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Sea-levels to be driven higher by warming currents undermining ice-shelves

By Martin Leggett - 03 Jul 2011 17:0:0 GMT
Sea-levels to be driven higher by warming currents undermining ice-shelves

(Image Caption: The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite captured this composite image of Antarctica's ice-covered landscape on January 27, 2009. The surface appears rough where the Transantarctic Mountains curve in a shallow "s" from the shore of the Ross Sea to the Ronne Ice Shelf. The Polar Plateau in the center of the continent is smooth, shaded only by the faint shadow cast by clouds. The Weddell Sea is textured with chunks of sea ice. Image Credit: NASA image courtesy the MODIS Rapid Response Team at NASA GSFC.)

Hidden layers of warming oceanic waters, snaking around the coast of Greenland and Antarctica, could drive forward the melting of their glaciers faster than previously predicted. This newly described mechanism, modeled in detail in a paper in Nature Geoscience, is the latest piece of evidence making a 3-foot rise in sea-levels - over this coming century - increasingly certain.

The interplay between mile-thick ice sheets, a warming atmosphere, and shifting oceanic currents has been a tough task for scientists to tackle. The IPCC famously left those complexities out of its sea-level predictions, back in 2007. But over the last few years, glaciologists and climate scientists have come to better understand how ice-sheets are likely to respond to a warming globe.

This latest research is significant, because it involves the use of 19 advanced computer climate models, ones that have had the effect of these warming tongues of the oceans, built in. Most previous models have focused how warming air melts ice-sheets and glaciers from the top-down. And recently, the effect of water flowing at the base of the sheets has also been better understood.

This view of the seaward edge of Antarctica's floating Ross Ice Shelf shows a region where the ice is cracking and may produce an iceberg.

(Image Caption: This view of the seaward edge of Antarctica's floating Ross Ice Shelf shows a region where the ice is cracking and may produce an iceberg. Image Credit: Michael Van Woert, NOAA NESDIS, ORA. National Oceanic and Atmospheric Administration/Department of Commerce.)

But one factor that scientists increasingly believe to be important is how sea-water and ice interact where glaciers flow into the oceans. 'Ocean warming is very important compared to atmospheric warming because water has a much larger heat capacity than air,' co-author Jianjun Yin said. 'If you put an ice cube in a warm room, it will melt in several hours. But if you put an ice cube in a cup of warm water, it will disappear in just minutes.'

Many glaciers in Greenland and Antarctica are held-back by ice-shelves that have frozen across parts of the coast. The worry is that warming sea-water underneath these ice-shelves is causing them to collapse. Once they are gone, that can cause the pent-up ice behind the shelves to flow out into the oceans, at a much faster rate. That is something scientists have already seen for several glaciers in Greenland and the Western Antarctic peninsula. The extent to which this will happen in the future depends, crucially, on how warm the sea-water lying under the ice-shelves is.

This satellite image shows Greenland's Helheim glacier where it meets the sea. The glacier is on the left. Large and small icebergs pack the narrow fjord in the right part of the images. Bare ground appears brown or tan, while vegetation appears in shades of red. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra satellite took the image in June 2005

(Image Caption: This satellite image shows Greenland's Helheim glacier where it meets the sea. The glacier is on the left. Large and small icebergs pack the narrow fjord in the right part of the images. Bare ground appears brown or tan, while vegetation appears in shades of red. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA's Terra satellite took the image in June 2005. Image Credit: NASA images created by Jesse Allen, Earth Observatory, using data provided courtesy of NASA/GSFC/METI/ERSDAC/JAROS, and the U.S./Japan ASTER Science Team.)

That is something the team from the University of Arizona believe they have a better handle on. They have incorporated new models of ocean circulation, focused in particular on the layer 200 to 500 meters (650 to 1,650 feet) beneath the surface. This layer is will warm, on average, by some 1.8°F (1°C) over the next century, they say - and that heat is likely to undermine ice-shelves and floating glaciers from underneath.

Interestingly, Antarctica and Greenland will act differently according to the models. Greenland's ocean currents, fed by the Gulf Stream, will rise by up to 3.6°F (2°C), speeding up the melt-rate considerably. Antarctica, by comparison, is protected by the cooler Antarctic Circumpolar Current, which will restrict the warming of its lower waters to 0.9°F (0.5°C). "No one has noticed this discrepancy before - that the subsurface oceans surrounding Greenland and Antarctica warm very differently," Yin said. 

But Antarctica, will continue to feed ice into the sea for much longer - much of its ice-laden landscape is below sea-level, and the penetrating sea will keep eating into its ice-sheets long after the 21st century. Co-author Jonathan Overpeck said, 'This does mean that both Greenland and Antarctica are probably going melt faster than the scientific community previously thought.'