Greenhouse gases spur warm, wet winters

If the current warming trend continues, Northern Hemisphere continents can expect winters that are warmer and wetter

June 3, 1999
Web posted at: 11:15 AM EDT

Warm, wet winters during recent decades in the Northern Hemisphere can be explained by the influence of greenhouse gases on atmospheric winds, Columbia University researchers report in Thursday's issue of the journal Nature.

During the last 35 years, temperatures have risen by seven to 10 degrees Fahrenheit over vast regions of northern Eurasia and North America, according to data compiled at the Goddard Institute for Space Studies in New York, N.Y.

"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," said Drew Shindell, a climatologist at Columbia University.

According to the research, increased concentrations of greenhouse gases have indirectly resulted in a dominant positive phase of an atmospheric wind pattern that ushers in warm, moist air from the oceans to the continents.

The wind pattern, known as the North Atlantic Oscillation, a weather phenomenon second in size only to the El Niño/La Niña cycle, is the vortex of west-to-east counterclockwise winds over polar regions.

While the pattern always exists, the vortex of west-to-east winds is stronger during its positive phase and weaker during its negative phase. In the Northern Hemisphere's winter months, the oceans retain more heat than the land, resulting in strong west-to-east winds, or a positive phase of the North Atlantic Oscillation.

The Columbia University researchers used a climate model, housed at the Goddard Institute for Space Studies, to demonstrate that the buildup of greenhouse gases in the atmosphere induces even stronger surface winds and warmer continental surface temperatures by giving the North Atlantic Oscillation an extra push.

Greenhouse gases trap heat at the Earth's surface, while cooling the stratosphere — a region of the atmosphere that extends from seven to 30 miles above the Earth's surface. This cooling has increased the speed of the stratospheric jet stream and strengthened the west-to-east winds over the polar region.

In other words, increased concentrations of greenhouse gases cause a pronounced positive phase of the North Atlantic Oscillation to prevail, resulting in warmer temperatures on Northern Hemisphere continents, said Shindell

"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," said Shindell.

In fact, the researchers conclude, 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 radiative trapping of heat at the surface.

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