It seems counterintuitive, even ironic, that global warming could cause some regions to experience colder conditions. But a new study explains the Rube Goldberg-machine of climatic processes that can link warmer-than-average summers to harsh winter weather in some parts of the Northern Hemisphere.
In general, global average temperatures have been rising since the late 1800s, but the most rapid warming has occurred in the past 40 years. And average temperatures in the Arctic have been rising at nearly twice the global rate, says Judah Cohen, a climate modeler at the consulting firm Atmospheric and Environmental Research in Lexington, Massachusetts. Despite that trend, winters in the Northern Hemisphere have grown colder and more extreme in southern Canada, the eastern United States, and much of northern Eurasia, with England's record-setting cold spell in December 2010 as a case in point.
A close look at climate data from 1988 through 2010, including the extent of land and sea respectively covered by snow and ice, helps explain how global warming drives regional cooling, Cohen and his colleagues report online today in Environmental Research Letters. In their study, the researchers combined climate and weather data from a variety of sources to estimate Eurasian snow cover, and then they speculated about how that factor might have influenced winter weather elsewhere in the Northern Hemisphere.
First, the strong warming in the Arctic in recent decades, among other factors, has triggered widespread melting of sea ice. More open water in the Arctic Ocean has led to more evaporation, which moisturizes the overlying atmosphere, the researchers say. Previous studies have linked warmer-than-average summer months to increased cloudiness over the ocean during the following autumn. That, in turn, triggers increased snow coverage in Siberia as winter approaches. As it turns out, the researchers found, snow cover in October has the largest effect on climate in subsequent months.
That's because widespread autumn snow cover in Siberia strengthens a semipermanent high-pressure system called, appropriately enough, the Siberian high, which reinforces a climate phenomenon called the Arctic Oscillation and steers frigid air southward to midlatitude regions throughout the winter.
"This is completely plausible," says Anne Nolin, a climate scientist at Oregon State University in Corvallis. The correlations between warm summers and cold winters that originally led the researchers to develop their idea don't prove cause and effect, but analyzing these trends with climate models in future studies could help researchers bolster what Nolin calls "an interesting set of connections."
"Northern Eurasia is the largest snow-covered landmass in the world each winter," she notes. It only makes sense, she argues, that it would have a big influence on the Northern Hemisphere's climate. Indeed, she adds, previous studies have noted the link between Siberian snow cover and climate in the northern Pacific.
The team's analyses suggest that climate cycles such as the El Niño-Southern Oscillation, the Pacific Decadal Oscillation, and the Atlantic multidecadal oscillation can't explain the regional cooling trends seen in the Northern Hemisphere during the past couple of decades as well as trends in Siberian snow cover do. If better accounts of autumn snow-cover variability are incorporated into climate models, scientists could provide more accurate winter-weather forecasts, the researchers contend.