Image 1 - Warm Ocean Currents Cause Ice Loss In Antarctica
April 26, 2012

Warm Ocean Currents Cause Ice Loss In Antarctica

Lee Rannals for

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An international team of scientists reported in the journal Nature on Thursday that warm ocean currents are the culprit behind recent ice loss in Antarctica.

The team, lead by British Antarctic Survey, used 4.5 million measurements made by a laser instrument mounted on NASA's ICESat satellite to map the changing thickness of Antarctica's floating ice shelves.

Of the 54 ice shelves they mapped, they revealed that 20 are melting due to warm ocean currents, mostly located in West Antarctica.

The inland glaciers that flow down to the coast and feed into the thinning ice shelves have accelerated, draining more ice into the sea.

"In most places in Antarctica, we can't explain the ice-shelf thinning through melting of snow at the surface, so it has to be driven by warm ocean currents melting them from below," Lead author Dr Hamish Pritchard from British Antarctic Survey said in a press release.

Pritchard said the researchers looked around the Antarctic coast and found that in all cases where ice shelves are melting, the inland glaciers are speeding up.

"It's this glacier acceleration that's responsible for most of the increase in ice loss from the continent and this is contributing to sea-level rise," Pritchard said.

He added that some ice shelves are thinning by a few feet per year, and glaciers drain billions of tons of ice into the sea due to this.

"This supports the idea that ice shelves are important in slowing down the glaciers that feed them, controlling the loss of ice from the Antarctic ice sheet," he said. "It means that we can lose an awful lot of ice to the sea without ever having summers warm enough to make the snow on top of the glaciers melt - the oceans can do all the work from below."

During the study, the researchers measured how ice shelf height changed over time, running computer models to discard changes in ice thickness due to natural snow accumulation.  They used a tide model that eliminated height changes due to rising tides as well.

On the other side of the continent, the researchers found that ice-shelf thinning is cause by warm summer winds directly melting the snow.

"This study shows very clearly why the Antarctic ice sheet is currently losing ice, which is a major advance," Professor David Vaughan is the leader of ice2sea, said in a press release.

He said the real significance of the study is that it shows the key to predicting how an ice sheet might change in the future.

"Perhaps we should not only be looking to the skies above Antarctica, but also into the surrounding oceans," Vaughan added.

Tom Wagner, cryosphere program scientist at NASA Headquarters in Washington, said that the study demonstrates how "space-based, laser altimetry" can deepen scientists understand of the Earth.

"Coupled with NASA's portfolio of other ice sheet research using data from our GRACE mission, satellite radars and aircraft, we get a comprehensive view of ice sheet change that improves estimates of sea level rise," Wagner commented in a prepared NASA statement.

Other studies have used satellite radar data to measure how ice shelves and glaciers have changed, but laser measurements can be more precise at detecting these changes.

Jay Zwally, ICESat project scientist at NASA's Goddard Space Flight Center, said that the study shows the importance of ICESat-2 to launch into space.

ICESat was the first satellite designed to use laser altimetry to study the Earth, lasting from its launch in 2003 to its mission end in 2009.  ICESat-2 is the successor of the first satellite, and is scheduled for launch sometime in 2016.

"We have limited information on the changes in polar regions caused by climate change," Zwally said. "Nothing can look at these changes like satellite measurements do."

The new research also links the observed increase in melting that takes place on the underside of a glacier or ice shelf.

Pritchard said as a result of warm water being funneled beneath the funnel ice, it has affected the strength and direction of ocean currents.  He said the new study suggest Antarctica's glaciers are responding rapidly to the changing climate.

Once the ice shelves completely collapse, the grounded glaciers that are found behind them will start to speed up, according to what Helen Amanda Fricker of Scripps Institution of Oceanography told BBC News.

"But what this study is showing, which is very new, is that you don't need to lose the shelf entirely for this to happen; just a reduction in the thickness of the ice shelf is enough to allow more of the grounded ice behind it to flow off the continent."

Pritchard told BBC that scientists didn't realize until now just how important the data they have uncovered is.

"Previously, you would have thought that we needed a lot of warming in the atmosphere to get a substantial loss of ice from Antarctica - because it's such a cold place," he told BBC. "But what we show is that that's not necessary; you don't need radical change."