Extreme Cold Winter of 2009–2010: Evidence against Climate Change?

Regional cold weather extremes during the winter of 2009-2010 raised many scientific questions and provoked popular skepticism about long-term climate change. Were these cold extremes an example of short-term climate variability, or were they so extreme relative to recent winters to call into question the general consensus of an overall warming trend? We analyze regional and northern hemispheric cold and warm temperature extremes for the winter of 2009-2010 in the historical context of all winters since 1948-1949. While Northern Europe, Siberia, and the Southeastern U.S. clearly experienced very cold temperatures, the extreme warm events were much more prevalent over the Northern Hemisphere continents in terms of both magnitude and spatial extent. Importantly, the record-breaking negative state of the North Atlantic Oscillation (NAO) explains the bulk of the observed cold anomalies, however the warm extremes are anomalous even accounting for the state of the NAO. The past winter's widespread and intense warm extremes together with a continuing hemispheric decline in cold snap activity is a pattern fully consistent with a continuation of the warming trend observed in recent decades.