Blame Warm Ocean Currents For Antarctic Ice Melt

(CORDIS) — Scientists from the Netherlands, the United Kingdom and the United States have discovered that Antarctica is losing ice primarily because of warm ocean currents hitting the underside of ice shelves. The findings, published in the journal Nature, can help researchers make more reliable estimates of future sea level rises.

The researchers used satellite measurements and models from the National Aeronautics and Space Administration’s (NASA’s) Ice, Cloud and Land Elevation Satellite (ICESat). These helped them distinguish between the two known causes of melting ice sheets: (a) warm ocean currents thawing the underbelly of the floating extensions of the ice sheets, and (b) warm air melting them from above.

They found that warm ocean currents are responsible for melting 20 of the 54 ice shelves. The majority of these melting ice shelves are in west Antarctica. The researchers observed that more and more inland glaciers are flowing down to the coast and feeding into the thinning ice shelves. The result is that more ice is drained into the sea, in turn swelling the level of the sea. According to the team, the ocean-driven thinning is causing the biggest and fastest ice losses in west Antarctica, and in Antarctica in general.

‘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,’ said Hamish Pritchard of the British Antarctic Survey in the United Kingdom, the lead author of the study. ‘The oceans can do all the work from below.’

The researchers mounted a laser instrument on ICESat over a 5-year period (2003 to 2008), producing a time series of 4.5 million surface height measurements, which in turn helped them map the changing thickness of most of the floating ice shelves around Antarctica. The team calculated how the ice shelf changed over time, and ran computer models to eliminate changes in ice thickness due to the accumulation and compaction of natural snow. Using a tide model as well, they were able to discard height changes triggered by tides that raise and lower the ice shelves.

‘This study demonstrates the power of space-based, laser altimetry for understanding Earth processes,’ explained Tom Wagner, cryosphere program scientist at NASA Headquarters in Washington. ‘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.’

Satellite radar data measured changes in ice shelves and glaciers in the past; better accuracy in detecting changes in ice shelf thickness through time was the result of measurements made by laser. Running from 2003 to 2009, ICESat was the first satellite to use laser altimetry to investigate the planet’s polar regions. ICESat-2 is expected to launch in 2016.

‘This study demonstrates the urgent need for ICESat-2 to get into space,’ said Jay Zwally, ICESat project scientist at NASA’s Goddard Space Flight Center in Greenbelt, United States. ‘We have limited information on the changes in polar regions caused by climate change. Nothing can look at these changes like satellite measurements do.’

Said Dr Pritchard: ‘Studies have shown Antarctic winds have changed because of changes in climate. This has affected the strength and direction of ocean currents. As a result warm water is funnelled beneath the floating ice. These studies and our new results suggest Antarctica’s glaciers are responding rapidly to a changing climate.’

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