A hotter planet could lead to tens of thousands more suicides by 2050 in the United States and Mexico alone, unless global warming is curbed, according to a study published on Monday.
Researchers examined decades worth of temperature data against suicide rates in U.S. counties and Mexican municipalities, some dating back to the 1960s, and found that hotter weather was linked to increases in deaths by suicide.
"Hotter temperatures are clearly not the only, nor the most important, risk factor for suicide," lead author Marshall Burke, an economist at Stanford University, said in a statement.
"But our findings suggest that warming can have a surprisingly large impact on suicide risk, and this matters for both our understanding of mental health as well as for what we should expect as temperatures continue to warm," he said.
According to their analysis, a 1 degree Celsius (1.8 degrees Fahrenheit) increase in average monthly temperature led to a 0.7 percent rise in suicide rates in the United States, and a 2.1 percent rise in Mexico.
The paper, published in the journal Nature Climate Change, also analysed the language used in over half a billion Twitter posts and found that suicide rates and depressive language increased during hot weather.
The study projected that if global warming is not capped by 2050, there could be at least an additional 21,000 suicides in the two countries alone.
Suicide rates rose in nearly every U.S. state from 1999 to 2016, with the rate spiking by more than 30 percent in half of the country, the Centers for Disease Control and Prevention reported in June.
Nearly 45,000 people committed suicide in 2016, making it one of three leading causes of death on the rise in the United States, along with Alzheimer's disease and drug overdoses.
"When talking about climate change, it's often easy to think in abstractions," said Burke, an assistant professor in environmental sciences.
"But the thousands of additional suicides that are likely to occur as a result of unmitigated climate change are not just a number, they represent tragic losses for families across the country," he said.
The past three years were the hottest on record, the United Nations' World Meteorological Organisation said in March.
The World Health Organization says that heat stress, linked to climate change, is likely to cause 38,000 extra deaths a year worldwide between 2030 and 2050.
In a heat wave in May, more than 60 people died in Karachi, Pakistan, when the temperature rose above 40C (104F).
In 2015, countries signing the Paris Agreement set a goal of limiting a rise in average world surface temperatures to "well below" 2C (3.6F) above pre-industrial times, while "pursuing efforts" to limit rising temperatures to 1.5C (2.7F).
U.S. President Donald Trump has vowed to pull out of the accord, which would make his country the only one to do so.