Sea ice on the Arctic Ocean shrank to its second-smallest extent since modern records began, in keeping with a long-term trend, the U.S. National Snow and Ice Data Center reported on Thursday.
The annual sea ice minimum was reached on September 9, the center said on its website here in a preliminary finding.
"Changing winds could still push ice flows together reducing ice extend further," the researchers said. A full analysis will be available in October, when monthly data are available for all of September, which is usually the month when the annual minimum is reached.
Arctic Sea ice is an important sign of a changing climate, and what happens in the Arctic has a major influence on global weather patterns.
At its apparent minimum, sea ice around the North Pole covered 1.67 million square miles (4.33 million square km). That measurement is 61,800 square miles (160,000 square km) above the all-time record low reached in 2007, the center said.
However, it is far below the average minimum for the period 1979 through 2000, according to NSIDC. The satellite record began in 1979.
These figures differ from those reported by the University of Bremen in German, which issued a statement that the Arctic ice reached a record low minimum on September 8.
PATCHES OF WATER AMID THE ICE
Both the University of Bremen and NSIDC use microwave sensors to observe Arctic ice, but these sensors are on different satellites. The Bremen report uses images with higher spatial resolution, according to Walter Meier of NSIDC.
"They can see in more detail, they can see these little patches of water, whereas we see these areas as just ice covered," Meier said by telephone. He said there can be higher potential for error with these high-resolution images, though there is no evidence of error in this case.
NSIDC's records go back to 1979; the records used by Bremen go back to 2003. Both indicate the last five years were the least icy in the Arctic sea ice satellite record.
It's not surprising that this year has not eclipsed the record year of 2007, Meier said.
That year was "a perfect storm" of ice-melting conditions in the Arctic, he said: warmer and sunnier than usual, with extremely warm ocean water and winds all acting in concert.
The fact that 2011 has seen the second-lowest ice extent without these extreme conditions shows a change in the character of the ice cover, Meier said.
Back in 2007, the ice was a consolidated mass which melted from the edges. This year, he said, the ice is more dispersed and the area is dominated by seasonal ice cover -- less hardy than multi-year ice -- which is more prone to melt.
"Now it doesn't take as extreme of weather conditions to get to the 2007 ballpark," Meier said.
(Reporting by Deborah Zabarenko in Washington, Editing by Cynthia Osterman)