October 28, 2008

Arctic Ice Thinning And Receding, Study Finds

Arctic sea ice has become thinner by as much as 49 centimeters, according to findings from a new study by UK researchers.

After stunning revelations from satellite images, the team from University College London reported the results provided the first definitive proof that the overall volume of Arctic ice was decreasing after being fairly constant for the previous five winters.

"The ice thickness was fairly constant for the five winters before this, but it plummeted in the winter after the 2007 minimum," said lead author Katharine Giles, whose study appears in the journal Geophysical Research Letters.

Up until last winter, the thickness of Arctic sea ice showed a slow downward trend during the previous five winters, but after the summer 2007 record low extent, the thickness of the ice also nose-dived.

Additionally, researchers noted that sea ice is not just receding but it is also thinning.

Sea ice in the Arctic shrank to its smallest size on record in September 2007, when it extended across an area of just 4.13 million sq km, beating the previous record low of 5.32 million sq km, measured in 2005.

The team from the university's Centre for Polar Observation and Modelling - part of the UK's National Centre for Earth Observation - found that last winter the ice had thinned by an average of 26cm below the 2002-2008 winter average.

Dr Giles added that the data also showed the western Arctic experienced the greatest impact, where the ice thinned by up to 49cm.

"The extent can change because the ice can be redistributed, increasing the amount of open water," said co-author Seymour Laxon. "But this does not reduce the overall amount of ice."

"To determine whether the reduction in sea ice extent is the result of ice being piled up against the coast or whether it is the result of melting, you need to measure the thickness."

The team is the first to measure ice thickness throughout the Arctic winter, from October to March, over more than half of the Arctic, using the European Space Agency's Envisat satellite.

Ice thickness can be calculated from the time it takes a radar pulse to travel from a satellite to the surface of the ice and back again.

"I think this is the first time that we can definitively say that the bulk overall volume of ice has decreased," said Dr Laxon.

"So this means melting; it doesn't mean that the ice has just been pushed up against the coastline."

Laxon added that the time when Arctic sea ice is going to disappear is up for debate.

"About five years ago, the average projection for the sea ice disappearing was about 2080," he said.

"But the ice minimums, and this evidence of melting, suggests that we should favor the models that suggest the sea ice will disappear by 2030-2040, but there is still a lot of uncertainty."


Image 2: Arctic sea ice extent as seen by Envisat's Advanced Synthetic Aperture Radar (ASAR) sensor during mid-September 2007 and mid-September 2008. The Arctic sea areas covered by ice in September 2008, but ice-free in September 2007, are visible in blue. The Arctic sea areas covered by ice in September 2007, but ice free in September 2008, are visible in dark brown. The Arctic sea covered by ice both in September 2007 and September 2008 are visible in light brown. Credits: ESA

Image 3: Circumpolar average winter season (October to March) ice thickness anomalies (from Katharine A. Giles, Seymour W. Laxon and Andy L. Ridout, Circumpolar thinning of Arctic sea ice following the 2007 record ice extent minimum, Geophysical Research Letters). Credits: Giles - Laxon - Ridout


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