When is it too hot to kill? Above 90 degrees.

After this summer’s many shooting incidents — the massacre in Aurora, Colo., followed by attacks near Texas A&M, at a Sikh temple in Wisconsin, at the Family Research Council in Washington and at the Empire State Building on Friday — Americans have pondered possible explanations. Gun laws. Psychological problems. Economic distress.

Consider one more. July was, after all, the hottest month on record. Might higher temperatures and global warming spur more violent crime?

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“There is good reason to believe that climate change will become one of the major forces driving crime as the century progresses,” Emory University sociologist Robert Agnew writes in “Dire Forecast: A Theoretical Model of the Impact of Climate Change on Crime,” a study published in Theoretical Criminology last year. Citing droughts, famine and extreme weather, Agnew predicts that “as individuals, groups, and states struggle to cope with the effects of climate change, their ability to legally adapt to further effects will decline.”

So, harsher climes could mean bloodier crimes. But is it ever too hot to engage in violence? A study this month in the Western Journal of Emergency Medicine says that high temperatures mean more violent crime — until they don’t.

“In Dallas from 1993–1999, increases in daily temperature in the low to moderate range were associated with increased aggravated assault, homicide, and rape,” Janet L. Gamble and Jeremy J. Hess write in “Temperature and Violent Crime in Dallas, Texas: Relationships and Implications of Climate Change.” “A temperature threshold appeared to exist (at approximately 90°F) where the positive relationship between mean temperature and violent crimes became negative.”

In other words, when the mercury rises above 90, criminals take a Ga­tor­ade break.

Such patterns could hold lessons for law enforcement officials, the authors write. “Regardless of the implications of a warming climate, the associations between temperature and violent crime offer opportunities for enhanced prevention and preparedness,” Gamble and Hess conclude.

— Justin Moyer, Outlook editorial aide

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