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Researchers in one survey found that, on average, every additional 1 degree Celsius of warming over five years was linked to an increase of mental health issues in those areas by 2%.
On Oct. 8, a consortium of 90 scientists from 40 countries published a report warning that inaction could lead to catastrophic climate change in the next 20 years. On the same day, a paper published in the Proceedings of the National Academy of Sciences by researchers from MIT, Harvard, and other top American research institutions concluded that climate change has the potential to worsen mental health on a mass scale. And such changes may already be measurable in the U.S. population.
The team used responses from the U.S. Centers for Disease Control’s long-running health survey of American adults, which samples randomly selected U.S. residents each year. One question on the survey asks people about their mental health:
“Now thinking about your mental health, which includes stress, depression, and problems with emotions, for how many days during the past 30 days was your mental health not good?”
The researchers used survey responses from 2002 to 2012, taking in about 2 million participants cumulatively. Since the survey includes data on where the respondents lived and when they answered the question, the team was able to pair the results with historical meteorological data.
They found that, on average, every additional 1 degree Celsius of warming over five years was linked to an increase of mental health issues in those areas by 2%. In already-hot places, where the average monthly temperature was between 25°C and 30°C (77°F and 86°F), just making the shift to an average monthly temperature above 30°C added 0.5% more mental health difficulties to the population.
To consider the impact of extreme weather disasters on mental health, they looked at the effect of Hurricane Katrina. The storm increased the prevalence of mental health difficulties among those affected by 4%, they found.
They also found that, on average, the mental health of low-income people was most harmed by hotter temperatures. Women, on average, were also harmed more than men. Given that this data was on people living in the US—a wealthy country with a relatively temperate climate—they note that countries with “less-temperate climates, insufficient resources, and greater reliance on ecological systems may see more severe effects of climate change on mental health.”
Of course, survey responses can’t indicate clinically diagnosed mental health conditions. But given that mental health conditions often go undiagnosed in a clinical setting, the researchers note that survey responses have the advantage of including instances of distress that would otherwise fly under the statistical radar.
What’s more, the study is about correlation, not causation—we still don’t know conclusively what, if anything, weather can do to causemental distress. Exposure to more extreme weather “may produce physiological stressors that precipitate poor mental health,” or “such extremes may initiate inflammatory processes that worsen mental health,” the researchers hypothesize.
Or, they write, the effects may be caused “entirely through reductions in health maintenance behaviors, like exercise and sleep.” Research already links warming nighttime temperatures to worse sleep quality, for example. And hotter summers in southern states is already projected to lead (pdf) to less exercise there. But more research is needed to truly determine the cause of the worsened mental health found in this study.
“Given the vital role that sound mental health plays in personal, social, and economic wellbeing—as well as in the ability to address pressing personal and social challenges—our findings provide added evidence that climatic changes pose substantial risks to human systems,” the researchers write.
This isn’t the first indication that mental hardship could stem from climate change: A consortium of scientists called the Climate Impact Lab has authored peer-reviewed studies that found higher temperatures increase suicide rates (pdf) in the U.S., Mexico, and India.
In the study on India, as reported by the Guardian, the researchers found that suicides go up particularly when heat damages crops during a particular growing season. When it’s hotter than 20°C (68°F), each degree increase in a single day correlates to about 70 additional people killing themselves, which implicates warming temperatures in an estimated 59,300 suicides in India over 30 years.
In 2017, the American Psychological Association issued a 69-page literature review that identified “ecoanxiety” as a legitimate mental health condition, noting that disasters like the “unrelenting day-by-day despair” of a prolonged drought, or more insidious changes like food shortages, rising sea levels, and the gradual loss of natural environments, will “cause some of the most resounding chronic psychological consequences.”
“Gradual, long-term changes in climate can also surface a number of different emotions, including fear, anger, feelings of powerlessness, or exhaustion,” the association wrote.
Zoë Schlanger is an environmental reporter at Quartz, where this article was originally published.