Winter Arctic sea ice this year was the second smallest area on record in a sign of greenhouse warming, U.S. climate scientists said on Wednesday.
Sea ice extent, or the area of ocean that is covered by at least 15 percent ice, was 5.7 million square miles in March, the Colorado-based National Sea and Ice Data Center said on Wednesday. March usually marks the end of winter in the Arctic, a period when sea ice recovers from the summertime minimum.
This March's ice level represented a slight recovery from the record low during the same month last year when the ice extent was 5.6 million square miles. But low sea ice levels this winter — the world's warmest on record, according to the U.S. government — are part of a trend toward less ice.
"This long-term trend, which seems to be accelerating, is really an indication of a warming, and the only way you get the warming is with greenhouse gases," said NSIDC research scientist Walt Meier. On a global level, carbon dioxide, the main greenhouse gas, is being released in growing amounts from tailpipes and smokestacks.
March 2007 Arctic sea ice extent was about 7 percent smaller than the average from 1979, the first full year satellites recorded it, to 2000.
As the world warms, the extent of sea ice surrounding both poles has concerned scientists as its melting can create a feedback loop that leads to ever more warming. Ice and snow reflect solar heat back into the atmosphere. But when more of it melts, the ocean absorbs more heat, which in turn can cause more warming.
A draft U.N climate report due on Friday said climate change at the poles could threaten indigenous populations with destruction of permafrost and cut habitat for migratory birds and mammals, with "major implications" for predators such as seals and polar bears.
The polar bear population in Canada's Hudson Bay has dropped to about 950 in 2004 from 1200 in 1989, a decline of 22 percent, according to scientists at NASA's Goddard Space Flight Center in Maryland.
Meier said low winter sea ice can threaten wildlife habitat because it means the ice is freezing up later and melting earlier in the year.
Over the last few winters sea ice has "safely" been at the lowest levels since at least a brief cooling period during mid-1800s, when the world emerged out of a period of warming during the Middle Ages, Meier said.
NSIDC, which is part of the University of Colorado and is affiliated with the U.S. government's National Oceanic and Atmospheric Administration, uses shipping logs to look at data that far back.
Meier said he expects winter sea ice levels to be low or lower in future years.