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| Vol. 24, No. 24 | June 4, 1999 |
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| Vol. 24, No. 24 | June 4, 1999 |
BY BOB NELSON
A team of scientists from Columbia has shown for the first time that warm winters in the northern hemisphere may be caused by the greenhouse effect because of a proposed connection to upper atmosphere winds.
Global mean surface temperatures have increased in the range of 0.6 to 1.2°F since the late 19th century. But far more severe warming has taken place over wide regions of northern Eurasia, Canada and Alaska, with temperatures averaging 7 to 10°F warmer in the last 35 years, according to data previously compiled by the NASA Goddard Institute for Space Studies in New York City.
The research, which appears in the June 3 issue of the British journal Nature, offers no predictions on what temperatures future winters will bring, but suggests a continuation of the current trend for three to four more decades. If warming trends continue, said Drew Shindell, associate research scientist at Columbia's Center for Climate Systems Research and lead author of the report, northern regions of Europe and Asia and, to a lesser extent, North America, can expect winters with increased rain and snow. Global warming's effects, some scientists say, could include melting of polar ice caps, coastal flooding and even reversal of ocean currents.
"Based on this research, it's quite likely that the warmer winters over the continents are indeed a result of the increasing amount of greenhouse gases in the atmosphere," Shindell said. "This research offers both a plausible physical mechanism for how this takes place, and reproduces the observed trends both qualitatively and even quantitatively." Other authors of the Nature paper were Gavin A. Schmidt, associate research scientist at Columbia's Center for Climate Systems Research; Ron L. Miller, associate research scientist in the Department of Applied Physics and Applied Mathematics at Columbia, and Lionel Pandolfo, assistant professor in the Department of Earth and Ocean Sciences at the University of British Columbia. Shindell, Schmidt and Miller also maintain an affiliation with the NASA Goddard Institute.
The physical mechanism, the authors suggest, is a redistribution of heat closely related to recent changes in atmospheric wind patterns, an indirect consequence of greenhouse warming. Greenhouse gases trap heat at the Earth's surface, while cooling the stratosphere, a region of the atmosphere that extends from about seven to about 30 miles above the planet's surface. This cooling has increased the speed of the stratospheric jet stream and has strengthened a lower atmosphere vortex of west-to-east, counterclockwise winds that naturally form over the polar region each winter.
During the winter, the ocean retains heat better than the land. So when the dominant west-to-east winds increase, they carry warmer air from the oceans to the continents, and colder continental air to the oceans. In North America, the Rockies intercept the warmer winds, so the effect is stronger west of the mountains and is mitigated in central and eastern portions.
"Despite appearing as part of a natural climate oscillation, the large increases in wintertime surface temperatures over the continents may therefore be attributable in large part to human activities," Shindell said. "The impact of greenhouse gases on climate through surface wind changes may be as large as, or in some areas larger than, the more direct impact of global warming."
The Columbia team used several versions of the NASA Goddard Institute's general circulation model, a computer program that predicts the Earth's climate when certain variables are changed. Model simulations suggest that much of the increase in surface winds and in continental surface temperatures during the winter months is induced by the buildup of greenhouse gases in the atmosphere. In the model, increasing greenhouse gas emissions lead to a warmer surface and, at the same time, a colder stratosphere. The large wintertime continental temperature increases produced in the model correspond quite well with what scientists actually observe. But when the researchers used a version of the climate model that did not adequately represent the stratosphere, the results did not jibe as well with reality.
Colder polar temperatures in winter, and warmer temperatures in the middle latitudes, are actually part of a natural cycle of climate variability, which made the warming trend more difficult for the scientists to isolate. The temperature differences are reflected in sea-level pressure, which decreases in the Arctic region and increases at the middle latitudes; this cycle is called the Arctic Oscillation and is second only to El Niņo in its effects on global weather. In the NASA Goddard Institute simulations, increasing greenhouse gases caused a preference for one phase of this cycle over another, with stronger west-to-east surface winds at the Northern Hemisphere middle latitudes, leading to the increased surface temperatures over land.
The research was supported by the National Oceanic and Atmospheric Administration and by the National Aeronautics and Space Administration.