(Bloomberg) — The layer of ice over the Arctic Ocean has thinned "dramatically" this decade, with its thin seasonal blanket for the first time making up a bigger portion of the total ice than the thicker, older coat, a study said.

Scientists from Nasa and the University of Washington in Seattle surveyed the ocean's ice sheet from 2003 through 2008 using observations from the Ice, Cloud and Land Elevation Satellite, or ICESat, to make the first estimate of its thickness and volume.

The study was published in the July 7 issue of the Journal of Geophysical Research-Oceans.

The ocean's ice layer gets larger each winter as the sun sets for several months and temperatures drop, the National Aeronautics and Space Administration said in a statement.

In the summer, much of that seasonal cover melts, while wind and ocean currents carry some away, the agency said.

Older ice, which averages about nine feet in thickness, is more likely to survive than seasonal sea ice, which is about 6 feet thick.

The researchers found that the Arctic Ocean's ice layer thinned by about 2.2 feet over four winters, or about seven inches a year, while the area covered by older, thicker ice shrank by about 42 percent, or 595,000 square miles — almost the land area of Alaska.

In 2003, 62 percent of the ocean's ice cover was older, thicker ice, with 38 percent in seasonal layers, the researchers found.

Five years later, 68 percent of the ice cap was made up of seasonal ice.