Arctic sea ice, a key component of Earth's natural thermostat, has thinned sharply in recent years with the northern polar ice cap shrinking steadily in surface area, government scientists said on Monday.

Thinner seasonal sea ice, which melts in summer and freezes again every year, now accounts for about 70 percent of the Arctic total, up from 40 to 50 percent in the 1980s and '90s, the researchers said, citing new satellite data.

At the same time thicker ice, which lasts two summers or more without melting, now comprises less than 10 percent of the northern polar ice cap in winter, down from 30 to 40 percent. Just two years ago, the thicker so-called perennial sea ice made up 20 percent or more of the winter cap.

Scientists have voiced concerns for years about an alarming decline in the size of the Arctic ice cap, which functions as a giant air conditioner for the planet's climate system as it reflects sunlight into space.

As a greater portion of the ice melts, it is replaced by darker sea water that absorbs much more sunlight, thus adding to the warming of the planet attributed to rising levels of heat-trapping greenhouse gases emitted into the atmosphere by human activity.

The ice cover plays a key role in the climate, Thomas Wagner, the chief snow and ice scientist for NASA, said in a conference call with reporters. The thicker ice particularly is very important, because it's the thicker ice that survives the summer to stay around and reflect that summer sunlight.

Walter Meir of the National Snow and Ice Data Center in Boulder, Colorado, added, We're getting an ice cover as we finish the winter and head into summer that's much more vulnerable to the summer melt and much more likely to melt completely and expose that dark ocean.

The decade-long trend of a contracting ice cap around the North Pole is continuing as well.

The maximum extent of Arctic sea ice for the winter of 2008-09 was measured at 5.85 million square miles (15.2 million square km), the fifth-lowest winter peak on record. That tally represents a loss of some 278,000 square miles (720,000 square km), about the size of Texas, from the winter peak averaged from 1979 to 2000.

The six lowest measurements since satellite monitoring began in 1979 have all occurred in the past six years.

Still, the total volume of sea ice in the Arctic during winter is estimated to contain enough water to fill Lake Michigan and Lake Superior combined.

Meir said there are consequences of an Arctic thaw beside a speed-up of global climate change and the survival of wildlife that depend on the polar ice.

Vanishing summer ice will open new navigation routes for shipping, opportunities to develop the region's natural resources and competition among northern nations to lay claim to parts of the Arctic, he said.

Meir said a strong consensus has emerged among climate scientists that the Arctic is headed for its first largely ice-free summer in the relatively near future, with forecasts running as early as 2013, though he sees that as too soon.

In any case, he added, It's not a matter of if, it's a matter of when.