The world has only a dozen years to act to keep global warming below 1.5 degrees Celsius, and stave off the most catastrophic effects of climate change, according to the latest report from the U.N.'s top climate science panel. Without rapid and drastic action, climate change will expose hundreds of millions more people to heat waves, sea-level rise, more extreme weather events — and, according to a new study published earlier this month in Proceedings of the National Academy of Sciences, worsened mental-health outcomes.

Previous studies have shown that rising temperatures can disrupt sleep patterns, worsen moods, and increase the risk of suicides, which led lead author Nick Obradovich and his colleagues to wonder if extreme temperatures might also lead to mental-health problems such as stress, depression, or anxiety. To find out, the researchers looked at self-reported mental-health data for some two million U.S. residents, collected by the Centers for Disease Control and Prevention's Behavioral Risk Factor Surveillance System between 2002 and 2012. The team compared health data to meteorological records over the same period to find out how extreme temperatures, gradual warming, and extreme weather events tracked with residents' self-assessments of mental health.

"Generally what we found was that exposure to hotter temperatures and [more] precipitation increased the reporting of mental-health problems," says Obradovich, a research scientist at the Massachusetts Institute of Technology's Media Lab. Months with at least 25 days of precipitation increased the probability of mental-health problems by 2 percent, while an increase in average monthly temperatures to above 30 degrees Celsius (or 86 degrees Fahrenheit) led to a 0.5 percent increase in the probability of mental-health issues. While half a percent may sound insignificant, on a population scale, the shift would lead to some two million more people in the U.S. reporting mental-health issues, which are already widespread and costly for American society.

The data also suggests that the risk may be slightly elevated for low-income populations compared to those with higher incomes, and for women compared to men. Those differences, though small, correspond with previous studies showing that low-income people and women are often disproportionately affected by climate change, according to Obradovich. "So those findings are not super surprising, but they are still concerning," he says.

Over the long term, every one-degree increase in average temperatures increased the risk of mental-health issues by 2 percent, while experiencing natural disasters like Hurricane Katrina led to a 4 percent jump in risk.

"If you look at the body of literature, you see that higher temperatures and exposure to extreme temperatures are associated with worsened outcomes across whole domains of human well-being," Obradovich says. "We know that increased temperatures worsen sleep and they worsen mood and they worsen cognitive performance and they worsen productivity." The big question now is why.

That's the question that Obradovich and his colleagues are tackling next, and it's a critical one for policymakers, he says, because interventions will look very different if all of the negative effects of temperature increases are driven by sleep disruptions, for example, or if daytime temperatures are also influencing mental health, mood, and cognition through some other means.

He's quick to point out that this study is just "one data point" in the growing body of literature showing climate change is costly to human health, and it has its limitations. This study looked at environmental stressors like heat, precipitation, and natural disasters, but Obradovich says that the existential threat of climate change might also have an effect on mental health that wasn't captured in this study. "For example, if people are really concerned about climate change because they live near the coast and that means maybe they'll have to move in coming decades, that's something we're not picking up at all in this study," he says, which means the study may be underestimating the risk of climate change to mental health.

There's also always uncertainty when looking to the future. We could be more resilient to climate change decades down the road. "Imagine a world where everyone in the U.S. has access to incredible, scientifically informed mental-health care," Obradovich says. "In that world, the effect between hot temperatures and mental-health outcomes might be reduced."

We could also act to prevent more warming. According to the U.N. report from the Intergovernmental Panel on Climate Change, avoiding another 0.5 to 2 degrees of warming is not about inventing new technologies, but scaling up the ones we already have. We have the means to prevent more catastrophic climate change; the question is whether or not we have the will.