Research Briefs

Hot summers increasing in the U.S.

June–August averages projected to climb further

A hot-looking yellow Sun over the ocean, with seagulls in the foreground.

A hot sun sets over Santa Ana, California. (©UCAR.)

A new study in the journal Climatic Change finds that extremely warm summers are not only occurring more frequently in the contiguous United States, but are likely to become normal by mid-century if the world continues its business-as-usual rate of greenhouse gas emissions.

NCAR visiting scientist Claudia Tebaldi (Climate Central) and Phil Duffy (Lawrence Livermore National Laboratory) analyzed observations and climate model results for the study. They used the model output mainly to address future changes, but also as a comparison with observed changes in order to test the accuracy of the models.

Tebaldi and Duffy first compared the period 1975–2000 to the preceding 25 years, finding that observations and results from 16 global climate models show that average summertime temperatures that were rare in the earlier period occurred more often in the 1975–2000 period in some regions of the United States. Through statistical analysis, they show that this increase is very unlikely to have occurred through natural weather variation.

The researchers then focused on the present climatology, using results from climate models for the period 1995–2024, a 30-year period centered around the current decade. Their analysis shows that summer temperatures that were extreme during 1950–1979 occur more often in the present period, another increase that is statistically very unlikely to be due to natural variation. This applies to the entire U.S. region.

Tebaldi and Duffy's last step was to focus on the future. They found that by 2035–2064, summertime average temperatures that would have ranked among the top 5% in 1950–1979 are projected to occur at least 70% of the time. The South, Southwest, and Northeast are projected to be especially prone to large increases in unusually hot summers

The research is based on the Intergovernmental Panel on Climate Change A2 emissions scenario, which assumes that global fossil fuel emissions will continue to increase at relatively high rates over the coming century.

P. B. Duffy, C. Tebaldi, "Increasing prevalence of extreme summer temperatures in the U.S.," Climatic Change, 2012; 111 (2): 487 DOI: 10.1007/s10584-012-0396-6