Heat Waves and Climate Change

A Science Update from Climate Communication – June 28, 2012


Climate Communication hosted a press conference featuring experts discussing the connections between extreme heat and climate change. Download the full audio recording of the conference »

Expert Reviewers:

There has been a remarkable run of record-shattering heat waves in recent years, from the Russian heat wave of 2010 that set forests ablaze to the historic heat wave in Texas in 2011 and the “Summer in March” in the U.S. Midwest in 2012. These events typify the on-going trend driven by climate change.

This report, Heat Waves and Climate Change, summarizes our current scientific understanding of the connection between climate change and the recent increase in extreme temperatures, as reported in peer-reviewed research articles published through May 2012.


Climate change is already affecting extreme weather. The National Academy of Sciences reports that the hottest days are now hotter.1  And the fingerprint of global warming behind this change has been firmly identified.2 3 4

Since 1950 the number of heat waves worldwide has increased, and heat waves have become longer.5  The hottest days and nights have become hotter and more frequent.6 7    In the past several years, the global area hit by extremely unusual hot summertime temperatures has increased 50-fold.8  Over the contiguous United States, new record high temperatures over the past decade have consistently outnumbered new record lows by a ratio of 2:1.9   In 2012, the ratio for the year through June 18 stands at more than 9:1.10  Though this ratio is not expected to remain at that level for the rest of the year, it illustrates how unusual 2012 has been, and how these types of extremes are becoming more likely.

The significant increase in heat extremes we have witnessed associated with a small shift in the global average temperature is consistent with climate change. The percentage change in the number of very hot days can be quite large.11  Global warming boosts the probability of very extreme events, like the recent “Summer in March” episode in the U.S. in which thousands of new record highs were set, far more than it changes the likelihood of more moderate events.12

Higher spring and summer temperatures, along with an earlier spring melt, are also the primary factors driving the increasing frequency of large wildfires and lengthening the fire season in the western U.S. over recent decades.13   The record-breaking fires this year in the Southwest and Rocky Mountain Region are consistent with these trends.

The impact of these changes can be devastating. The drought, heat wave and associated record wildfires that hit Texas and the Southern plains in the summer of 2011 cost $12 billion.14

Fig. 1: Summer in March. The colors show the difference from average temperatures from March 13-19, 2012 in degrees C. The dark red areas were 15° C (27°F) above average for this period. Source: NASA

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(Full List of References)


  1. National Research Council of the National Academies Committee (2010)
  2. Min, S.K., Zhang, X., Zwiers, F.W. and Hegerl, G.C. (2011)
  3. Dai, A. (2011)
  4. Seneviratne, S.I., et al. (2012)
  5. Trenberth, K.E., et al. (2007)
  6. Trenberth, et al., op. cit.
  7. Gutowski, W.J., et al. (2008)
  8. Hansen, Sato, and Ruedy. (2012)
  9. Meehl, G. A., C. Tebaldi, G. Walton, D. Easterling, and L. McDaniel. (2009)
  10. NCDC. (2012)
  11. Peterson, T.C., et al. (2008)
  12. Rahmstorf, S. and D. Coumou. (2012)
  13. Global Climate Change Impacts in the United States. (2009)
  14. NCDC. (2012)