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It’s a relationship that defies the cliché of summer as a time of easy living: For those who are vulnerable, it appears to be warmer weather, not cold, that increases the rate of death due to suicide.

Now a large-scale study is offering new evidence that the connection is real. And based on the anticipated impact of global warming, researchers are forecasting tens of thousands of additional fatalities than would otherwise be expected if temperatures were stable.

“We think there is a mental-health aspect to living in a warmer and more variable climate,”said Patrick Baylis, an environmental economist at the University of British Columbia and a co-author on the study. “It’s part of the social cost of climate change.”

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In their results, published Monday in the journal Nature Climate Change, the researchers show a small but measurable correlation that translates into a one per cent increase in suicide rate for an average temperature increase of 1° Celsius.

The correlation appears to be independent of income or access to air conditioning and it holds true for populations who live in northern climates as much as for those in the south.

The researchers caution that their results do not suggest that climate change can be isolated as the root cause of any individual death by suicide. The small effect they discern is easily obscured by a variety of complex social and psychological factors and only emerges when looking across a large population.

In this case, the study drew on 40 years of public health records in the United States and 20 years from Mexico. As further evidence, researchers also combed through more than 600 million social media posts and found that instances of “depressive language” increase with local monthly temperatures.

“We had a lot of data to throw at this, which is important if you want to separate the effects of temperature from all the other important factors that cause suicide risk,” said Marshall Burke, a Stanford University resource economist who led the study.

Based on historic trends the researchers then used climate models to project an excess 20,000 deaths by suicide in the two countries by the year 2050.

The study does not shed light on why warmer weather would have such an effect. Various explanations have been suggested, ranging from social and environmental factors to the physiological effect of temperature on brain chemistry.

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The researchers point out that their findings are in agreement with other mental health studies that show social conflict and violent behaviour also track with temperature.

While Canadian data was not included in the study, Dr. Burke said that there is good reason to think Canada would be similarly affected.

“What we do find is that it doesn’t matter where you are. A one-degree temperature increase has the same effect. To us, that implies that this is something fundamental about how humans respond and that appears hard to adapt to,” he said.

Mark Sinyor, an associate scientist at the Sunnybrook Research Institute in Toronto who was not involved in the study, said that it offered some of the most persuasive evidence yet that increases in temperature are associated with more suicide deaths.

In 2014, Dr. Sinyor was part of a team that published a smaller study comparing mortality statistics from Toronto with that from two counties in Mississippi. Their results similarly showed that suicide rates were higher during weeks that were unusually warm in both locations.

“Since every death by suicide is a tragedy both for the person and his or her loved ones, we have an obligation to try to prevent all of them and, sadly, it appears that climate change may be another obstacle in that effort that we must work to overcome,” he said.

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Dr. Sinyor emphasized that while the new study’s forecast for an increasing suicide rate driven by a warming climate is something policy makers should take seriously, there are steps that can be taken to change that outcome.

“We can educate people about the signs of mental disorders, decrease stigma associated with seeking help and make sure that everyone has timely access to high quality mental health-care,” he said.

“By paying attention to those things as well as taking steps to preserve our environment, we can prevent many deaths − and that should be our focus.”

Personal crises that can lead to thoughts of suicide are usually temporary and those who are experiencing such thoughts are urged to seek help by by contacting a regional crisis centre. A list of of centres across Canada and be accessed through the Canadian Association for Suicide Prevention.

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