Extreme cold and global warming

"Hey! There was some crazy blizzards in the US this year and the last. How can you scientists still say that the Earth is warming when it was unusually cold in the winter of 2010 and 2011?" This is a common kind of question from laymen in regards to the feasibility of climate change. Luckily, some scientists actually looked at this in a recent article in Geophysical Research Letters. (gated beyond the abstract, unfortunately)

These scientists divided the Northern hemisphere into eight regions: USA, Canada, Alaska/Yukon, Siberia, Far East, Central Asia, Nothern Europe/Russia, and Mediterranean/Middle East. Then, using statistical techniques, they examined the temperature record for extreme cold and warm events for the winters of 2010 and 2011 and compared it to the historical record for 1950-2009. Indeed, the USA, Northern Europe, and Siberia did experience winters that were unusually cold in 2010 and 2011. However, the number and extent of unusually cold days in these two regions was offset by an even greater number of unusually hot days in every other region of the Northern Hemisphere! Not only were there are larger number of extremely hot days, but these hot spells also lasted much longer and were more extreme compared to any of the extremely cold days in the United States, Europe, or Siberia.

What accounted for the cold extremes experienced in parts of the Northern Hemisphere then? Similar to the natural El Niño/El Niña phenomenon in the Southern Pacific ocean, another natural phenomenon called the North Atlantic oscillation causes variability in local climates in the Northern Hemisphere. During the winters of 2010 and 2011, this oscillation exhibited an unusual and persistent pattern that caused cold air from the Arctic to move down to the United States and Europe, while warm air traveled up to the Arctic. This made it so that while much of North America and Europe was unusually cold, the Arctic was, in fact, much warmer during the winter. Even now, the total area of Arctic sea ice is near record lows this year.

In contrast, the extremely hot weather in the rest of the Northern Hemisphere cannot be explained by natural variations in climate, but require an outside explanation. This paper demonstrates that while there may be locally extreme cold events in weather than run counter to the overall trend of anthropogenic climate change, the overall trend for the entire planet is warming.