Unusually Warm Winters

It's not your imagination

Over the long term, records show the world is warming. Global average surface temperature crept up by 1.3 degrees Fahrenheit (0.7 degrees Celsius) between 1900 and 2000, and the first decade of this century ranked as the warmest on record, according to the U.S. National Oceanic & Atmospheric Administration's State of Climate in 2010 report.

So mild weather this winter hasn't come out of the blue. "It's consistent with the idea that global warming is going on," said Kevin Trenberth, a senior scientist at the independent National Center for Atmospheric Research (NCAR) in Boulder, Colo.

Natural variability and just plain luck, however, may play a prominent role in why winter is warmer this year, scientists say.

When attempting to describe how they think global climate change will affect weather, climate scientists tend to use metaphors along the lines of playing with a loaded dice or a baseball player on steroids. Both describe a shift in the overall results, a shift toward more extreme weather and warmer temperatures. 

Our fingerprints?
The general trend toward a warmer planet may also be indirectly affecting weather. New research points to melting Arctic sea ice — with the last summer, arguably, setting a record for lowest extent—as a contributor to shifts in weather in the Northern Hemisphere.

The decline in sea ice, as well as earlier snow melt in the Arctic, appears to be weakening the jet stream, resulting in more persistent weather (think extended snow or stints of heat waves) and more influxes of cold air to the south, as well as more warmth to the north, Jennifer Francis, a research professor at Rutgers University, writes in Yale Environment 360.

Other research has linked decreasing Arctic sea ice to unusually large snowfalls and cold surges in the Northern Hemisphere.

Other factors, man-made or not, may also play roles, Trenberth said.
The sun has become more active recently, after an unusually long quiet phase. And there is also speculation that the new Arctic ozone hole may have some influence. Both affect the stratosphere, a layer of the atmosphere, which, in turn, affects the Northern Annular Mode.
These factors "are rearing their heads in perhaps ways we don't understand," Trenberth said.