Satellite data shows Arctic on thinner ice

* Arctic sea ice thinning sharply

* Ice cap functions as giant air-conditioner

By Steve Gorman

LOS ANGELES, April 6 (Reuters) - 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."