The frozen Arctic has shrunk to its second lowest level since satellites began measuring it in 1979, capping a decade of "rapidly decreasing summer sea ice," US scientists said Friday.

The Colorado-based National Snow and Ice Data Center (NSIDC) said the ice cap appeared to have reached its lowest level for the summer, at 4.33 million square kilometers (1.67 million square miles) on September 9.

That is the second lowest level since the center began tracking the ice mass by satellite more than 30 years ago, with the lowest level recorded in 2007.

The NSIDC cautioned that the measurement was based on preliminary results and that changing winds could push the ice flows closer together, resulting in a smaller figure when final numbers are released in early October.

"The last five years (2007 to 2011) have been the five lowest extents in the continuous satellite record, which extends back to 1979," the NSIDC said.

"While the record low year of 2007 was marked by a combination of weather conditions that favored ice loss... this year has shown more typical weather patterns but continued warmth over the Arctic," it said.

Using a different set of data, German researchers said last week that the area covered by Arctic Sea ice had reached its lowest point since the start of satellite observations in 1972, calling it a "new historic minimum."

Arctic ice cover plays a critical role in regulating Earth's climate by reflecting sunlight and keeping the polar region cool.

Retreating summer sea ice -- 50 percent smaller in area than four decades ago -- is described by scientists as both a measure and a driver of global warming, with negative impacts on a local and planetary scale.

Satellite tracking since 1972 shows that the extent of Arctic sea ice is dropping at about 11 percent per decade.