If you live in a region of the Northern Hemisphere that is not experiencing record-breaking heat waves this summer, consider yourself lucky. The US, the UK, Japan, Northern Europe, Western Europe, China and Mexico (phew!) are all suffering from abnormally hot and dangerous summer temperatures in 2018.

The vast majority of climate scientists agree that climate change is making these extreme temperatures more intense and more frequent, putting global public health at risk. Still, when it comes to mental health - an important but overlooked aspect of public health - the effects of a red-hot weather map are rarely considered.

Now, a ground-breaking new study has found a disturbing link between climate change, extreme temperatures and suicide. By comparing temperature and suicide data from thousands of US counties and Mexican municipalities over several decades, the study has revealed strong evidence that hotter weather increases suicide rates.

The findings reveal that if the world warms by 2.5 degrees Celsius by 2050, it could result in a 1.4 percent increase in America's suicide rate and a 2.3 percent increase in Mexico's suicide rate.

After taking into account population growth and no less than 30 climate models, the researchers predict that hotter temperatures could result in an additional 21,000 suicides in the US and Mexico by 2050.

This is no small matter. As the authors point out, these effects are as large in size as the influence of economic recessions (which increase the suicide rate) or gun restriction laws (which decrease the suicide rate).

One of the authors of the paper, Marshall Burke, described the findings as "brutal."

"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," said Burke, a professor of Earth system science at Stanford.

While the study does not suggest that temperature is the only factor associated with suicide, suicide rates were not significantly altered by gun ownership, sex, population size, average income, or the availability of air-conditioning.

The study is also supported by previous research, which has found seasonal patterns in suicide rates, with a spike generally occurring at the beginning of summer.

"Hotter temperatures are clearly not the only, nor the most important, risk factor for suicide," Burke emphasized.

"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."

The consequences of the findings are disquieting. Right now, suicide alone causes more deaths globally than all forms of interpersonal and intergroup violence combined. In fact, suicide is among the top 5 causes of death globally, and among the top ten in the US.

"Thus, even modest changes in suicide rates due to climate change could portend large changes in the associated global health burden, particularly in wealthier countries where current suicide rates are relatively high and/or on the rise," the study concludes.

What the researchers found is not a modest change. If nothing is done to combat greenhouse gas emissions, they predict that anywhere from 14,000 to 26,000 suicides could take place in the US by 2050.

"We've been studying the effects of warming on conflict and violence for years, finding that people fight more when it's hot," said co-author Solomon Hsiang.

"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."

Exactly how hot temperatures impact the human mind was beyond the scope of this study, however, the authors do have a few theories. So far, the most probable explanations have to do with the side effects of thermoregulation or other neurological responses to temperature, which could, theoretically, impact mental health.

"Studies suggest that some components of brain chemistry, in particular certain neurotransmitters, are important in both mental health and in how the body regulates its internal temperature," Burke told the Atlantic.

"That to us suggests at least there's a plausible biological linkage between temperature, thermal regulation and how the brain regulates its own emotion."

This hypothesis certainly fits within the study's findings. After collecting and analyzing over 6 million location-specific Twitter updates in the US, the study also found a significant link between "depressive" language on Twitter - such as "lonely," "trapped" or "suicidal" - and unusually hot local temperatures.

Specifically, it was revealed that for each additional 1 degrees Celsius increase above average, the likelihood of a tweet containing depressive language rose by 0.79 percent. 

As interesting as these results are, the underlying mechanism here is again poorly understood. Until researchers can better understand the link between hotter temperatures and suicide, the only way to solve this deadly global issue is to halt the rapid warming of our planet.

"If there is going to be adaptation in the future, it has to be unprecedented. It has to be unlike anything we've seen over the past half century in the United States," Burke added.

The study has been published in Nature Climate Change.

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