Summer temperatures once considered exceptionally high have, in recent decades, become more frequent across the United States as a result of anthropogenic climate change.

Philip Duffy, currently at the Lawrence Livermore National Laboratory in Livermore, California, and Claudia Tebaldi at the National Center for Atmospheric Research in Boulder, Colorado, compared summer temperature extremes from 1950 to 1999 with simulations derived from 16 global climate models. Model projections suggest that US summer temperatures will continue to rise as the century progresses. Even in regions that have warmed relatively little so far, the chances of extreme temperatures — seen only once in 20 years in the second half of the past century — will be at least 70% in any given year by 2064.

Clim. Change http://dx.doi.org/10.1007/s10584-012-0396-6 (2012)