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Study upends theory of how Arctic ice shelves lose mass

Warm ocean water is causing the Antarctic ice shelves to melt from the bottom up, a new study says, contradicting the previously accepted notion that the shelves lose the most mass due to iceberg break-off.

In the study from University of California, Irvine, in conjunction with NASA, researchers said that this bottom-up melting accounted for 55 percent of all Antarctic ice shelf mass loss from 2003 to 2008, which is a much higher amount than previously predicted. This type of melting is called "basal melt."

Previously, scientists had looked at basal melting from each individual ice shelf, but in this study researchers looked at the melting caused by warm ocean water as a comprehensive survey of all Antarctic ice shelves.

"The traditional view on Antarctic mass loss is it is almost entirely controlled by iceberg calving," Eric Rignot, who is part of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the University of California, Irvine, said in a statement. "Our study shows melting from below by the ocean waters is larger, and this should change our perspective on the evolution of the ice sheet in a warming climate."

Ice shelves themselves form due to a combination of events. They begin as land-ice that flows out to sea, and then they grow as snow falls onto their surfaces. The researchers took this into account when calculating basal melt and combined a regional snow accumulation model with a new map of Antarctica's bedrock for their research. Antarctica holds around 60 percent of Earth's fresh water in its ice shelves.

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"Ice shelves mostly melt from the bottom before they even form icebergs," said Rignot. "This has profound implications for our understanding of interactions between Antarctica and climate change. It basically puts the Southern Ocean up front as the most significant control on the evolution of the polar ice sheet."

According to NASA, Antarctic ice shelves lost 2,921 trillion pounds of ice per year in 2003 to 2008 through basal melt, while iceberg formation accounted for 2,400 trillion pounds of mass loss each year.

One reason that this difference is significant is that basal melt can have a greater impact on ocean circulation than iceberg calving. Icebergs break off and discharge large quantities of fresher, lighter water, yet basal melting is a lower-density water and does not mix well with the saltier Ocean water. Scientists predict that this may be changing the rate of bottom water renewal.

"Changes in basal melting are helping to change the properties of Antarctic bottom water, which is one component of the ocean's overturning circulation," Oceanographer Stan Jacobs from Columbia University's Lamont-Doherty Earth Observatory said in a statement. "In some areas it also impacts ecosystems by driving coastal upwelling, which brings up micronutrients like iron that fuel persistent plankton blooms in the summer."

The findings are published in the June 14 issue of Science.