By Fred Pearce
5 August 2013
(New Scientist) – Arctic ice is losing its reflective sheen. It's common knowledge that each summer, more and more of the ice melts leaving the dark waters of the ocean uncovered – a process that accelerates global warming by reducing the amount of solar radiation reflected back into space. Now it turns out that the surviving sea ice is also becoming darker and less reflective.
For the first time, a detailed analysis of 30 years of satellite data for the Arctic Ocean has quantified how much the albedo, or reflectivity, of Arctic ice is diminishing. Aku Riihela of the Finnish Meteorological Institute told New Scientist he estimates that darker ice means the Arctic Ocean's albedo at the end of the summer is of the order of 15 per cent weaker today than it was 30 years ago.
The cause of the darkening, says Riihela, is partly due to thinning ice and the formation of open water fissures, and partly because in the warmer air, ponds of liquid water form on the surface of the ice. The shallow ponds on the ice can dramatically reduce reflectivity and increase the amount of solar radiation that the ice absorbs. "This shows that the increasing melt affects the inner Arctic sea ice, too," said Riihela.
Earlier this year, Marcel Nicolaus of the Helmholtz Centre for Polar and Marine Research in Bremerhaven, Germany, reported a trans-polar study aboard a German icebreaker, which found that "more than 50 per cent of the ice cover now consists of thin one-year ice on which the meltwater is particularly widespread".
The melting and darkening of the Arctic is a major factor in climate change. It acts as a positive feedback, because the more ice melts or darkens, the more the Arctic warms and the more ice melts.
It may help explain the speed of Arctic ice loss, which far exceeds the predictions of existing climate models, including those used in the 2007 climate assessment of the Intergovernment Panel on Climate Change.
The authors of the new paper have not yet calculated the effect of their findings on those predictions. But they can only hasten the day when the Arctic is ice-free in summer.
Journal reference: Nature Climate Change: DOI: 10.1038/nclimate1963