Higher Temperatures Tied to Rising Suicide Rates in US, Mexico

George W. Citroner

July 30, 2018

Rising average temperatures may lead to a significant increase in suicide rates in both the United States and Mexico, new research suggests.

After comparing historical temperatures with suicide data in the two countries, investigators found a strong association between warming temperatures and rising suicide rates. They note that this effect may amount to thousands of additional deaths caused by climate change throughout the region by 2050.

"Suicide is a leading cause of death globally, and suicide rates in the US and other wealthy countries have been rising," coauthor Sam Heft-Neal, PhD, research fellow at the Center on Food Security and the Environment and in the Department of Earth System Science at Stanford University, California, told Medscape Medical News.

"Better understanding the causes of suicide is a public health priority," said Neal.

The study was published online July 23 in Nature Climate Change.

Decades of Weather, Suicide Data

"Strong seasonal patterns in suicides (typically, an early summer 'peak') were recognized in the nineteenth century," write the investigators. In addition, "suicide alone causes more deaths globally than all forms of interpersonal and intergroup violence combined."

What was not known previously is whether this pattern is caused by warmer temperatures; seasonally varying meteorologic factors, such as daylight exposure; or social or economic factors, which vary seasonally.

To address this, the researchers compared decades' worth of temperature and suicide data from thousands of US counties and Mexican municipalities to isolate the role temperature had on suicide rates from other factors.

The research team also carefully examined the language used in more than half a billion Twitter updates and tweets to further confirm the effect that hotter weather may have on mental well-being.

Tweets were analyzed to find a correlation between higher temperatures and words such as "suicidal," "lonely," or "trapped."

"Our findings suggest that for a one-degree Celsius rise in monthly average temperature, suicides in the US rise by almost 1% and suicides in Mexico rise by just over 2%," said Neal.

"These effects have not diminished over time, suggesting people have not historically adapted. We project that unmitigated climate change could result in tens of thousands of additional suicides across the United States and Mexico by 2050," he said.

He admits that although they found evidence that the effect is a biological response, they were unable to reject nonbiological explanations, such as how a change in temperature could affect social mediators of suicide.

Still, the investigators were surprised to find that "the effects differed very little based on whether a population was used to hot weather or their economic status," said Neal.

"Past research has shown an association between hotter temperatures and conflict and violence," he added. "We now show that there also appears to be a relationship between hotter temperatures and self-harm."

Although previous research, including a study published April 2015 in Psychology, Health and Medicine, has also shown that suicide rates peak during warmer months, because of the difficulty in excluding other factors, not all studies have identified this effect.

Not a "Direct Motivator"

Commenting on the findings for Medscape Medical News, Joseph H. Baskin, MD, staff psychiatrist at the Cleveland Clinic Foundation and assistant professor at the Cleveland Clinic/Case Lerner School of Medicine, Ohio, said there are "myriad factors that can lead to suicidality," including individual, economic, mental health, and societal factors, as well as access to lethal means.

"The authors did a good job reporting their findings," said Baskin, who was not involved in the research.

"They stress that rising temperature and climate change shouldn't be viewed as the direct motivations for suicide. Rather, they point out that temperature and climate may increase the risk of suicide by affecting the likelihood that an individual situation leads to an attempt at self-harm," he said.

Asked whether a biological linkage between temperature, thermal regulation, and how the brain regulates its emotions may account for the increased rate of suicide, Baskin noted that that's difficult to determine.

"Perhaps at higher temperatures one's ability to navigate difficult situations is impaired because cognition is dulled. With less of the rational mind to rely on, an individual's impulses are more prone to be given free reign," he said.

Neal noted that the researchers discovered a consistent relationship between rising temperature and increased suicide risk.

"Hotter temperatures are clearly not the only, nor the most important, risk factor for suicide, but our findings suggest that warming can have a surprisingly large impact on suicide risk. This matters for both our understanding of mental health as well as our expectations as temperatures continue to rise globally," he said.

"According to our findings, projected changes in suicide rates under future climate change could be as important as other well-studied determinants of suicide rates, like a rise in unemployment or a recession," said Neal.

However, Baskin said that more studies are needed.

"This study is a retrospective analysis; it looks backwards. The gold standard of research is a double-blind, controlled perspective study. What these findings do suggest is the need for more research," he concluded.

The study was partially funded by the Stanford Woods Institute for the Environment. Dr Neal and the other investigators have disclosed no relevant financial relationships.

Nat Clim Change. Published online July 23, 2018. Abstract

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