If You Thought It Was Hot Last Summer . . .

By Andrew Freedman

If you thought last summer was hot, just wait. What are now considered to be unusually hot summer temperatures are likely to be the new normal in just a few decades.

In a trend that fits with manmade global warming, a new study shows the odds of unusually warm summers have already increased in much of the United States, and will rise much more in the near future.

Other studies have looked at changes in short-term heat waves, but this is the first to look at the odds of changes in average summer temperatures overall — the new “normal” temperatures we can expect more of the time as global warming continues.

The research, published in the journal Climatic Change, found that warming average summer temperatures are more consistent with the influence of manmade global warming than natural variability. 

Exceedance frequencies during 2035-2064 for summer-mean temperatures that were 95th percentile values during 1950–1979, from the an ensemble of 16 computer models. Credit: Duffy and Tebaldi/Climatic Change.

Claudia Tebaldi, a senior scientist at Climate Central and a co-author of the study, said the results are also consistent with other recent research showing summertime warming trends in the U.S. 

By examining historical weather observations and simulations from 16 state-of-the-art climate models, the research showed that extremely warm average summer temperatures that used to be quite rare are already occurring more often in certain regions, particularly in the Western U.S., upper Midwest and Atlantic Coast. 

Tebaldi said the computer model simulations that include rising amounts of global warming gases such as carbon dioxide closely matched the actual warming trends and shift in odds towards extreme summer temperatures that have occurred during the past several decades, which not only adds confidence to projections for the future, but also suggests that manmade climate change is playing a key role.

Tebaldi and her coauthor, Phil Duffy of the Lawrence Livermore National Laboratory in California, used those model-based results to project the likelihood that summer temperatures would exceed a certain threshold all the way out to the year 2064. What they found was especially worrisome for the Southwest, where warming is expected to be the most significant, while other parts of the country may not warm quite as much. However, even in the Midwest, which shows less intense warming so far and in the coming decades, the change in mean summertime temperatures will be very noticeable, Tebaldi said. 

“By that time the map is all practically red, which means in every region what was a 5 percent chance event becomes more like a 70 percent chance event,” she said.

Warmer summers can place a greater strain on the electrical grid as well as the human body, with increased heat-related illnesses and deaths. Also, hotter summers can affect agricultural production and water resources, which is a particular concern in the already parched Southwest.

Children cool off in the hot summer heat. Credit: fulchismo/flickr

A recent study by researchers at Stanford University found that permanently hotter summers, in which the coolest summer temperatures will be hotter than the hottest temperatures of past summers, may occur in the U.S. far sooner than was previously anticipated. The study found a 50 percent likelihood that a regime shift into an era of hotter summers would reach North America by midcentury, after starting in the tropics. 

Last summer may have been a preview of what’s to come. Oklahoma and Texas set records for the hottest average temperatures for June, July and August in recorded history. In fact, Oklahoma set a record for the highest average summer temperature in any state in U.S. history, with an average temperature close to 87°F. In Texas and Oklahoma, many communities smashed all-time records for the most 100 degree days. Intense heat also enveloped the Mid-Atlantic, Northeast, and Midwest last summer.

Tebaldi said her study does show an area of more intense summertime warming that extends into West Texas, but noted that Oklahoma has seen a smaller increase in the odds of very warm summertime temperatures. 

Still Oklahoma, like the rest of the country, is likely to see unusually hot summers become the norm by mid to late century.