Perennial Arctic Ice Cover Diminishing, Officials Say
Wednesday, March 19, 2008
The amount of long-lasting sea ice in the Arctic -- thick enough to survive for as much as a decade -- declined sharply in the past year, even though the region had a cold winter and the thinner one-year ice cover grew substantially, federal officials said yesterday.
Using new data from NASA's ICESat satellite, researchers over the past year detected the steepest yearly decline in "perennial" ice on record. As a result of melting and the southward movement of the thicker ice, the percentage of the Arctic Ocean with this stable ice cover has decreased from more than 50 percent in the mid-1980s to less than 30 percent as of last month.
"Because we had a cold winter, the public might think things have gotten better," said Walter Meier of the National Snow and Ice Data Center at the University of Colorado at Boulder. "In fact, the loss of the perennial ice makes clear that they're not getting better at all."
The surprising drop in perennial ice makes the fast-changing region more unstable, because the thinner seasonal ice melts readily in summer.
The Arctic lost an unprecedented amount of ice during last summer's unusual warmth, and Meier said conditions are right for a similarly large melt if the temperatures are at all above normal this year. The area of thick Arctic ice lost over the past two decades equals 1 1/2 times the size of Alaska.
While normal weather variation plays a role in yearly ice fluctuations, officials said the dramatic decline in perennial ice -- which can range from 6 feet thick to more than 15 feet thick -- appears to be consistent with the effects of global warming.
Officials said the loss of long-lasting ice was less the result of warming of the atmosphere than of a long-term rise in ocean temperatures and the effects of the "Arctic oscillation," a variable wind pattern that can either keep icebergs in the Arctic (when the wind pattern is "negative") or push them south (when it is "positive"). Climate experts believe that both the rising water temperature and increasingly frequent "positive" oscillations are a function of global warming.
Josefino Comiso of NASA's Goddard Space Flight Center, the lead author of a related 2007 study, said Arctic Ocean temperatures appear to be rising quickly because less of the water is covered by ice, which reflects sunlight and keeps water temperatures lower. After last summer's very warm weather, the amount of ice cover shrank dramatically, and the water became warmer.
He said climate experts have concluded that the Arctic oscillation, which is a natural climate phenomenon, is also being modified by global warming. The dynamics are not yet understood, but it appears that higher temperatures in the tropics and elsewhere make it more likely that the oscillation will push icebergs down past Greenland and into the Atlantic.
Arctic sea ice always grows and shrinks, ranging from an average minimum in September of 2.5 million square miles to an average winter maximum in March of 5.9 million square miles. Instruments on NASA's Aqua satellite, as well as Defense Department satellites, showed that the maximum sea ice extent in March increased by 3.9 percent over that of the previous three years because of the winter.
Nonetheless, the total ice coverage was still 2.2 percent below the long-term average. And the very old ice, which remains in the Arctic for at least six years, made up more than 20 percent of the Arctic in the mid- to late 1980s, but by this winter it had decreased to 6 percent.
Flying over the Arctic, one might perceive the sea ice cover as broad, Meier said, but that apparent breadth hides the fact that the ice is so thin. "It's a facade, like a Hollywood set," he said. "There's no building behind it."
While the Arctic sea ice is changing fast, the same is not true in Antarctica. Comiso said the amount of ice surrounding the continent is little changed over recent decades, although some ice loss has been occurring around the continent's peninsula and on some glaciers. Antarctica is significantly less tied to the world's weather patterns and is considered to be less subject to the effects of global warming so far.
The report drew concern from Rafe Pomerance, president of the environmental group Clean Air-Cool Planet.
"This is another startling and serious indicator of massive changes in the Arctic due to climate change," he said in a statement. "It is one more reminder that we must address the global warming with a level of commitment and resources equal to the problem."
With the behavior of Arctic sea ice becoming an increasingly important issue, NASA is planning to launch a follow-on satellite mission, ICESat II, in 2015.