Around the world, spikes in human violence correlate with unforgiving and ceaseless heat. As the planet gets hotter, scientists are questioning how peaceful societies will be increasingly disrupted by oppressive hot spells. A study released Wednesday in Nature Climate Change suggests things are only going to get worse. Not only is the heat instigating violence between people, but the sweltering temperatures also appear linked to people hurting themselves.

In the study, the international team argues that suicide rates are already influenced by rising temperatures and predicts that impending shifts in climate will result in thousands more suicides directly linked to the hotter weather. Their projected model predicts that temperature increases through 2050 could lead to an additional 21,000 suicides in the United States and Mexico alone.

“Our research speaks to the potential human cost of climate change, and to the benefit we will see if we can reduce future temperature increases through mitigation,” first author and Stanford University Earth system science professor Marshall Burke, Ph.D., tells Inverse. “We find that climate change, if left unchecked, will result in tens of thousands of additional suicides — which is tens of thousands of families that will lose loved ones.”

A new study says that suicide rates are likely to rise as the planet gets hotter. PX Here

Suicide is a leading cause of death globally, and suicide rates in the US have increased by 25 percent over the last 15 years. Meanwhile, warmer months have been linked to increased cases of self-harm. A recent Public Health review concluded that “high ambient temperatures have a range of mental health effects,” and a 2007 research paper from the British Journal of Psychiatry on suicide rates in Britain and Wales concluded that “there is an increased risk of suicide during hot weather.”

The new study is an attempt to establish whether suicide spikes are really directly linked to temperature and use the findings to build a predictive model of how climate change will affect mental health in the future. First, Burke and his team looked at the role of temperature in suicides, comparing historical temperature and suicide data across thousands of US counties and Mexican municipalities that spanned several decades. From 1968 to 2004 in the US and from 1990 to 2010 in Mexico, suicide rates rose by a respective 0.68 and 2.1 percent as monthly temperatures rose by 1.8 degrees Fahrenheit.

During these periods of time, the effect of temperature on suicide did not decrease over time. Notably, the effect also didn’t decrease when the researchers took rising incomes or the adoption of air conditioning into account.

Next, the scientists analyzed 600 million tweets in an effort to see whether hotter-than-normal days elevated the likelihood of a depressive state. Searching for words like “lonely,” “trapped,” and “suicidal,” they found that these words were used more often during hotter days. This demonstrated a consistent positive relationship between warmer temperatures and the likelihood that someone would attempt self-harm, says Burke, though he is careful to note: “While poor mental health is a risk factor in the vast majority of suicides, we cannot say that the people who appeared depressed on Twitter were more likely to commit suicide.”

A NASA map of the Earth in 2017. Blue indicates areas cooler than average and red indicates warmer than average. NASA

With the assumption that future suicide rates would continue to rise with temperatures, the team built a model designed to forecast future suicide rates in relation to changing global temperatures. The projections were stable, linear, and tragic. By 2050, the scientists calculated, temperature increases would increase suicide rates by 1.4 percent in the US and 2.3 percent in Mexico.

Despite knowing that the correlation exists, scientists are still debating why suicides are linked to rising temperatures. In 2017, UC Berkeley researcher Tamma Carleton, Ph.D., argued that failing harvests and increased poverty due to rising temperatures have caused more than 59,000 suicides in India over the last 30 years. Burke and his team, however, are more focused on the effect of heat on an individual’s psychology.

“A main hypothesis is that hotter temperatures have a direct effect on human emotion and impulsivity,” says Burke. “For instance, we also see other types of human violence rise with temperature, suggesting that the propensity to violence is simply higher when temperature is hotter.”

More research is needed to know for sure. But what we do know is that the link is there — and that the planet is getting hotter. Whether or not the rise in temperature will be mitigated by human intervention remains to be seen.

“We have the power to do something about this by slowing the future rise in temperatures,” says Burke.