A man wipes his forehead in a park on a hot summer day.
A man sweats in New York's Central Park in July 2012. Eric Thayer/Reuters

Average monthly highs above 86 degrees Fahrenheit increase the probability of mental-health issues, a new study finds.

Scientists have predicted many troubling consequences of global warming for Earth’s ecosystems and human health and welfare. Among them is an increased risk to our mental health. A new study published in the Proceedings of the National Academy of Sciences finds that short-term exposure to extreme weather, multiyear warming, and tropical-cyclone exposure are all associated with worse mental health.

“The environmental stressors that are likely to be produced by climate change—added exposure to heat, natural disasters—we have evidence that links those environmental stressors to worsened mental-health states,” Nick Obradovich, one of the researchers, told CityLab.

In the study, researchers examined meteorological and climatic data in combination with the responses of almost 2 million randomly sampled U.S. residents from the CDC’s Behavioral Risk Factor Surveillance System, a long-running health survey. Between 2002 and 2012, survey respondents answered the question: “Now thinking about your mental health, which includes stress, depression, and problems with emotions, for how many days during the past 30 was your mental health not good?” The question does not measure the incidence of psychiatric disorders, but mental-health status more broadly, including that of individuals who experience “subclinical” distress.

Compared with temperatures of 10 to 15 degrees Celsius (50 to 59 degrees Fahrenheit), average monthly maximum temperatures higher than 30 degrees Celsius (86 degrees Fahrenheit) increase the probability of mental-health issues by about 1 percent. That might seem tiny without context, but a shift from monthly average high temperatures between 25 and 30 degrees Celsius (77 and 86 degrees Fahrenheit) to averages above 30 degrees Celsius would result in nearly 2 million additional individuals reporting mental-health difficulties over a 30-day period, extrapolated to current U.S. population numbers. The study also finds that higher precipitation increases the risk of mental-health problems.

It’s unclear what mechanism or mechanisms are at work. Obradovich noted a number of possibilities: Heat can disrupt sleep; climate-change-fueled weather is less pleasant to experience; warmer temperatures can be a physiological stressor. The study determined correlations, but more research is needed to understand exact causes.

“One of the hardest things with studies like this is really narrowing down precisely why we observe what we do,” said Obradovich. “There are a variety of reasons why this could be happening. And I think one of the ones that we know relatively the least about is how much sleep is playing a role in this. But we don’t know very well exactly how that works—how much, in terms of hours per night, sleep is disrupted by higher temperatures, [and] how people can adapt to that.”

Obradovich and his fellow researchers also found that exposure to Hurricane Katrina increased reports of mental-health issues by approximately 4 percent, which they determined by comparing reported mental health in federal disaster areas to non-disaster areas, before and after Katrina.

The study does not imply that a general mental-health crisis is inevitable. The researchers note that adaptation is possible, and will depend on what factors are driving the trends. For example, if heat’s detriment to sleep is the culprit, better and more widespread cooling might help us adapt to higher temperatures. Or we might go out at different times of the day. “There are a variety of small-scale behavioral adaptations,” said Obradovich, that could allow us to better cope with climate change.

Although he was hesitant to suggest targeted approaches to mitigate the effects of climate change on mental health because of the need for more research on the cause(s), Obradovich noted that in general, more resources for dealing with mental-health problems will improve society’s resilience.

“It’s not going to be a bad thing for governments and NGOs to put added resources toward doing a better job with ... improving the mental health status of the citizens of the U.S. and around the world, as well,” he said. “We’re not going to go wrong with that approach.”

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