Arctic Ocean sea ice has melted to the second lowest minimum since satellite observations began, according to scientists at the National Snow and Ice Data Center.
Sea ice melt recorded on Monday exceeded the low recorded in 2005, which had held second place.
With several weeks left in the melt season, ice in summer 2008 has a chance to diminish below the record low set last year, according to scientists at the National Snow and Ice Data Center.
Environmental groups said the ice melt was another alarm bell warning of global warming.
"It's an unfortunate sign that climate change is coming rapidly to the Arctic and that we really need to address the issue of global warming on a national level," said Christopher Krenz, Arctic project manager for Oceana.
"This is not surprising but it is alarming," said Deborah Williams, a former Interior Department special assistant for Alaska. "This was a relatively cool summer, and to have ice decrease to the second lowest minimum on record demonstrates that global warming's ongoing impact is profound."
The National Snow and Ice Data Center, based at the University of Colorado, reported the ice Monday melted below the 2005 minimum of 2.05 million square miles set on Sept. 21 that year. Exact figures will be released Wednesday.
Through the beginning of the melt season in May until early August, daily ice extent for 2008 closely tracked the values for 2005, the center said.
In early August 2005, the decline began to slow. In August 2008, however, the decline has remained steadily downward at a brisk pace.
The most recent ice retreat primarily reflects melt in the Chukchi Sea off Alaska's northwest coast and the East Siberian Seas off the coast of eastern Russia, according to the center.
The Chukchi Sea is home to one of two populations of Alaska polar bears.
Federal observers flying for a whale survey on Aug. 16 spotted nine polar bears swimming in open ocean in the Chukchi Sea. The bears were 15 to 65 miles off the Alaska shore. Some were swimming north, apparently trying to reach the polar ice edge, which on that day was 400 miles away.
Polar bears are powerful swimmers and have been recorded on swims of 100 miles but the ordeal can leave them exhausted and susceptible to drowning in high seas.
Sea ice is the primary habitat of polar bears. They depend on it to hunt their primary prey, ringed seals, which create lairs on ice for breeding maintain breathing holes with powerful claws.
Summer sea ice last year shrunk to about 1.65 million square miles, nearly 40 percent less than the long-term average between 1979 and 2000. Most climate modelers predict a continued downward spiral, possibly with an Arctic Ocean that's ice free during summer months by 2030 or sooner.
Krenz said the announcement Tuesday showed that last year's record low sea ice was not an anomaly. As ice covers fewer square miles of ocean, he said, warming will accelerate.
"It's going to accelerate climate change through changes in the reflectance of the Arctic," he said. "It's going from bright ice to a much darker ocean."
More square miles of dark ocean will absorb more heat. More warmth will accelerate melting of Arctic permafrost, allowing organic matter now frozen to melt and add to the greenhouse gas problem, he said.
"That allows for the breakdown of that by bacteria and other organisms that release CO2 or methane, depending on how the breakdown occurs," he said.
The effects faced by people in the Arctic eventually will reach the rest of the nation and the world, he warned.