for National Geographic News
The Arctic's fastest known August ice retreat has left the region with just a little more ice than in 2007, when ice hit the lowest levels on record, scientists say.
This week the ice has already started freezing back up for winter, but the rapid loss in August points to long-term, significant declines in perennial—or year-round—ice cover.
"I expected that the perennial ice this year would be similar to that of 2005 and 2006," said Josefino Comiso of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
"But in August, it started going at a much more rapid rate. It almost got to as low as 2007," he said.
(Related: "Arctic Ice in 'Death Spiral,' Is Near Record Low [September 17, 2008].)
The ice cover at the end of last summer, Comiso said, was 38 percent less than average and 27 percent less than the previous lowest perennial ice cover.
Reasons for Decline
As temperatures warm due to global warming, Arctic ice is getting thinner, which makes it easier to melt—especially in warmer water.
The oldest ice can be meters thick, having grown over the course of many years. A significant portion of these ice chunks have floated out of the Arctic into the warmer Atlantic Ocean and melted, Comiso said.
Thick ice that melts or floats away in summer is replaced by thin ice, formed over the course of one winter, and this thin ice melts even quicker the following summer, he explained.
Meanwhile, Arctic Ocean water that used to be covered by ice—which deflects much of the light and warmth of the sun—now absorbs that solar energy, causing it to warm further.
Comiso noted that according to scientists in the field, Arctic ice has been melting even faster from the bottom than it has from the top.
This suggests that the warming of the Arctic water is a more important factor affecting the decline of summer ice, he said.
Since the satellite data used for this study tracks only the two-dimensional ice cover, bottom decline is not observable until the ice cover totally melts. This may also have contributed to the rapid August decline.
(Explore a vanishing Arctic sea ice interactive.)
Strong and Steady
Mark Serreze is a senior research scientist at the National Snow and Ice Data Center (NSIDC) based at the University of Colorado, Boulder, who also monitors Arctic sea ice.
"In most years [indeed even in 2007] the pace of ice melt slows in August in response to declining sunlight," Serreze said in an email.
"This summer, however, ice melt stayed strong and steadily through the month, in part due to a pattern of atmospheric circulation fostering warm conditions over the ocean north of Siberia."
Serreze and his colleague's found ice loss in spring and early summer concentrated in the Beaufort Sea, while the August retreat was greatest in the Chukchi and East Siberian Seas, according to an NSIDC update posted at the center's Web site.
"Ice loss in the Chukchi and East Siberian Seas averaged 14,000 square kilometers (5,400 square miles) per day faster than in 2007," the report reads.
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