This week, the Intergovernmental Panel on Climate Change released a Special Report on the effect of the world warming 1.5°C or more above pre-industrial levels. Their conclusion – to avoid climate change caused catastrophe, we must keep warming below the Paris agreement's 2°C target. Or, as Global Climate and Energy Lead Scientist at WWF Chris Weber told IFLScience: “1.5°C is the new 2°C."

There are many reasons why climate change is categorically bad, from sea level rises to food and water shortages to deathly heatwaves and more. Now, thanks to a paper published in Proceedings of the National Academy of Sciences, we can add something else to the list: our mental health. 

Previous studies have linked natural disasters to PTSD and acute depression, shown that psychiatric hospital admissions increase during hotter weather, and found that heat and drought raise rates of suicide.

For this study, researchers examined the mental health records of 2 million randomly selected US citizens using data collected by the Centers for Disease Control and Prevention’s Behavioral Risk Factor Surveillance System between 2002 and 2012, comparing the responses to meteorological and climatic data from the same period. 

Respondents were asked to answer the following: "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?" Responses were then split into two categories. One signified there had been mental health difficulties over the 30 days. Zero implied there had not. This vagueness meant the researchers could capture a wide range of mental health difficulties from the clinical to the unreported and the severe to the mild. 

So, what did they find? A clear correlation between climate change – higher temperatures, greater precipitation (in some regions), and natural disasters – and worsening mental health standards. 

When average monthly temperatures rose above 30°C, the likelihood of mental health problems climbed 0.5 percent. It sounds minimal but that 0.5 percent translates into an additional 2 million people in the US alone. What's more, when the researchers compared the mental health of individual cities from 2002 to 2012, they found a 1°C increase in average maximum temperature was associated with a 2 percent increase in the prevalence of reported mental health difficulties. 

Women and low-income households were the worst affected. When temperatures exceed 30°C, women were 60 percent more likely than men to report mental health difficulties while those from low-income households were 60 percent more likely than those from the highest-income households to do so. 

Gray and drizzly weather had a similar effect. Months with 25 days of precipitation or more increased reports of mental health difficulties by 2 percent compared to months with zero. 

The most drastic shifts in mental health, perhaps unsurprisingly, came in the wake of natural disasters. The researchers analyzed data from Hurricane Katrina and found it increased the risk of mental health difficulties by 4 percent. 

"Warming is likely to amplify the frequency and intensity of natural disasters, which often cause physical injury, psychological trauma, infrastructure damage, and societal disruption in affected regions," the authors explained.

The study faced certain limitations. Not only was it correlational, but the researchers didn't consider either the severity or the type of mental health difficulties reported. This was intentional (to keep the results straightforward) but it's something they hope to explore in future studies. It was also US-centric. Reports show that the world's poorest countries will be among the most affected by climate change. 

It's not all gloom and doom. If we act now, we can we prevent the worst effects of climate change and, as the authors point out, we may also learn to adapt physiologically and technologically to our changing environment.