The Troubling Ways Heatwaves Warps Minds

August 18, 2020

Warm weather can leave many of us dreaming of ice cream, sunbathing and beaches, but the Sun can also make us violent, grumpy and depressed. Why?

It was July 1988. Across the United States, the land was simmering in the warmest summer on record. City dwellers swarmed onto beaches, electricity use was higher than ever as people cranked up the air conditioning and the freeways were lined with broken-down, overheated vehicles. Ice lollies melted before they could be eaten.

But something else was happening too.

In fact, 1988 wasn’t just a year of record-breaking sunshine, but also record-breaking violence. There were an unprecedented number of murders, rapes, armed robberies and assaults – around 1.56 million of them. While it is hard to link any single incident to the heatwave, could there have been a link between the weather and the general trend for violence?

People have suspected that warm weather can alter our behaviour for centuries. The idea is embedded into our very language – we talk of tempers “flaring”, “incandescent” rage, getting “hot under the collar” – and Shakespeare described “mad blood stirring” in the oppressive heat of a Verona summer back in 1597.

The earliest studies into the phenomenon emerged in the late 19th Century, coinciding with the first reliable crime statistics. According to one analysis, offences against people tended to peak in the summer months, while crimes against property were found to be more common in the winter.

Since then, the evidence has been piling up.

Every year, as the mercury rises, we undergo a collective transformation. Some of the symptoms are relatively minor – people are more likely to honk their horns when they’re stuck in traffic; the police usually notice a spike in disorderly behaviour; and we’re less likely to help strangers out.

But others are more disconcerting.

The global heatwave of 2018 – which led to widespread droughts and unusually high numbers of Arctic wildfires; drove reindeer to Finland’s beaches; and even shrunk a mountain in Sweden – was also associated with some alarming human occurrences. In the UK, there were a record number of 999 calls, with one police officer commenting that the public react “very strangely” to that kind of weather and in some areas, police reported that calls were up 40%.

Of course, this is all heavily anecdotal – and there are plenty of alternative explanations for these individual incidents. But the wider correlation appears to be supported by a weight of academic research from around the world.

In South Africa, for every degree that the temperature goes up, there is a 1.5% increase in the number of murders

In the UK, between April 2010 and 2018, there was 14% more violent crime at 20C than there was at 10C. In Mexico, there is more organised crime in warmer weather – and some academics suspect this is because it creates a “taste for violence”. In South Africa, scientists have discovered that, for every degree that the temperature goes up, there is a 1.5% increase in the number of murders. In Greece, one study found that more than 30% of 137 homicides reported in a particular region occurred on days with an average temperature of more than 25C.

Similar patterns involving violent crime and heat have also been observed Sub-Saharan Africa, Taiwan, the United States, Finland, and Spain… the list goes on. In all, the effect has been demonstrated in hundreds of scientific studies.

Then there are the uprisings. In one study, scientists tracked uprisings around the world from 1791 to 1880 – and found that the overwhelming majority occurred in the summer months. Whichever end of the planet they looked at, the relationship still held up. For example, in Europe they were most likely to happen in July – while in South America they were more likely to happen in January.

More recent studies have confirmed the link between social movements and the weather. An analysis of more than 7,000 events over 36 years found that they tended to happen on more clement days, and as the temperature went up, they were more likely to get violent. Anecdotally, it seems to fit. Just last week, rioting broke out in the Netherlands after the hottest week since the nation started keeping records; a building was set alight, fireworks were hurled at police officers and 27 people were arrested.

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