Page last updated at 11:49 GMT, Tuesday, 7 April 2009 12:49 UK

Arctic ice shows winter thinning

By Richard Black
Environment correspondent, BBC News website

Iceberg
Recent summers have seen on average less ice floating in the Arctic Ocean

Arctic ice reached a larger maximum area this winter than in the last few years, scientists say, but the long-term trend still shows it declining.

The 30-year trend shows the maximum annual sea-ice cover, usually seen in March, is shrinking by 2.7% per decade.

Only 10% of the cover consists of relatively durable ice that has formed over more than two years, a record low.

Scientists from the US National Snow and Ice Data Center (NSIDC), say the thin ice is prone to summer melting.

"Thickness is important, especially in the winter, because it is the best overall indicator of the health of the ice cover," said NSIDC scientist Walt Meier.

We're not set up well for summertime; we're in a very precarious situation
Walt Meier

"As the ice cover in the Arctic grows thinner, it grows more vulnerable to melting in the summer."

In the 1980s, thick multi-year ice made up 30-40% of the cover, the scientists say.

The summer minimum area is changing much faster than the winter maxima, shrinking by about 0.7% per year. Last year UK researchers showed that the ice has also markedly thinned in recent years.

Space eyes

The Arctic sea-ice reached its maximum extent this year on 28 February, slightly earlier than usual, and remained roughly constant through March.

Averaged over March, the sea-ice covered 15.16 million sq km (5.85 million sq miles).

Graph
The maximum sea ice extent is declining by about 2.7% per decade

By comparison, this was 590,000 sq km (228,000 sq miles) below the average for the years 1979 to 2000, and 730,000 sq km (282,000 sq miles) above the record low of 2006.

The winter saw big variations in Arctic air temperatures, with some areas much warmer and others cooler than average.

Some parts of the region including the Barents Sea experienced air temperatures 4C above the long-term average, while others, including the Bering Sea, were as much as 2C below the average - a pattern reflected in the areas where the thick, multi-year ice accumulated.

The data comes principally from two Nasa satellites.

IceSat measures the height by which the ice rises above the surrounding ocean, which can be used to calculate the overall ice thickness. Meanwhile, the Quikscat satellite can distinguish between multi-year and newly formed ice using differences in the way they scatter light.

The shrinkage in Arctic ice area and volume carries implications for climate change globally. Dark water absorbs more of the Sun's energy than reflective white ice, so the decline of sea-ice will act to amplify a warming trend.

NSIDC researchers believe that a warm summer could see a major melt.

"We're not set up well for summertime," said Dr Meier. "We're in a very precarious situation."

Forecasts of the date by which Arctic summers will be ice-free range from five years to several decades, with natural climatic cycles playing an important role.

Richard.Black-INTERNET@bbc.co.uk



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