SAN FRANCISCO -- Arctic ice and snow hit new record lows this year, more evidence that man-made warming is transforming the frigid far north into a warmer, greener place, scientists said yesterday.
Arctic snow extent dipped to a new minimum last spring, and sea ice cover fell to less than half its long-term average in late summer, at the end of its annual melt. A brief but dramatic surface thaw across most of the Greenland ice sheet shocked scientists in July. And permafrost reached new record-high temperatures in northern Alaska.
"Conditions in the Arctic are changing in both expected and in sometimes surprising ways," said National Oceanic and Atmospheric Administration chief Jane Lubchenco, whose agency released its sixth annual "Arctic report card" yesterday at the American Geophysical Union fall meeting.
The peer-reviewed analysis, compiled by 141 scientists in 15 countries, describes a region undergoing a dramatic transformation.
"Multiple observations provide strong evidence of widespread, sustained changes that are driving the Arctic environmental system into a new state," said Martin Jeffries, Arctic science adviser at the Office of Naval Research, who helped edit the report.
Those developments are particularly striking, experts said, because temperatures across most of the Arctic were not unusually warm this year compared to conditions over the past decade.
Greenland, however, bucked that regional trend. Last summer was the warmest there since record-keeping began 170 years ago. And it was punctuated by a period of short-lived but intense melting over most of Greenland's ice sheet surface, including high-elevation "dry-snow zones" that have not thawed in recent memory.
For a handful of days in mid-July, 97 percent of the ice sheet surface showed signs of thaw, covering a larger area than any melting event since satellite records began in 1979, NASA reported. A similar but shorter event followed later that month.
"I've studied Greenland for 20 years now," said Jason Box, a glaciologist at Ohio State University. "I've devoted my career to it. And 2012 was an astonishing year, especially in summer."
Warming twice as fast as the globe
As its melt accelerated, Greenland's massive ice sheet also became less reflective. Bright snow disappeared, leaving behind bare ice that is less reflective and therefore absorbs more heat from the sun.
Similarly, as Arctic sea ice headed to a new record-low summer extent in mid-September, highly reflective ice gave way to darker ocean water. Sea ice cover ultimately dropped 49 percent below the 1979-2000 average, besting the previous record set in 2007 by 16 percent.
Both trends have helped accelerate warming in the Arctic, where temperatures are rising twice as fast as the global average.
"The Arctic is one of the Earth's mirrors, and that mirror is breaking," said Don Perovich, a sea ice expert at Dartmouth College.
Those changes are affecting the region's biology, as tundra marches north, sending up green shoots in areas where moss and lichen or even ice once dominated. Scientists are also concerned about the fate of species that depend on sea ice for hunting and breeding grounds, such as polar bears and walruses, as that ice disappears.
In the Arctic Ocean, thinning, shrinking sea ice cover is allowing more light to penetrate the water column, sparking massive blooms of microscopic phytoplankton, with unknown consequences for the larger marine food web. "If we are not already there, we are surely on the verge of seeing a new Arctic," Jeffries said.
But the effects of Arctic warming also reach beyond the region's borders, scientists believe.
Over the past seven years, a new weather pattern has emerged, sending blasts of warm southern air into the Arctic each June. One recent analysis linked those shifting summer winds to record thaws of the Greenland ice sheet, unusually wet European summers and Rocky Mountain wildfires.
Others have suggested that Arctic warming is changing the behavior of atmospheric circulation over mid-latitudes in fall and winter, fueling extreme weather like the powerful snowstorms that buried the U.S. East Coast during the winters of 2009-2010 and 2010-2011.
"The impact of adding that heat on the planet does influence mid-latitude weather and storms," said Jim Overland, an Arctic expert at NOAA's Pacific Marine Environmental Laboratory. "Rather than the normal jet stream going just straight from west to east, it creates a wavier pattern that can paradoxically add more snow and heat at mid-latitudes."
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