When the temperature spikes, so too do suicide rates, crime, and violence. Twitter sees hate tweets and online aggression increase during heatwaves, along with phrasing that researchers have linked to anxiety and depression. Our very language captures the confluence of heat and emotion—when we are annoyed, we get “hot under the collar;” when we are angry our “blood boils;” and when something gets to be too much, we have to “let off steam.” Spike Lee’s seminal exploration of racial tension in 1989’s Do The Right Thing unfolds during the hottest day of the year, when scorching temperatures stoke violent reactions. Doctors and scientists are now starting to unravel the complex interplay between extreme heat and poor mental health outcomes.
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As another early-season heatwave hammers the U.S. northwest, with experts predicting more for the summer to come, it is increasingly vital to understand how high temperatures affect the brain, and, more importantly, how we can protect ourselves and others. “It’s easy to understand how going through a traumatic experience like a hurricane can impact mental health. The connection between heat and mental illness is not so intuitive,” says Shabab Wahid, a mental health expert at Georgetown University’s Department of Global Health. Wahid recently co-published a study in The Lancet Planetary Health showing that even a one degree increase in ambient temperature above the norm contributes to a higher probability of experiencing depression and anxiety. While his research focused on Bangladesh, the findings apply globally, he says. “There is a growing body of scientific literature that is identifying this link between climate-related factors and adverse mental health outcomes. And every indication is that as the climate change continues to worsen, these links will gain in strength.”
Indeed, according to a 2018 study by Stanford economist Marshall Burke published in Nature Climate Change, a 1.8°F (1°C) increase in average temperature in the U.S. and Mexico correlates to a 1% increase in suicides—translating into thousands of additional deaths every year. The Burke study projects that if temperatures continue to climb as climate scientists predict they will, the resulting increase will be enough to wipe out the combined efforts of suicide-prevention programs and gun-control policies in the United States.
The number of extreme heat days is increasing every year due to climate change, fundamentally changing social interactions and personal well-being in a serious threat to mental stability, says Robin Cooper, an associate clinical professor at the University of California San Francisco and the president of the Climate Psychiatry Alliance. “We have to start thinking about climate change as a mental health crisis. If we ignore climate change as a public health threat, we are abdicating our role as healthcare providers.” That means investing more in research. While it is well established that heat impacts brain function, the exact mechanisms are poorly understood. Scientists point to a multitude of interrelated psychological, social, and biological factors ranging from disrupted sleep to the heat-impaired function of vital neurotransmitters and hormones.
Spikes in suicides and events related to mania and post-traumatic stress disorder (PTSD) tend to occur in the late spring and early summer when temperatures are more volatile, according to Josh Wortzel, who studies the intersection of climate change, heat waves, and mental health at Brown University. “It’s not necessarily the hottest days of the year that are associated with the greatest number of suicides and suicide attempts,” he says, “but actually when the temperature changes dramatically.” Severe swings, like this week’s 15°-30°F spike in temperatures in parts of the Pacific Northwest, are typically the most dangerous.
Much of that can be traced back to sleep. Anyone that has lived through a heatwave without the benefit of air conditioning knows that quality sleep becomes elusive. Over time, the accumulative effects can lead to memory loss, lack of focus, and increased irritability, says Cooper. “Sleep is a profoundly complex function, and a lack of restorative sleep has so many different ramifications for mental health.” Impaired sleep is often a trigger for manic episodes in those with bipolar disorder, she notes, an indication that it serves an important function in mood regulation. “Poor quality sleep may be one of the driving factors” behind the link between extreme heat days and mental health declines.
Heat also impacts the neurotransmitter serotonin, one of our most important mood regulators, closely linked with keeping aggression in check, according to Wortzel. Serotonin helps relay information about skin temperature to the brain’s hypothalamus, which goes on to control shivering and sweating responses when necessary. Patients with depression often have difficulties with this thermoregulation process; the fact that these problems can be ameliorated when patients take selective serotonin reuptake inhibitor antidepressants suggests a relationship between heat exposure and serotonin production.
Climate trauma plays an important role as well, says Brit Wray, director of the Stanford medical school’s program on climate change and mental health. “It’s not like everyone who survives a wildfire is going to develop PTSD. But it’s a lot harder when floods come that take away anything that might have been left. And then you’re also dealing with other social stresses, maybe a financial downturn, maybe a pandemic.” The compounding stresses to the mental system wear away at resilience, at which point maladaptive coping mechanisms—substance abuse, domestic violence, suicidal ideation—take root. Then you add in the neurophysical impacts of extreme heat on the brain, and the very real threat of heat waves on patients already suffering from mental health disorders, and the mental-health repercussions escalate. “It becomes a vicious cycle,” says Wray, on the sidelines of the 2023 Frontiers Forum, an annual event focused on society, health and science. “We definitely do have a mental health crisis within the climate crisis that we need to get ahead of before too many of these events add up.” That means better support in communities that are most affected, and improved understanding of how climate change, trauma, and mental health interact.
Over the past few years there’s been growing interest in how temperature and climate change affect mental health among psychiatrists, says Wortzel. The problem is that funding for further research is limited. “Climate change is now considered the number-one public health concern. But there is not enough understanding of how it impacts mental health. For us to not be investing more right now in how to understand the impacts of heat on the brain is unfortunate.” Unfortunate for research, but also for the billions of people at risk from extreme heat in the coming years. Heat waves are a fact of life in a warming world; more research can help us prepare.
Environment and Climate Change