The CDC's surveillance survey basically asked participants, "How, over the recent period, has your mental health status been?" Nick Obradovich, study author and scientist at the Massachusetts Institute of Technology, told CNN.
“Exposure to hotter temperatures and higher rates of precipitation in that period produced increases in the probability that people were going to report some mental health problem in that period,” he said.
Obradovich and his colleagues found that shifting monthly temperatures between 25 degrees Celsius (77 degrees Fahrenheit) and 30 degrees Celsius (86 Fahrenheit) to monthly averages greater than 30 degrees Celsius (86 Fahrenheit) related to a 0.5 percentage point increase in the prevalence of mental health difficulties.
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“If this exact change in temperature were generalized across the nation,” Obradovich explained to CNN, “‘that would produce approximately 2 million additional individuals reporting mental health difficulties.’”
Over the past five years, the team found that an increase of just 1 degree Celsius was associated with a 2 percentage point increase in the prevalence of mental health difficulties.
The most vulnerable populations include those with lower incomes, existing mental health issues and women, according to the study.
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Obradovich’s work isn’t the first to address a link between rising temperatures and mental health problems.
A study published in July in the journal Nature Climate Change found that the shifting seasons may even lead to 26,000 more suicides in the United States by 2050.
If monthly temperatures are 1 degree Celsius warmer than usual, researchers estimate suicide rates in the U.S. will increase by 0.7 percent and by 2.1 percent in Mexico.
"'Climate change is going to generate winners and losers'—this is a phrase you hear all the time," study author and Stanford University professor Marshall Burke told the Atlantic. "But for this outcome, it's all losers. There are no winners. We find these strong linear relationships everywhere when you crank up the temperature."
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A 2012 study on the impact of extreme heat on morbidity in Milwaukee, Wisconsin, also concluded that climate change may increase heat-related hospital admissions and impact admissions specifically for self-harm, including suicide attempts.
“The most important point of this [new] study is that climate change, indeed, is affecting mental health, and certain populations (women and the poor) are disproportionally impacted,” Jonathan Patz, a professor and director of the Global Health Institute at the University of Wisconsin-Madison who was not involved with the study, told CNN.
You can purchase the full PNAS study for $10 at pnas.org.