The first comprehensive survey of Antarctica’s ice shelves finds that melting is occurring at a rate unprecedented in modern history. Warming oceans are literally dissolving away the ice from underneath, according to lead author Eric Rignot, a professor at the University of California, Irvine, and researcher with NASA’s Jet Propulsion Laboratory in Pasadena, Calif. The study is published today, June 14, in the journal Science.
Before this new study, scientists believed that iceberg calving–when large chunks of ice break off a glacier and fall into the sea–was the dominant form of ice loss in Antarctica. Now, researchers find that 55 percent of all ice shelf loss from 2003 to 2008 was due to basal ice melt, an amount much greater than previously thought. According to Rignot, the finding has significant implications for scientists’ understanding of how the frozen continent responds to climate change and demonstrates how the Southern Ocean is the most important control on the evolution of the polar ice sheet.
Armed with an array of measurement techniques, including satellite observations, computer models of regional snow accumulation, a new map of Antarctica’s bedrock, and data from NASA’s Operation IceBridge, Rignot and his colleagues were able to determine the proportions of glacial ice lost through icebergs and ocean melting. Although there were regional differences, overall the loss of ice from basal melt between 2003 and 2009 exceeded that from iceberg calving, with the continent’s ice shelves losing 2,400 trillion pounds (1,089 trillion kilograms) of ice through iceberg formation and 2,921 trillion pounds (1,325 trillion kilograms) from under-shelf thawing.
Ocean circulation is more greatly affected by basal melt than by glacier calving, according to a NASA press release issued June 13. While icebergs gradually melt as they drift into the sea, melting of ice shelves’ underbellies discharges large amounts of fresher, lighter water near Antarctica’s coastline. Because lower-density water mixes and sinks less readily than colder, saltier water, it may alter the rate at which bottom water is renewed.
“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 co-author Stan Jacobs, an oceanographer at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York. This also has an impact on ecosystems in some areas, Jacobs explained, by causing an upwelling in coastal waters with a resulting increase in micronutrients that feed plankton blooms during the summer months.
While ice shelf melt can be compensated for by ice flow and does not necessarily mean an ice shelf is completely breaking down, Rignot cautioned that there are a number of places around Antarctica where ice shelves are melting too quickly, resulting in a change in glaciers and in the entire continent.
The discovery comes as international policy makers around the world are considering laws aimed at limiting the emissions of greenhouse gases, including carbon dioxide. Earlier this month, President Barack Obama and his chinese counterpart reportedly reached a deal to curb greenhouse gas emissions, which was widely seen as a significant deal for supporters of policies aim at thwarting climate change.