[24.10.2003]
Satellite data reveals rapid Arctic warming
A NASA satellite survey of the Arctic has revealed just how rapidly the region is warming. The overall trend of rising temperature over the past 20 years is eight times higher than that recorded by ground measurements over the past century.
The satellite observations are vital because they can cover the whole Arctic, not just the regions accessible to researchers on the surface. The data also shows that summer sea ice cover is continuing its retreat.
"Climate is changing, the Arctic is changing rapidly, and it has significant effects on lower latitudes," said Mark Serreze, of the University of Colorado in Boulder, at a press conference on Thursday.
Climate models predict global warming will have its strongest effects in polar regions, making them a valuable laboratory to understand climate variations, says David Rind, of the NASA Goddard Institute for Space Studies in New York.
Uneven retreat
Serreze's analysis shows sea ice coverage in 2002 was the lowest in the 20 years of satellite observations. The retreat is uneven, showing up particularly in areas north of Alaska, where temperature data confirms the warming predicted by climate models. It is also consistent with reports that sea ice is growing thinner.
The analysis of Arctic surface temperatures was conducted by Josefino Comiso, of the NASA Goddard Space Flight Center in Maryland and reported in the Journal of Climate. His data show that sea-ice temperatures during the summer - the most critical season for ice cover - increased 1.22ýC per decade. The annual sea ice trend was smaller, 0.33ýC degree per decade. Although winters have cooled, that effect was more than offset by rising spring, summer and autumn temperatures, which combined to stretch the melt season by between 10 and 17 days. Annual land-surface temperatures increased most over North America - 1.06ýC per decade - and rose 0.50ýC per decade over Eurasia.
Feedback loop
The retreating summer sea ice has knock-on effects. The exposure of more open water, which absorbs more solar energy than ice, means further warming is likely. More ocean open ocean also means winds can build up stronger waves that are eroding Arctic coasts.
"There are communities in Alaska that are having to move their villages" to escape erosion of low-lying coasts, says Michael Steele, an oceanographer at the University of Washington in Seattle.
One push behind the warming is a natural cycle called the North Atlantic Oscillation. For the past 20 years, it has been stuck in a phase where low pressure over the Arctic is increasing heat transport from middle latitudes. Part of the effect may be natural, but Serreze adds that there is growing evidence that human-caused changes in greenhouse gas and stratospheric ozone concentrations may shift the oscillation into the Arctic-warming mode.
That is evidence is unlikely to be welcomed by the US Bush Administration, which remains officially sceptical about global warming. But Rind warns the evidence shows rapid change now: "We can't afford to wait long times for technological innovation" to control greenhouse emissions. Journal reference: Journal of Climate (vol 16, p 3498)
Web links:
Arctic warming, NASA Goddard Space Flight Center
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