Dr. Laurence Padman, ESR Senior Scientist

Antarctic Ice Loss Caused by Warm Ocean Currents

ESR Senior Scientist, Laurie Padman, is a co-author on a recent paper in the journal Nature entitled "Antarctic ice-sheet loss driven by basal melting of ice shelves". The international team of researchers has established that warm ocean currents are the dominant cause of recent ice loss in Antarctica. New measurement techniques have been used to differentiate, for the first time, between the two causes of thinning ice shelves – warm ocean currents melting the underside, and warm air melting from above. This finding brings scientists a step closer to providing reliable projections of future sea-level rise.

The researchers, led by Dr. Hamish Pritchard at the British Antarctic Survey (BAS) in Cambridge, UK, used measurements made by a laser instrument mounted on NASA’s ICESat (Ice, Cloud and Land Elevation Satellite) to estimate the changing thickness of almost all the floating ice shelves around Antarctica, revealing the pattern of ice-shelf melt around the continent. Of the 54 ice shelves studied, warm ocean currents are melting 20, most of which are in West Antarctica. In every case, the inland glaciers that flow down to the coast and feed into these thinning ice shelves are also draining more ice into the sea, contributing to sea-level rise. Only Larsen Ice Shelf, on the eastern side of the Antarctic Peninsula (the long stretch of land pointing towards South America), is thinning because of warm air above it instead of melting from ocean currents. Co-authors Drs. Stefan Ligtenberg and Michiel van den Broeke, both at Utrecht University in the Netherlands, used their model of snowfall and ice-shelf surface melting in Antarctica to determine how much of the ice shelf thinning was caused by atmospheric conditions. With the exception of Larsen Ice Shelf, surface thinning was small, confirming that ice shelf thinning measured by the satellite laser must be caused by warm water circulating under the ice shelves and melting the base.

Dr. Helen A. Fricker at Scripps institution of Oceanography, La Jolla CA, and Laurie Padman contributed to the analysis of ICESat altimeter measurements. Correcting altimetry data for ocean tides and other causes of rapid ocean height change is critical to being able to measure long-term trends in elevation. ESR has a long history in developing accurate polar tide models and corrections for how ice shelves respond to changing atmospheric pressure. Dr. Padman was a Science team member for the ICESat mission.

Discussing the significance of the new research, Dr. Fricker commented: "In the last decade or so we have seen some inland glaciers accelerate and thin dramatically when an ice shelf completely disappears. In this study we also see that glaciers are losing mass to the ocean even while the ice shelves are still there, but getting thinner. This suggests that ice loss is very sensitive to quite small changes in the ocean circulation around Antarctica."

Visualization of the western Antarctic ice shelves. The shelves are indicated
by the rainbow color; red is thicker (greater than 550 meters), while
blue is thinner (less than 200 meters). Credit: NASA/Goddard CGI Lab

Dr. Padman added: "Previous studies have shown that the rate of ice-shelf thinning is sensitive to quite small changes in the flow of warm ocean currents around Antarctica. Circulation changes are mostly driven by changes in the pattern of winds around Antarctica which, in turn, are related to global changes in climate. So, predicting future ice loss from Antarctica requires that we understand changing global climate, how it affects ocean circulation around Antarctica and near and under the ice shelves, and exactly how basal melting happens. The fresh water released by the glaciers then contributes to changes in ocean circulation and formation of sea ice which affects climate. So, it's a really challenging and important problem to be working on."

David Vaughan, another co-author of the paper and the leader of ice2sea - a major polar research program funded by the European Union, summed up the research: “This study shows very clearly why the Antarctic ice sheet is currently losing ice, which is a major advance. But the real significance is that it also shows the key to predicting how the ice sheet will change in the future is in understanding the oceans.”

NASA chief scientist Waleed Abdalati, an expert in Earth's ice systems who wasn't involved in the research, said the study "makes an important advance" and provides key information about how Antarctica will contribute to global sea level rise.

Another outside expert, Ted Scambos of the National Snow and Ice Data Center, said the paper will change the way scientists think about melt in Antarctica. Seeing more warm water encircling the continent, he worries that with "a further push from the wind" newer areas could start shrinking.

This summary was based on press releases from BAS and Scripps and additional comments from Huffington Post. A NASA animation of ocean currents around Antarctic ice shelves can be found here.

Further discussions of the paper