Sea levels worldwide will rise higher than anticipated, thanks to a new once-stable region of Antarctica that is suddenly melting, and at a fast rate. Analysis of satellite data shows that although the massive ice sheet on the southern Antarctic Peninsula, made up of multiple glaciers, was rock solid from 2000 to 2009, since then it has begun to melt rapidly. The glaciers, stretching along 750 kilometers of coastline, are shedding 60 cubic kilometers of ice into the ocean each year—about 70,000 Empire State Buildings of ice annually.
The onset of such rapid loss “came as a surprise to us," says Bert Wouters, a fellow at the University of Bristol in England, who led the analysis, published in the May 22 Science. In just a few years, he says, the dynamics of the region “completely shifted."
The surface of some of the glaciers is dropping by as much as four meters each year, as measured by remote-sensing instruments on the CryoSat 2 satellite. The ice loss is so great it is also causing changes in Earth’s gravitational field, which have been detected by GRACE (Gravity Recovery and Climate Experiment) satellites.
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Wouters says the glaciers’ quick disappearance is not caused by a reduction in annual snowfall or by warmer air temperatures. It is caused by thinning ice shelves; bulky glacial sheets on land transition into large, flat ice shelves that float on the ocean. When the shelves are thick, they slow or even stop the glaciers they are connected to from gently sliding into the sea, at the mercy of gravity. But if the shelves thin too much they can no longer hold back the enormous ice mass on land, and the glaciers accelerate their march into the ocean. This mechanism already has allowed glaciers in other regions of Antarctica to speed up their progress toward the sea.
Overall, the ice shelves along the southern Antarctic Peninsula have lost almost one fifth of their thickness since the early 1990s. Scientists say the likely cause is a change in winds across the Southern Ocean, a result of climate change. The shifting winds are pushing warmer water toward the ice shelves, melting them from below, and against the glacial ice along the coast, melting it as well. Around 2009, Wouters says, thinning of the ice shelves and melting of the glaciers “passed a critical threshold which triggered the sudden ice loss.”
The rate of melting could continue to rise as the ice sheets continue to thin. And another geographic feature could play a bigger role. The land under the glaciers, already below sea level at the coast, drops even more in elevation inland. As glaciers melt at the sheet’s front edge, seawater could flow downhill under the retreating ice mass, melting it further.
Because of these factors, in a scant few years the southern Antarctic Peninsula has become the second largest contributor to sea level rise in Antarctica, Wouters’ team found. First is the Amundsen Sea sector, which is adding 100 to 120 cubic kilometers of ice into the oceans each year, he says. Wouters is concerned that the southern peninsula could become an even bigger player in the near future. The Amundsen Sea sector, he says, probably began losing ice in the 1970s, and the rate of loss increased gradually. In the southern peninsula, he notes, the change has been “much more rapid.” And “once dynamic ice loss has been initiated,” he adds, “it is hard to stop.”