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Summer can be bad for your (mental) health

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A recent paper by Stanford University's Marshall Burke and collaborators estimates that a 1 degree Celsius increase in average monthly temperature produces a 0.68 per cent increase in the monthly suicide rate in the US and a 2.1 per cent increase in Mexico.

[NEW YORK] It's sunny and hot, colleagues are turning up at work in shorts, restaurants' summer verandas are teeming with customers. That sounds great. And yet for some people, those conditions trigger depression and even suicidal thoughts.

A recent paper by Stanford University's Marshall Burke and collaborators estimates that a 1 degree Celsius increase in average monthly temperature produces a 0.68 per cent increase in the monthly suicide rate in the US and a 2.1 per cent increase in Mexico. Mr Burke also found that each additional degree of average temperature boosts the likelihood of depressive language in Tweets by 0.79 per cent. This means heat waves like the current one in the US, with temperatures 10 degrees or more above average, result in dozens of additional suicides and in emotional lows for hundreds of thousands of people – one reason why climate change is dangerous even to people living in countries that aren't threatened by flooding and, to a superficial eye, would only benefit from a slightly warmer climate.

The Burke study is significant because of its painstaking design; it takes in the period from 1968 through 2004, uses granular data from US counties and Mexican municipalities and takes care to weed out other possible factors, including gun ownership rates, inequality and economic cycles. But it's far from the first in establishing a connection between higher temperatures and people's taking their own lives (a 2016 paper by Konstantinos Fountoulakis, which finds that climate explains more of the variability in suicide rates across Europe than do economic factors, contains a particularly impressive list of studies showing the temperature connection).

Since the 19th century, researchers have looked and often found seasonality in suicide rates. Most saw peaks in late spring and early summer (for example, data for Sweden and Finland since the 1750s show that the greatest share of suicides have occurred in the summer), some noted a second peak in October. In recent years, however, the findings focused on heat as a catalyst. In 2007, Lisa Page of the London Institute of Psychiatry and her collaborators found no evidence of suicide seasonality in England and Wales, but discovered that with each degree above 18 degrees Celsius, the suicide rate increased 3.8 per cent. In 2011, Viktoria Likhvar and collaborators showed that increases in temperature resulted in an immediate bump in self-inflicted mortality due to more violent suicides, such as shootings and hangings (as opposed to quieter methods such as drug overdoses and poisonings).

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Life scientists have struggled to find a universal biological explanation for the phenomenon, but they've suggested some theories. "Although speculative, perhaps the most promising mechanism to link suicide with high temperatures is a psychological one," Ms Page wrote. "High temperatures have been found to lead individuals to behave in a more disinhibited, aggressive and violent manner, which might in turn result in an increased propensity for suicidal acts."

Researchers have also suggested that suicide rates may have more to do with alcohol consumption than with temperature; though relatively few suicides occur during the Christmas season, when people also drink a lot, Ms Page did find a Jan 1 peak in the UK. And in many countries, summer is the season when people drink the most outside of the Christmas season. That's a possibility for which the recent Stanford paper doesn't account.

Whatever the causality, however, it's clear that there's a strong correlation between hot weather and depressive states of mind, as well as hot weather and suicide, especially the violent kind.

This should make us worry more about climate change and its implications. Even if we want it to be warmer where we live, we might end up with increases in aggression and suicide rates as temperatures go up. Mr Burke and collaborators calculated that a 2.5 degree increase in US average temperatures by 2050 compared with the 2000 level – a likely scenario based on current climate change models – would drive up the suicide rate by 1.4 per cent, causing more than 14,000 excess suicides. Such a prospect makes it important for scientists to figure out what actually causes the correlation and how to mitigate it: If climate change cannot be completely reversed, we should at least make a better effort to deal with its effects by improving mental health provisions and early-warning systems.

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