April 6, 2009
Satellites Show Arctic Sea Ice Continues To Shrink
NASA and the National Snow and Ice Data Center say that the decade-long trend of shrinking sea ice cover in the Arctic is continuing and new evidence suggests that the ice cap is thinning as well.
Arctic sea ice naturally cools air and water masses and plays an important role in ocean circulation as well as reflecting solar radiation back into space.
Satellite monitoring of Arctic sea ice showed that this winter had the fifth lowest maximum ice extent on record. And those who monitor the ice say the six lowest maximum events since satellite monitoring began in 1979 have all happened between 2004-2009.
Charles Fowler, the leader of a team of researchers at the University of Colorado, Boulder, said the majority of Arctic sea ice survived at least one summer and often several until the last few years.
Researcher Walt Meier said seasonal ice, which melts and re-freezes every year, now comprises about 70 percent of Arctic sea ice in winter, up from 40 to 50 percent in the 1980s and 1990s. Thicker ice that has survived two or more years now comprises just 10 percent of ice cover, down from 30 to 40 percent in years past.
The maximum sea ice extent for 2008-09 was 5.85 million square miles"”278,000 square miles less than the average extent for 1979 to 2000, according to researchers from the National Snow and Ice Data Center in Boulder.
Meier said ice extent is an important measure of the health of the Arctic, but it only gives a two-dimensional view of the ice cover.
"Thickness is important, especially in the winter, because it is the best overall indicator of the health of the ice cover. As the ice cover in the Arctic grows thinner, it grows more vulnerable to melting in the summer," he said.
Each winter, the sun sets for several months and intense cold sets in, causing the Arctic ice cap to grow. While some of that ice is naturally pushed out of the Arctic by winds, most of it melts in place during summer.
Experts say the thicker, older ice that survives one or more summers is more likely to persist through the next summer.
In the past, scientists have typically used estimates of ice age to approximate its thickness since it has been hard to measure directly.
However, last year saw the first map of sea ice thickness over the entire Arctic basin. Ron Kwok of NASA's Jet Propulsion Laboratory in Pasadena, Calif. produced the map with a team of researchers.
Kwok's team estimated the thickness and volume of the Arctic Ocean ice cover for 2005 and 2006 by using two years of data from NASA's Ice, Cloud, and land Elevation Satellite (ICESat).
Kwok warned that the older, thicker sea ice is declining and is being replaced with newer, thinner ice that is more vulnerable to summer melt.
Seasonal sea ice averages about 6 feet in thickness, while ice that had lasted through more than one summer averages about 9 feet.
The researchers are currently attempting to extend the ICESat estimate further, from 2003 to 2008, to see how the recent decline in the area covered by sea ice is mirrored in changes in its volume.
Kwok said that with the new data on both the area and thickness of Arctic sea ice, they would be able to better understand the sensitivity and vulnerability of the ice cover to changes in climate.
The Arctic ice cap grows each winter as the sun sets for several months and intense cold sets in. The total volume of winter Arctic ice is equal to the volume of fresh water in Lake Superior and Lake Michigan combined.
Image 1: Data from NASA satellites show younger, thinner Arctic sea ice is replacing multi-year ice. Credit: James Maslanik, University of Colorado
Image 2: This data visualization from the AMSR-E instrument on the Aqua satellite show the maximum sea ice extent for 2008-09, which occurred on Feb. 28, 2009. Credit: NASA Goddard's Scientific Visualization Studio
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