Washington This year will see the second-biggest loss on record of Arctic sea ice - a sign that the area of ice coverage is shrinking at a pace faster than once expected.
The trend also suggests that global warming is likely to increase, polar bear habitat will decline, and previously icebound areas could be opened to oil and gas exploration.
Mark Serreze, a senior scientist at the National Snow and Ice Data Center, said Tuesday that the sea-ice minimum, which will be reached later this month, won't hit last year's record because the amount of daylight is decreasing in the Arctic and a new freeze is beginning.
But the minimum amount of ice at summer's end this year will be near last year's total. In 2007, the extent of Arctic ice was 23 percent lower than the previous record in 2005. That 2007 total, which set a record, was 1.65 million square miles. As of Monday, satellite observations showed 1.78 million square miles.
Arctic sea ice is important because it reflects most sunlight, keeping polar regions cool and producing a cooling effect at lower latitudes. Open water, in contrast, absorbs the sun's energy, and the warm ocean waters mean more ice melting, the National Snow and Ice Data Center says.
If ice is lost, more sun is absorbed, and heat from the ocean is released back to the atmosphere, Serreze said.
The understanding of the impacts of these changes is still in its infancy, Serreze said. The center's measurements show the ice has declined dramatically during the past 30 years.
"The real issue is, what's the long-term trend - and it's negative," Serreze said. He said summer could be ice-free in the Arctic by 2030. Some scientists give it just another decade.
Two years of extreme ice minimums indicate that the low in 2007 wasn't random, said James Overland, a scientist who researches the arctic climate at the National Oceanic and Atmospheric Administration (NOAA).
The cause of the ice loss is a combination of global warming from an increase of greenhouse gases and natural variations - winds that brought warm temperatures and blew ice to the Atlantic side, Overland said. If the cause was global warming alone, most scientists thought they wouldn't see this kind of ice loss for 30 years, he said.
This summer conditions were more normal in the Arctic, but the ice was thinner to begin with.
"The question was, would the new ice last through the summer - and it did not," Overland said.
The relationship between ice and ocean conditions will make it hard for sea ice to return to what it was before, he said. Instead, the long-term trend will be decline in ice cover.