As global temperatures rise because of climate change, suicide rates are likely to rise as well, according to a new analysis by Stanford University and UC Berkeley researchers.

The study, published today in the journal Nature Climate Change, concluded that projected temperature increases over the next few decades could lead to an additional 21,000 suicides in the United States and Mexico by 2050. That’s an increase of several percentage points over rates today, which are actually rising as other causes of death decline.

While people have recognized for centuries that suicides tend to peak during warmer months, many factors beyond temperature also vary seasonally, such as unemployment rates or the amount of daylight. Until now, it has been difficult to disentangle the role of temperature and other risk factors.

“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,” said Solomon Hsiang, study co-author and an associate professor at the Goldman School of Public Policy. “We’ve been studying the effects of warming on conflict and violence for years, finding that people fight more when it’s hot. Now we see that in addition to hurting others, some individuals hurt themselves. It appears that heat profoundly affects the human mind and how we decide to inflict harm.”

The researchers compared historical temperature and suicide data across thousands of U.S. counties and Mexican municipalities over several decades, and also analyzed the language used in over half a billion Twitter updates – tweets – to determine whether hotter temperatures impact mental well-being. They looked, for example, at whether people’s tweets expressed feeling ‘lonely,’ ‘trapped,’ or ‘suicidal’ more often during hot spells.

“We found very strong evidence that abnormally hot weather increases both suicide rates and the use of depressive language on social media,” said lead author Marshall Burke, an assistant professor of Earth system science in the School of Earth, Energy & Environmental Sciences at Stanford who received his Ph.D. from UC Berkeley in 2014. “Surprisingly, these effects differ very little based on how rich populations are or if they are used to warm weather. For example, the effects in Texas are some of the highest in the country.”

Based on global climate models, the team calculated that temperature increases by 2050 could raise suicide rates by 1.4 percent in the U.S. and 2.3 percent in Mexico. These effects are roughly as large as the influence of an economic recession, which increases the rate, or suicide prevention programs and gun restriction laws, which decrease the rate.

The researchers emphasize that suicide is complex, with many factors contributing to people’s overall risk. But, while temperature alone is probably not a main motivation, temperature likely affects how individuals perceive, evaluate and act on their own personal situation.

“When talking about climate change, it’s often easy to think in abstractions. But 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,” Burke said.

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