, you have to pay attention to what’s happening elsewhere,” he says. “The tropics are where there is a large source of uncertainty.”

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