Climatologists consider volcanic aerosols, polar ozone depletion, solar radiation and greenhouse gases to be important factors in climate warming.

NASA scientists input all of these factors in a climate model and concluded that greenhouse gases are the primary factor driving warmer winter climates in North America, Europe and Asia over the last 30 years. They found that greenhouse gases, more than any of the other factors, increase the strength of the polar winds that regulate northern hemisphere climate in winter.

Using a computer model that simulates climate through interactions of ocean and atmosphere, scientists input current and past levels of greenhouse gases such as carbon dioxide, methane, water vapor and nitrous oxide. They found that greenhouse gases such as those increase the strength of polar wind circulation around the North Pole.

The polar winds play a large role in the wintertime climate of the northern hemisphere. The winds blow from high up in the stratosphere down to the troposphere and eventually the Earth's surface.

When they strengthen, as they do from increases in greenhouse gases, they blow stronger over the warm, moist oceans picking up and transporting warmer air to the continents. Thus, warm air from the Pacific Ocean warms western North America, and the Atlantic Ocean warmth is shared with Eurasia. When winds are stronger, winters are warmer because air picks up heat as the winds blow over the oceans. When winds become weak, winters become colder.

The findings by Drew Shindell, Gavin Schmidt, and other atmospheric scientists from NASA's Goddard Institute for Space Studies and Columbia University appeared in the April 16 issue of the Journal of Geophysical Research – Atmospheres.

Shindell noted that increases in greenhouse gases make the stronger polar winds last longer into the springtime and contribute to a warmer early spring climate in the northern hemisphere.

The stronger wind circulation around the North Pole creates a large temperature difference between the pole and the mid-latitudes. Shindell noted that the Southern Hemisphere isn't affected by increasing greenhouse gases the same way, because it's colder and the polar wind circulation over the Antarctic is already very strong.

"Surface temperatures in the Northern Hemisphere have warmed during winter months up to 9 degrees Fahrenheit over the last three decades, over 10 times more than the global annual average 0.7 degree Fahrenheit," says Shindell. "Warmer winters will also include more wet weather in Europe and western North America, with parts of western Europe the worst hit by storms coming off the Atlantic."

Year-to-year changes in the polar winds are quite large, according to Shindell. But over the past 30 years, we have tended to see stronger winds and warming, indicative of continually increasing greenhouse gases.

Shindell looked at volcanic activity from 1959 to 2000 and identified volcanically active and non-active years. The researchers concluded that because volcanic forcing is intermittent and decays rapidly, it seems unlikely to have contributed greatly to the long-term observed warming trend.

Large volcanic eruptions such as Mount Pinatubo in 1991 inject aerosols into the atmosphere and have a global cooling effect during the years following an eruption.

Also included in the model were the 11-year solar cycle and the effects of solar radiation on stratospheric ozone. Schmidt noted that long-term changes in solar irradiance have influenced the upper atmosphere.

"However, it is unlikely that solar variability has been responsible for much of the observed trend in increasing the polar winds," Schmidt said.

Because the upper polar atmosphere becomes colder when ozone is depleted, the winds circling the pole are slightly enhanced.

"However," Shindell noted, "greenhouse gases have the biggest impact on the strengthening of the polar winds, and in turn, the warming of the northern hemisphere during winter months."

Shindell said that the warming trend would likely continue over the next 30 years as greenhouse gases continue to increase in the atmosphere. - By Cynthia O'Carroll

(Editor's Note: For the abstract, go to this URL.)

[Contact: Cynthia O'Carroll]

24-Apr-2001