New suspect in trend toward warmer winters: wind
Scientists say greenhouse gases may be confining cold air to polar
SAN FRANCISCO — Scientists say a newly documented shift in Arctic wind patterns could be evidence of one way that green-house gases are heating up the planet.
They suggest that the buildup of carbon dioxide and other greenhouse gases in the atmosphere has fed a 30-year trend of stronger winds encircling the Arctic and bringing warmer winters to the Northern Hemisphere.
Natural causes can't be ruled out, but computer-based climate models cannot simulate the effect unless they include the influence of the buildup of such gases, says David Thompson of the University of Washington at Seattle.
Indeed, the trend "seems to be unprecedented," noted Seattle colleague John Wallace at a recent meeting here of the American Geophysical Union.
The shift is in what is called Arctic Oscillation. Arctic Oscillation refers to two patterns of atmospheric pressure that occur in the region between the north pole and north mid-latitudes.
The "weak" phase of the oscillation occurs when the average Arctic pressure is higher than normal while the mid-latitude average pressure is below normal. The "strong" phase occurs when Arctic pressure is relatively low and mid-latitude pressure relatively high.
In this mode, winds between the mid-latitudes and the Arctic tend to form a pattern called the Polar Vortex. This usually confines colder air to high latitudes. Warmer, wetter conditions then prevail over Scandinavia, Siberia, and North America east of the Rockies.
The Seattle scientists have found, to their surprise, that the Arctic Oscillation has been increasingly favoring this strong mode during the past 30 years. Something seems to be biasing its fluctuations. They suspect greenhouse gases are forcing this bias, but they can't yet prove it.
The oscillation operates independent of other major weather factors such as El Nio, and changes in the oscillation can explain much of what has been happening in the Northern Hemisphere climate.
The oscillation's the thing
The change in the oscillation can account for half of the 5.4 degree F. rise in Eurasian average surface temperature during the past 30 years. It accounts for nearly all of the small drop in Arctic average sea level atmospheric pressure. It accounts for three-quarters of the 45 percent rise in Norwegian rainfall.
Other scientists have identified such trends with swings in Atlantic and Pacific sea-surface temperatures. Dr. Wallace says these are just part of the oscillation.
In a perplexing discovery, scientists also discovered that these oscillations begin six miles up in the stratosphere and move downward - instead of starting low and moving upward like all other known weather phenomena.
Moreover, Mark Baldwin of Northwest Research Associates Inc. in Bellevue, Wash., notes that the stratosphere is relatively cold in the strong phase of the oscillation. This provides further evidence that greenhouse gases could be influencing the oscillations, he says, because greenhouse gases cool the stratosphere as they warm the surface.
Another climate discovery
In another announcement at the meeting with major implications for climate-change study, researchers said new data indicate that the dirt and soot humans put in the air are as important as greenhouse gases in driving global warming.
Climatologists don't yet know whether they enhance the warming or tone it down, but the find suggests that efforts to curb the emission of greenhouse gases are not enough to control human-induced climate change. A global effort will have to be made to control aerosol pollution as well.
"This is a sort of new thing thrown at us," says climatologist V. Ramanathan with the Scripps Institution of Oceanography in La Jolla, Calif. It introduces unexpected complexity into the climate-change puzzle.
Scientists had thought aerosols would have a cooling effect because they reflect incoming sunshine. But the haze layers contained so much soot they absorbed some of the radiation and warmed up.
Scientists say they want to study whether the haze is having an effect on tiny marine algae that underlie the ocean food chain. But the main effect of aerosols may be on rainfall and clouds, says Dr. Ramanathan. They may change cloud structure in ways that significantly change - probably reduce - rainfall.
(c) Copyright 1999. The Christian Science Publishing Society