Wednesday, July 8, 2009

Nasa satellites reveal extent of Arctic sea ice loss

Study - based on satellite measurements - among first to estimate the thickness of the Arctic ice, rather than surface area

The Earth is going thin on top. A new study has revealed that the Arctic Ocean's permanent blanket of ice around the North Pole has thinned by more than 40% since 2004.

Scientists said the rapid loss was "remarkable" and could force experts to reassess how quickly the Arctic ice in the summer may disappear completely. They blame the loss on global warming, which has driven temperatures in the Arctic to record highs and summer ice extent to recent lows.

The study, based on satellite measurements, is among the first to estimate the thickness of the Arctic ice, rather than just its surface area.

Ron Kwok, senior research scientist at Nasa's Jet Propulsion Laboratory in California, said: "Even in years when the overall extent of sea ice remains stable or grows slightly, the thickness and volume of the ice cover is continuing to decline, making the ice more vulnerable to continued shrinkage."

The study looked at measurements taken of the Arctic region by the Icesat satellite, launched in 2003.

Overall, the experts found that the ice, typically up to about 3m thick, thinned by 67cm over the last four winters.

Converting to ice volume, the scientists worked out the amount of so-called multiyear ice, which persists through Arctic summers, had decreased in the winter by up to 6,300 cubic kilometres since 2005 – a decline of more than 40%. The research is published in the Journal of Geophysical Research: Oceans.

Kwok said: "Ice volume allows us to calculate annual ice production and gives us an inventory of the fresh water and total ice mass stored in Arctic sea ice. Our data will help scientists better understand how fast the volume of Arctic ice is decreasing and how soon we might see a nearly ice-free Arctic in summer."

Earlier this year, scientists warned that sea ice volume reached a record low in 2008 due to an unusually high proportion of the thinner first year ice.

Donghui Yi, a scientist with Nasa's Goddard Space Flight Centre in Maryland who worked on the study, said: "The main reason [for the ice thinning] is the growth in melting of the multiyear ice, which is caused by the warmer weather. More ice melts in summer and then you get less in winter."

The Arctic ice cap fluctuates with the seasons, growing in the freezing winter and shrinking over the summer. An important finding of the study is that the majority of Arctic ice no longer survives the summer. In 2003, this multiyear ice made up 62% of the region's total ice volume. By 2008, this was down to 32%. The remaining 68% was "first-year" seasonal ice, which was open water during the summer, so is thinner and more likely to melt away.

Yi said it would be dangerous to try to use the results to predict when the Arctic might be ice-free in summer.

"Some people might do that but I wouldn't like to do those kinds of linear predictions." More years of data are needed to make an informed judgement, he said.

Unfortunately, the Icesat's data gathering days are nearly over. Two of its three lasers, used to judge the sea ice height, have already failed and the third is on its last legs. Icesat2, the satellite's replacement, is not planned for launch until 2014.

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