Icy signs of warming emerge in Arctic.

Satellite measurements of Arctic sea ice reveal evidence of increased melting since 1979 in the far north -- the strongest sign yet that temperatures are climbing across the region.

"There has definitely been a warming and it's been a large-scale one, but we can't say whether it will continue," says Douglas M. Smith of the United Kingdom Meteorological Office in Bracknell. He reports his findings in the Feb. 15 Geophysical Research Letters.

Sea ice, which can reach a thickness of 4 meters or more, forms when temperatures drop enough for the surface of the ocean to freeze. A blanket of permanent ice covers the central Arctic Ocean year round, but the surface layer melts during summer and refreezes in winter. Climate researchers are watching sea ice closely because they regard it as a bellwether of greenhouse warming -- and one that may sound an alarm before changes in the rest of the globe grow obvious.

Annual surface melting shows up clearly in satellite recordings taken by microwave-sensing instruments, says Smith. When the top of the ice gets moist, it emits more microwave radiation than unmelted sea ice.

Smith exploits these signals to look for changes in the length of the summer melt season.

From 1979 through 1996, summer melting has gradually begun somewhat earlier in the year and autumn freezing somewhat later, he has found. Overall, the period each summer during which melting occurs has increased at the rate of 5.3 days per decade.

The findings corroborate other satellite studies of the Arctic's periphery, which show a decline in the area of ocean covered by sea ice. Researchers have treated such measurements with caution because the data are sensitive to subtle degradations in the satellite sensors. This so-called instrumental drift could introduce artificial trends into the satellite readings.

The changes in the sensors do not significantly skew the estimates of melting, however, so the new measurements offer the clearest sign so far of warming in the Arctic, says Smith.

"This is an important addition to the other studies," says Claire Parkinson, a climatologist who investigates sea ice at NASA's Goddard Space Flight Center in Greenbelt, Md. "His results, which avoid the instrument drift issue, help confirm the other results where people have had to worry about instrument drift."

It is difficult to draw conclusions about the future from such a short period of measurements, says Smith. His findings are consistent with the theory that greenhouse gases are warming the planet, but they may also reflect natural cycles in the Arctic climate.

Since 1991, he notes, the number of days with melting has actually decreased, indicating a cooling over this span. One explanation is that sunlight-blocking sulfur from the eruption of Mount Pinatubo in 1991 temporarily halted the greenhouse warming trend in the Arctic, says Smith. Another possibility is that a natural warming cycle peaked in the late 1980s and has since given way to a cooling cycle in the north.