Global warming may be responsible for the Northern Hemisphere’s recent bout of severe winters. As Arctic sea ice melts, it funnels cold air toward the equator and sets the stage for snow, a new study finds.
“When we have a dramatic reduction in sea ice, we end up with more snow,” says climate scientist Jiping Liu of the Georgia Institute of Technology in Atlanta and coauthor of the study, published online February 27 in the Proceedings of the National Academy of Sciences.
Despite rising global temperatures, extreme winters have blasted much of the Northern Hemisphere during the last decade. Unusually large snowstorms pummeled the United States’ east coast during the winters of 2009 to 2010 and 2010 to 2011. Parts of Japan saw record levels of snow this winter, while in Europe both the Danube and Venice’s canals froze over, a rare sight.
To explain this bitter cold and snow, some scientists have turned to natural climate fluctuations — including El Ni±o, a periodic warming of the eastern Pacific Ocean thought to portend warmer and drier winter conditions. But since some severe winters coincided with El Ni±o years, Liu’s team looked instead to sea ice floating in the Arctic, a region that has been warming twice as quickly as the average rate for the Northern Hemisphere.
Satellite observations show that the amount of sea ice during autumn months, after the summer melt, declined by 27.3 percent between 1979 and 2010. In its worst year, 2007, sea ice covered 4.13 million square kilometers in September, down 1.19 million square kilometers from the previous record low in 2005. Years with less autumn ice tended to be followed by more winter snow in many parts of the Northern Hemisphere. The researchers’ computer simulations suggest that losing 1 million square kilometers of ice can increase snowfall by 3 to 12 percent in some places, including parts of the United States, Europe and China.
Liu and his team “confirm a link between sea-ice cover and snow cover,” says Ralf Jaiser, a climate scientist at the Alfred Wegener Institute for Polar and Marine Research in Potsdam, Germany. “That is something I always expected but was not able to prove.”
Jaiser and other scientists have been studying the atmosphere to work out how changes in sea ice could chill faraway places. When the reflective ice disappears, the darker ocean that remains absorbs more of the sun’s energy. Both the surface of the water and the air above it heat, changing the way that winds circulate through the atmosphere and forming a high-pressure system.
Computer simulations published in 2009 in Geophysical Research Letters found that such a pressure system can push cold air out of the Arctic and into Eurasia. A case study of Europe’s 2005-2006 winter, reported in 2010 in the Journal of Geophysical Research, suggested that cold air blowing in from the Arctic increases by threefold the chance of cold winter extremes in Europe.
Disappearing sea ice may also provide more moisture for forming snow, says Liu. In further simulations by his team, open water no longer covered by ice released vapor that traveled to parts of Europe and Asia.
But Stephen Vavrus, a climate scientist at the University of Wisconsin-Madison, doubts that this humidity plays much of a role. Most of the United States would have abnormally low humidity during the winter, based on the data of Liu’s team, in precisely the areas where lots of snow falls. “Whether there is enough moisture to cause heavy snowfall during a cold interval is probably controlled by other factors,” says Vavrus.
And disappearing sea ice isn’t only thing driving the cold and the snow. The United States had a particularly warm and snowless winter this year, probably thanks to a periodic flip in Arctic winds that trumped the effects of sea ice loss, says Liu.
Still, if sea ice melts further, big snowstorms may be in the forecast more often than not. “If this pattern of reduced sea ice continues, in the short term we may see more of these cold, snowy conditions,” says Radley Horton, a climate scientist at Columbia University who collaborated with Liu.