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Warmer and Wetter Winters in Europe and Western North America Linked to Increasing Greenhouse Gases
Why are winters warming up so much faster over Northern Hemisphere continents than over the rest of the globe? A new study by NASA researchers in the June 3 issue of the journal Nature is the first to link the well-documented large degree of North America and Eurasia winter warming and the associated wind changes to rising greenhouse gas levels in the atmosphere.
Drew Shindell, an atmospheric scientist from NASA's Goddard Institute for Space Studies and Columbia University, NY, found that the large warming that has occurred during Northern Hemisphere winters over the last 30 years — up to nine degrees Fahrenheit, a full 10 times warmer than the global average 0.9 degree Fahrenheit warming — is likely a result of human activities that increase greenhouse gases. Warmer winters will bring more wet weather to Europe and Western North America, Shindell said, with Western Europe the worst hit by storms coming off the Atlantic.
Using a computer climate model to try and understand where the warming over the past three decades is coming from, Shindell and colleagues discovered that in the model, increasing greenhouse gases cause stronger spiraling winds over the North Pole, a phenomenon called the polar vortex, that change the normal Northern Hemisphere climate. "Northern Hemisphere winters have been warming up for the past 30 years," Shindell said. "It's a big concern to know why this happens."
The normal Northern Hemisphere climate is run by a system similar and second in size to El Niño. The pressure system, called the Arctic Oscillation, and encompassing the more familiar North Atlantic Oscillation, is the reason why some winters are much colder or more severe than others. The temperature see-saw happens because at times, there are very strong westerly winds blowing across Northern Hemisphere mid-latitudes. Air picks up heat as the winds blow over the oceans.
"The ocean retains heat much better than the land. And if the winds blow more rapidly, they bring more warm, wet air onto the land and warm up the continents," said Shindell. Thus, warm air from the Pacific Ocean warms Western North America and Atlantic Ocean warmth is shared with the Eurasian continent. The stronger the wind, the warmer the winter. At other times, the winds become weak, and winters become much colder.
The flip-flop from cold to warm happens every few years, according to Shindell. But over the past 30 years, he said, the system has tended more toward strong winds and warming. And even during colder periods, temperatures have risen by several degrees. "Since the cycle is a natural phenomena," said Shindell, scientists have to ask, "is the increased warming part of the natural oscillation, or are humans causing the warming?"
"The computer models we were using weren't able to provide the answer, said Shindell." said Shindell. Models can easily generate the Arctic Oscillation, but adding the swelling greenhouse gas factor to previous models didn't make the warm periods increase like they've been doing since the 1960s.
Shindell found a probable reason for warming Northern Hemisphere winters by studying the polar vortex in his climate model. Over the North Pole, there is a polar vortex created in the stratosphere, the part of the atmosphere that starts more than six miles above the Earth's surface. The vortex arises because the North Pole is completely dark and extremely cold in the winter, creating a large temperature difference between the polar region and the mid-latitudes.
The temperature difference makes for a large pressure contrast that forces east-west traveling winds into a spiraling whirlpool in the stratosphere. And as the mid-latitudes warm up due to greenhouse gases, the temperature difference gets larger and the vortex stronger.
"If we include the stratosphere in the climate model, we're finding that the increasing greenhouse gases lead to an enhanced polar vortex," said Shindell. It is the strengthened polar vortex that drives winds all the way from the stratosphere to the lower atmosphere, creating stronger westerlies and increasing the warm periods of the Arctic Oscillation, said Shindell. A realistic representation of the stratosphere, including the polar vortex, is typically not included in climate models. These results suggest that the impact of global warming may therefore be incompletely represented by most current models, with this altered circulation effect making for greater regional warming during wintertime.
Shindell's model predicts that if greenhouse gases continue to increase, winter in the Northern Hemisphere will continue to warm. "In our model, we're seeing a very large signal of global warming and it's not a naturally occurring thing. It's most likely linked to greenhouse gases," he said.
Shindell, D.T., R.L. Miller, G.A. Schmidt, and L. Pandolfo 1999. Simulation of recent northern winter climate trends by greenhouse-gas forcing. Nature 399, 452-455.
Leslie McCarthy, NASA Goddard Institute for Space Studies, New York, N.Y., 212-678-5507, firstname.lastname@example.org