Located in the northern hemisphere and mostly in the polar region, Arctic is covered by ice right through the year, the ice cover melting or freezing depending on seasonal variations.
Now it has been found that the area of floating ice in the Arctic has shrunk more than in any summer since satellite tracking began in 1979 and it reached that record point a month before the annual ice pullback typically peaks.
The cause is probably a mix of natural fluctuations, like unusually sunny conditions in June and July, and long-term warming from heat-trapping greenhouse gases and sooty particles accumulating in the air, according to several scientists.
William L. Chapman, who monitors the region at the University of Illinois Urbana-Champaign and posted a Web report on the ice retreat, said that only an abrupt change in conditions could prevent far more melting before the 24-hour sun of the boreal summer sets in September.
"The melting rate during June and July this year was simply incredible," Chapman said. "And then you've got this exposed black ocean soaking up sunlight and you wonder what, if anything, could cause it to reverse course."
But Mark Serreze, a sea-ice expert at the National Snow and Ice Data Center in Colorado estimates differed somewhat from those of the Illinois team, and by the ice center's reckoning the retreat had not yet surpassed the satellite-era record set in 2005.
Still it was close even by their calculations he said, adding that it is almost certain that by September, there will be more open water in the Arctic than has been seen for a very long time. Ice experts at NASA and the University of Washington echoed his assessment.
Dr. Serreze said that a high-pressure system parked over the Arctic appeared to have caused a "triple whammy" keeping away clouds, causing winds to carry warm air north and pushing sea ice away from Siberia, exposing huge areas of open water.
The progressive summertime opening of the Arctic has intensified a longstanding international tug of war over shipping routes and possible oil and gas deposits beneath the Arctic Ocean seabed.
Last week, Russians in two mini-submarines planted a flag on the seabed at the North Pole. On Wednesday, Stephen Harper, the Canadian prime minister, kicked off a tour of Canada's Arctic holdings, pledging "to vigorously protect our Arctic sovereignty as international interest in the region increases."