Polar sea ice on the wane.

Satellite measurements reveal that the extent of sea ice has decreased in both the Arctic and Antarctic since 1978, reports a team of scientists from the Nansen Environmental and Remote Sensing Center in Bergen, Norway.

For climate scientists, Earths polar regions play the role of the canary sent down a coal mine to test the air. As extremely sensitive regions of the planet, the Antarctic and the Arctic are expected to warm more and possibly sooner than other parts of the globe in response to greenhouse gas pollution. A thinning of polar sea ice might therefore provide an early warning of greenhouse warming.

NASA scientists had previously detected evidence of reduced sea ice extent in the Arctic while studying satellite data collected from 1978 to 1987. Antarctic sea ice showed no statistically significant trend during that period.

To continue the sea ice record, Ola M. Johannessen, Martin W. Miles, and Einar Bjorgo merged the data from 1978 to 1987 with measurements collected by a different satellite sensor between 1987 and 1994. The two satellite sensors overlapped by 6 weeks, enabling the Norwegian scientists to match the data sets.

In the July 13 Nature, they report that ice reductions in the Arctic accelerated in the later period. The area covered by sea ice, which includes some regions of open water, dropped by 2.5 percent per decade in the earlier period and 4.3 percent per decade in the later one. The area of water actually covered by ice fell 2.8 percent per decade in the earlier record and 4.5 percent per decade later.

When they combined the two data sets, Johannessen and his coworkers found a slight, but statistically significant, drop in Antarctic sea ice extent, amounting to 1.4 percent per decade over the last 16 years. The area actually covered by ice also declined but not by a statistically significant amount, they announced last month at a conference in Florence.

Although the change matches the pattern expected from greenhouse warming, the team says it is too early to pinpoint the cause. With only 16 years of satellite data, the scientists cannot rule out the possibility that natural changes are reducing the ice cover, says Miles.