Number of extremely hot temperatures are increasing

http://www.pnas.org/content/109/37/E2415.abstract

James Hansen, Makiko Sato (National Aeronautics and Space Administration Goddard Institute for Space Studies and Columbia University Earth Institute, New York, NY 10025)
Reto Ruedy (Trinnovim Limited Liability Company, New York, NY 10025)

March 29, 2012


Abstract

“Climate dice,” describing the chance of unusually warm or cool seasons, have become more and more “loaded” in the past 30 y, coincident with rapid global warming. The distribution of seasonal mean temperature anomalies has shifted toward higher temperatures and the range of anomalies has increased. An important change is the emergence of a category of summertime extremely hot outliers, more than three standard deviations (3σ) warmer than the climatology of the 1951–1980 base period. This hot extreme, which covered much less than 1% of Earth’s surface during the base period, now typically covers about 10% of the land area. It follows that we can state, with a high degree of confidence, that extreme anomalies such as those in Texas and Oklahoma in 2011 and Moscow in 2010 were a consequence of global warming because their likelihood in the absence of global warming was exceedingly small. We discuss practical implications of this substantial, growing, climate change.