Warm ocean waters causing basal melting of Antarctic ice shelves are responsible for most of the continent's ice shelf mass loss, a new study by NASA and university researchers has found. Basal melting from seawater only a few degrees above the freezing point is distributed unevenly around the continent and have a direct impact on ocean circulation.
Scientists have studied the rates of melting from underneath of individual ice shelves, the floating extensions of glaciers that empty into the sea. But this is the first comprehensive survey of all Antarctic ice shelves. The study found basal melt accounted for 55 percent of all Antarctic ice shelf mass loss from 2003 to 2008, an amount much higher than previously thought.
Antarctica holds about 60 percent of the planet's fresh water locked into its massive ice sheet. Ice shelves buttress the glaciers behind them, modulating the speed at which these rivers of ice flow into the ocean. Determining how ice shelves melt will help scientists improve projections of how the Antarctic ice sheet will respond to a warming ocean and contribute to sea level rise. It also will improve global models of ocean circulation by providing a better estimate of the amount of fresh water ice shelf melting adds to Antarctic coastal waters.
The study used reconstructions of ice accumulation, satellite and aircraft readings of ice thickness, and changes in elevation and ice velocity to determine how fast ice shelves melt and compared the mass lost with the amount released by the calving or splitting of icebergs.
"The traditional view on Antarctic mass loss is it is almost entirely controlled by iceberg calving," said Eric Rignot of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the University of California, Irvine. Rignot is lead author of the study that was published in the June 14 issue of the journal Science. "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," he added.
In total, Antarctic ice shelves lost 1,325 trillion kilograms of ice per year in 2003 to 2008 through basal melt, while iceberg formation accounted for 1,089 trillion kilograms of mass loss each year.
Basal melt can have a greater impact on ocean circulation than glacier calving. Icebergs slowly release melt water as they drift away from the continent. But strong melting near deep grounding lines, where glaciers lose their grip on the seafloor and start floating as ice shelves, discharges large quantities of fresher, lighter water near the Antarctic coastline. This lower-density water does not mix and sink as readily as colder, saltier water, and 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," said author Stan Jacobs, an oceanographer at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y.
The three giant ice shelves of Ross, Filchner and Ronne, which make up two-thirds of the total Antarctic ice shelf area, accounted for only 15 percent of basal melting. Meanwhile, fewer than a dozen small ice shelves floating on 'warm' waters produced half of the total melt water during the same period.
The researchers also compared the rates at which the ice shelves are shedding ice to the speed at which the continent itself is losing mass and found that, on average, ice shelves lost mass twice as fast as the Antarctic ice sheet did during the study period.
Rignot said that "In a number of places around Antarctica, ice shelves are melting too fast, and a consequence of that is glaciers and the entire continent are changing as well."
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