January 8, 2010 – Original Source: New Scientist
Antarctica is warming, but not melting anything like as much as expected. In fact, during the continent’s summer this time last year, there was less melting than at any time in the 30 years that we have had reliable satellite measurements of the region.
The apparent contradiction is explained by the seasonal pattern of warming, say two glaciologists writing in Eos, the weekly newspaper of the American Geophysical Union. The continent’s winters and springs have warmed most, but it is still too cold in these seasons for anything to melt. Melting in Antarctica happens almost entirely in the summers, which have warmed very little, say Andrew Monaghan of the National Center for Atmospheric Research in Boulder, Colorado, and Marco Tedesco of the City College of New York.
John King of the British Antarctic Survey, based in Cambridge, warned against misinterpreting the lack of summer warming. “Climate change denialists will use this work as evidence that Antarctica is not warming, despite the authors saying their works show no such thing,” he said.
Every year is different, says Tedesco. “In 2005, we had summer melting occurring inland as well as over the coastal ice shelves, and over areas up to 2500 metres above sea level.” And even during the exceptionally low melt of last summer, ice on the Antarctic peninsula, which stretches out towards South America, continued to melt. The Wilkins ice shelf, which is attached to the peninsula, has been collapsing rapidly since February 2008.
The continent’s huge ice sheets contain enough frozen water to raise sea levels globally by around 60 metres. Tedesco and Monaghan say the main factor in how much they warm each summer is the strength of the winds that circle the continent. Circumpolar winds act as a barrier to warm air. They have become stronger over the past four decades, effectively sealing off most of the continent each summer from the effects of global warming.
The circumpolar winds appear to have strengthened because the ozone layer in the stratosphere has thinned. This has made the lower stratosphere cooler and generated stronger winds beneath. But Tedesco warns that as the ozone hole heals in the coming decades, the winds will weaken, the continent will become much warmer in summer – and melting will increase.