STANFORD, Calif. — Global warming doesn’t just hurt the planet’s health — it may harm people’s too. An alarming new study from Stanford University brings one potential major human-afflicting consequence of climate change into focus by exploring the link between rising temperatures and suicide rates.

Researchers discovered a positive association between the two, but a shocking one at that. They predict something to the tune of 21,000 suicides in just the U.S. and Mexico by 2050.

The study, led by Stanford economist Marshall Burke, gathered a broad set of data, including historical temperature and suicide rate data across thousands of regions, along with half a billion tweets and Twitter updates, to help establish the correlation between hot weather and suicide rates. Suicide is now one of the world’s leading causes of death, making this research more timely than ever.

While previous research had established that warm weather was linked to a higher incidence of suicide, there was no consensus on whether balmy temperatures were the cause of the epidemic or merely correlated. Extra daylight hours or high unemployment rates, for example, could be the true culprit.

Hot weather, on its own, the researchers found, does increase both suicide rates and the use of depressive language on social media, which confirmed their hypothesis.

“Surprisingly, these effects differ very little based on how rich populations are or if they are used to warm weather,” says Burke, an assistant professor of Earth system science at the university.

This means that air conditioning and other modern conveniences have not negated the effects of hot weather; suicide rates, in their midst, have continued to rise.

The researchers came to their headline figure of 21,000 added suicides by combining their quantitative data with established climate model projections. In percentage terms, anticipated temperature increases could lead to a 1.4 percent hike in suicide rates in the U.S., and a 2.3 percent increase in Mexico.

Put another way, climate change may have as outsize an effect on suicide rates as an economic recession.

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“We’ve been studying the effects of warming on conflict and violence for years, finding that people fight more when it’s hot,” explains Solomon Hsiang, the study’s co-author. “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 add that heat in itself acts as less of a risk factor for suicide than a multiplier. In other words, heat on its own doesn’t lead to self-harm, but can increase the chances of one inflicting bodily harm to themselves, given other adverse circumstances.

“Hotter temperatures are clearly not the only, nor the most important, risk factor for suicide,” Burke emphasizes. “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.”

Burke et al. published their findings on July 23, 2018 in the journal Nature Climate Change.