Friday, 07 October 2011 09:59
First a recap: The summer destruction of the polar cap officially ended on Sept. 9 with the second smallest extent ever observed during the age of satellites, according to the newest analysis posted by the National Snow & Ice Data Center. Plunging polar temperatures have been refreezing the Arctic for weeks.
Ice covered about 1.78 million square miles during September -- more than 40 percent lower than the monthly average recorded between 1979 and 2000 and only 120,000 square miles above the minimum seen in 2007.
To put that data in perspective, it's as though a marine habitat almost the size of Alaska, Washington, Oregon and California combined has disappeared from the Arctic's September world. But 2011 still retained more ice than the minimum seen in 2007 by NSIDC calculations, holding onto floes that would cover an area about the size of New Mexico.
Don't exhale with relief just yet, scientists say. Ice shrink in 2011 may have been more severe than it might appear (and a German team of scientists say the record did fall; more on that below).
Although sea ice melted at double the average rate during early summer -- opening the Northern Sea Route and Northwest Passage earlier than ever before -- 2011 still lacked the sort of unusual weather thought to have triggered the 2007 record. In fact, much of the late summer favored ice preservation, which raises some troubling questions about the true meaning of this year's second-place finish in the race to the bottom.
"Why did ice extent fall to a near record low without the sort of extreme weather conditions seen in 2007? One explanation is that the ice cover is thinner than it used to be," the NSIDC says here.
Ice that was less than two years old covered 80 percent of the Arctic basin last March -- much less than the long-term average. While new ice has generally been surviving summer melt since 2007, the polar sea has been steadily losing the oldest and thickest ice at the same time, especially in the Beaufort and Chukchi seas north of Alaska.
Thus, 2011 flirted with a new record partly because Alaska's royal blue, multi-year ice cap -- that frigid wilderness of epic floes that once wiped out Yankee whaling fleets and inspired the international rescue of trapped gray whales -- is almost gone.
"Continued loss of the oldest, thickest ice has prevented any significant recovery of the summer minimum extent," federal scientists concluded. "In essence, what was once a refuge for older ice has become a graveyard."
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The extent of summer ice in the Arctic -- defined as the area covered by at least 15 percent ice -- has been sliding for almost 30 years. Some scientists say the Arctic will lose virtually all of its summer ice cover within decades, and they blame this loss on rising temperatures and complex shifts in Arctic storm patterns.
The ongoing summer meltdown stresses animals that need the solid floes for hunting and denning platforms. When sea ice melts out of existence, habitat necessary to maintain healthy populations of polar bears, walruses and seals simply disappears. The loss of summer ice in the Beaufort Sea led the United States to list polar bears as threatened under the Endangered Species Act and designate a span of ocean north of Alaska as critical habitat.
Loss of summer ice also directly contributes to climate change because darker, open water absorbs more solar energy than white floes. The exposed ocean heats up faster and then causes even more ice to melt. The complete elimination of a summer ice cap -- something that's never occurred during the entire era with modern humans as a species -- could alter global climate on scales unseen in a geologic age.
Although the 2011 season might not have set a new record according to the satellite used for decades by the NSIDC, other indicators suggest that sea ice has never fared worse.
The total volume of sea ice -- size of the surface footprint plus all the ice hidden beneath the surface -- appears to have plunged to about the same unprecedented levels seen last year and possibly lower, according to the newest monthly analysis posted by the Polar Science Center at the University of Washington. At an estimated 1,000 cubic miles, the latest sea ice volume was two-thirds less than the 30-year average and 75 percent lower than the maximum seen in 1979.
Different estimates of sea ice extent by a European team found that 2011 did in fact drop slightly below the 2007 level, according to this mid-September story posted by the European Space Agency.
"The University of Bremen reports that their data shows the Arctic Sea ice extent reached a new historic minimum ... on Sept 8, 2011," reports the Oct. 5 post on the Arctic Theme page sponsored by NOAA. "Both the Northwest Passage and the Northern Sea route appear to be open."
Using a microwave system aboard the NASA satellite Aqua, the Bremen scientists say that ice extent dipped some 10,400 square miles below the minimum seen in 2007 -- breaking the record with the elimination of ice covering an area about the size of Vermont.
"It seems to be clear that this is a further consequence of the man-made global warming with global consequences," according to this English translation of "Alerting message from the Arctic" posted last month by the German team. "Directly, the (livelihood) of small animals, algae, fishes and mammals like polar bears and seals is more and more reduced."
The two reports of sea ice minimum -- one slightly above the record and other slightly below -- don't necessarily contradict, scientists have explained. Their satellites use different capacities to gather data about ice conditions over a vast region and are reflecting the same overall trend.
"The common and important point to our understanding is that all methods find consistently that all minima since 2007 have been lower than all minima before," the German scientists explained. "The last five minima (2007-2011) are the four lowest on record."
Thrills and chills: Sorting out 2011 vs. 2007
So what drove the two years so low? Don't look for simple answers. The dynamic of air and water, weather and sea conjures a complicated picture.
For instance, the 2007 record year experienced unusually high pressure over the Beaufort Sea -- with clear skies and warm conditions, the NSIDC explained, and unusually low pressure on the other side of the Arctic in the Kara Sea.
"This pattern resulted in strong southerly winds from the Bering Strait region across the North Pole, which brought warmer winds and ocean waters northward to melt the ice edge and push the ice northward," the NSIDC explained. "In addition, especially strong high pressure over the Beaufort and Chukchi Seas in June 2007 resulted in less than average cloudiness, allowing more sunlight to reach the ice."
A similar pattern occurred in 2011, but it was not as strong or as persistent. Winds blew east to west rather than north, tending to spread ice out during August and increase the extent. At the same time, sea surface temperatures were above average but not as high as 2007.
"These lower temperatures may be the result of less solar heating of the exposed ocean surface or less transport of warm waters from the south," the NSIDC reported.
"In 2007, ice retreated early from the shores of Alaska and Siberia, allowing the ocean mixed layer to heat up and enhance melting of the ice from below. In contrast, ice was slower to retreat in this region in summer 2011, and less bottom melt was observed."
Originally posted October 6, 2011