Extreme US summer temperatures occurring more frequently

Extreme US summer temperatures are already occurring more frequently in the United States, and could become normal by mid-century, according researchers at the Lawrence Livermore National Laboratory.

A recent analysis of observations and results obtained by climate models showed that previously rare high summertime (June, July and August) temperatures are already occurring more frequently in some regions of the 48 contiguous United States.

The report suggests that extreme high temperatures could become the norm by midcentury if the world continues on a business as usual schedule of emitting greenhouse gases. Phil Duffy, lead author of the report, said:

The observed increase in the frequency of previously rare summertime-average temperatures is more consistent with the consequences of increasing greenhouse gas concentrations than with the effects of natural climate variability. It is extremely unlikely that the observed increase has happened through chance alone.

The geographical patterns of increases in extreme summer temperatures that appear in observations are consistent with those that are seen in climate model simulations of the 20th century, Duffy said.

According to the report, climate models project that previously rare summer temperatures will occur in well more than 50 percent of summers by mid-century throughout the lower 48 states. Duffy said:

The South, Southwest and Northeast are projected to experience the largest increases in the frequency of unusually hot summers. The strong increase in extremes in the Southwest and Northeast are explained by strong historical and projected warming there. This result is based upon assuming a commonly used scenario for future emissions of carbon dioxide, the main driver of human-caused climate change.

What was historically a one in 20-year occurrence will occur with at least a 70 percent chance every year. This work shows an example of how climate change can affect weather extremes, as well as averages.

Bottom line: According to researchers at the Lawrence Livermore National Laboratory, extreme summer temperatures are already occurring more frequently in the United States, and could become normal by mid-century.

Read more from the Lawrence Livermore National Laboratory

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