Pools of melted ice and snow that form on the surface of the Arctic sea ice explain why it is melting faster than predicted, scientists say
Scientists have been struggling to understand why the northern sea ice has been retreating at a faster rate than estimated by the most recent assessment of the Intergovernmental Panel on Climate Change (IPCC), in 2007.
The IPCC's computer models had simulated an average loss of 2.5% in sea ice extent per decade from 1953 to 2006. But in reality the Arctic sea ice had declined at a rate of about 7.8% per decade.
Arctic sea ice has retreated so much that in September 2007 it covered an all-time low area of 4.14m km sq, surpassing by 23% the previous all-time record set in September 2005.
And during the summer of 2008, the north-west and north-east passages - the sea routes running along the Arctic coastlines of northern America and northern Russia, normally perilously clogged with thick ice – were ice-free for the first time since records began in 1972.
Part of the reasons for the discrepancy has to do with melt ponds, which are pools of melted ice and snow that form on the sea ice when it is warmed in spring and summer. As they are darker than ice and snow, they absorb solar radiation rather than reflect it, which accelerates the melting process.
"Melt ponds were not taken into consideration by global climate models as sea-ice albedo [the ratio of reflected to incident solar radiation] is a complex process that is poorly described in these models," explains Christina Alsvik Pedersen from the Norwegian Polar Institute, the first author of the research which has been accepted for publication in the Journal of Geophysical Research.
"The inclusion of the melt ponds in the models goes towards explaining why the sea ice in the Arctic melts faster than the models can predict," she says.
Melt ponds may play a growing role in the melting of the Arctic sea ice in future, Pedersen adds, as first-year ice - which melts in summer and freezes in autumn – is replacing the old multi-year ice – which stays frozen regardless of the seasons.
"First-year ice has normally a smoother surface than multi-year ice, which tends to have rougher, ridged surfaces," says Pedersen. "That allows melt ponds to cover a wider area on first-year ice, which extends the surface on which the solar radiation is absorbed, and that will accelerate the melting process."
The research helps our understanding of the physical processes behind the melting of the Arctic, according to Pål Prestrud, author of a 2007 UN report on the melting of the ice and snow and the director of the Centre for international climate and environmental research in Oslo.
"The global climate models have been good at predicting temperature, but when it comes to sea ice, they are not good enough," he said. "This research is one piece of the puzzle that will help us understand the physical process involved in the melting of the Arctic and predict better what will happen in future."
"Another piece of the puzzle we need to understand better is what happens with oceans currents in the Arctic Ocean and the warming of the oceans as a whole," he said.
Researchers at the Max Planck Institute in Hamburg and the SINTEF institute in Trondheim were also part of the team.