Rising temperatures linked to human-caused climate change could lead to increasing suicide rates in the U.S. and Mexico, a study suggested Monday.
By comparing historical temperature and suicide data going back decades, researchers found a strong correlation between warm weather and increased suicides, according to new research published in Nature Climate Change, a peer-reviewed British journal.
Researchers have known for centuries that conflict and violence tend to peak during warmer months.
"Now we see that in addition to hurting others, some individuals hurt themselves," said Solomon Hsiang, study co-author from the University of California-Berkeley. "It appears that heat profoundly affects the human mind and how we decide to inflict harm."
The study found climate change could lead to 9,000 to 44,000 additional suicides across the U.S. and Mexico by 2050.
“This may be the first decisive evidence that climate change will have a substantial effect on mental health in the United States and Mexico, with tragic human costs,” Hsiang said.
The study found suicide rates rise 0.7 percent in U.S. counties for each 1.8-degree increase in monthly average temperature.
"The thousands of additional suicides that are likely to occur as a result of unmitigated climate change are not just a number, they represent tragic losses for families across the country," said lead author Marshall Burke of Stanford University.
In the U.S., suicides claim nearly 45,000 lives a year, twice the number of homicides, and they are the 10th-leading cause of death. Suicide rates in the U.S. have risen nearly 30% since 1999, according to a report in June from the Centers for Disease Control and Prevention.
Globally, about 800,000 people die as a result of suicide every year, the World Health Organization said.
"Suicide is one of the leading causes of death globally, and suicide rates in the U.S. have risen dramatically over the last 15 years," Burke said. "So better understanding the causes of suicide is a public health priority."
Mental health experts caution other factors should be considered when studying suicide rates.
Medication, prescription costs and the economy are key parts of the equation, said Daniel Reidenberg, executive director of Suicide Awareness Voices of Education, a non-profit organization.
"If in fact suicide rates were truly corresponding to the temperature, would that suggest that we keep people who are suicidal in cooler climates or temperature-controlled settings and that would reduce the risk of their death?" Reidenberg said.
Study co-author Burke said hotter temperatures are "clearly not the only, nor the most important, risk factor for suicide."
"But our findings suggest that warming can have a surprisingly large impact on suicide risk, and this matters for both our understanding of mental health as well as for what we should expect as temperatures continue to warm," he said.
The past three years have been the globe's hottest on record, the National Oceanic and Atmospheric Administration said, just as carbon dioxide levels are at their highest in 800,000 years.
A study last year in the Proceedings of the National Academy of Sciences found that "warming temperature trends over the last three decades have already been responsible for over 59,000 suicides throughout India."
Looking at social media, the authors found higher monthly temperatures also were associated with an increased use of "depressive language" on Twitter. Looking at more than a half-billion Twitter posts, they found that tweets contain language such as "lonely," "trapped" or "suicidal" more often during hot spells.
Hotter temperatures were nearly the same as the influence of economic recessions, which increase suicide rates, or suicide prevention programs and gun restriction laws, which decrease suicide rates. The study recommends policymakers "implement policies to mitigate future temperature rise."
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