A couple of years ago Solomon Hsiang, assistant professor of public policy at University of California, Berkeley, became fascinated by an important issue: what effect does heat have on our brains – and emotions? It is a pertinent question right now, given that temperatures have soared this summer across most of Europe and the US – currently, some 70 million Americans are still under some level of “heat advisory” notice. 

Some readers will have welcomed the warmer weather. If you live in Scotland or Sweden, where you don’t often get the chance to spend time on the beach in August, summer sunshine has historically been associated with joy. Conversely, if you live in a city with minimal air conditioning (London, for example) or in one of those sweltering US states, a sharp rise in temperature is greeted with horror. 

But what Hsiang and his research team wanted to know is whether there is something about hot weather that destabilises our brain and makes us violent. This is not a new issue: a recent meta-analysis of 60 prior studies showed that unstable temperatures tend to be correlated with conflict.

In the US, for example, there is good evidence that road rage, domestic assault and murder are higher during heatwaves. In the Netherlands, scientists have shown that police are more likely to attack suspects if they are in a hot room (say 27C) rather than a cool one (21C). 

In Tanzania, there is evidence that the brutal murder of elderly women, dubbed “witch killings”, soar after periods of drought, or other unusual weather patterns. Meanwhile, scientists and historians have shown that temperature change has been implicated in conflict in the modern-day Middle East, various conflicts of the 17th century and even the fall of Rome. 

What Hsiang and his colleagues – a seven-strong team led by Marshall Burke at Stanford – were particularly interested in discovering was whether heat also led to suicide. And so they crunched through extensive data on suicides and weather in the US and Mexico, and analysed more than half a million social-media posts to assess the wider social mood. 

Their paper, published in July, showed that not only was hot weather associated with a sharp rise in mental instability – as measured by the use of keywords linked to suicide on social media – but it also went hand in hand with higher suicide rates. Most notably, a 1 degree change in the temperature was correlated with a 0.7 per cent increase in the suicide rate in US counties, and a 2.1 per cent increase in Mexican cities – irrespective of whether the normal base temperature was hot or cold. 

“Mental well-being deteriorates during warmer periods,” the paper reports, noting that “unmitigated climate change” could result in “a combined 9,000–40,000 additional suicides across the United States and Mexico by 2050, representing a change in suicide rates comparable to the estimated impact of economic recessions, suicide prevention programmes or gun restriction laws”.

A sceptic might argue that much of the data is still not extensive enough to be truly definitive. They might also point out that the projected rise in suicides does not necessarily look that dramatic when you look at some of the shocking projections that are being bandied around about the economic cost of climate change, and its associated geopolitical upheaval (which will undoubtedly loom large at the UN General Assembly meeting later this month).

But one reason why Hsiang, Burke and others explored suicide was to make a bigger point: you cannot look at climate change solely in terms of economics or simply explain the correlated conflict through that lens. Yes, changes in weather can sometimes cause fights over scarce resources; just look at how water shortages have sparked conflict around Gaza and are increasingly stoking tensions between Iraq, Syria and Turkey.

However, the fact that we know more heat corresponds to higher levels of domestic violence, road rage – and now suicide – suggests that climate not only affects our tangible environment, but may also be impacting our brain chemistry, albeit in ways that researchers admit that they do not fully understand. “The pattern is very striking, but the research is at an early stage,” says Hsiang, who likens the situation to the early days of investigating the link between lung cancer and smoking. “We know there is a correlation, but we don’t quite know why,” he says.

This has several implications. One is that scientists need to do more research into human brains and heat. Another is that people running companies, hospitals, schools, prisons – or, indeed, any place where humans study or work – need to think about how temperature might affect their staff.

There is a third, more personal, implication: we could probably all benefit from reflecting a little more on how heat changes our minds. Most of us already know, for example, that being overheated makes us more irritable and tired; but do we also make worse decisions in hotter months? Do we get more aggressive in ways we sometimes ignore? It is a curious question to ponder; particularly as cooler days loom.

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Letter in response to this column:

Heat and humidity do scramble the brain / From Alice Bray, New York, NY, US

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