The Effects of Rising Global Temperatures on Mental Health

The climate crisis is creating a growing mental health crisis. In 2017, the American Psychological Association formally codified the term eco-anxiety to describe “a chronic fear of environmental doom” that is experienced by a growing number of persons of all ages. Thousands are already seeking treatment for psychological traumas caused by wildfires and other natural disasters, which will become more frequent and severe. Yet, beyond these indirect effects of global warming, rising temperatures have a direct impact on neurotransmitter function and temperature homeostasis that will impact psychiatric patients.

A number of psychiatric conditions fluctuate seasonally. For example, seasonal increases in ambient temperature are linked to increased emergency room visits for depression, bipolar disorder, bulimia, and posttraumatic stress disorder (PTSD). Hotter days in a given location are correlated with higher rates of violence, and violent suicides are more common during periods of increased temperature and sunlight.

These trends are likely, in part, due to seasonal variations in brain serotonin levels, which are affected by temperature and light. The major serotonin metabolite in cerebrospinal fluid (5-HIAA) decreases when daily temperatures increase, and serotonin signaling in the brain’s limbic regions is correlated with ambient light exposure. This is likely because serotonin plays an important role in maintaining the body’s temperature homeostasis. While acute increases in ambient temperature decrease brain serotonin levels, these levels slowly return to baseline after days of continued exposure to higher temperatures. This phenomenon is called acclimation.

Ecological studies of the prevalence of mental health disorders reflect the effects of acclimation. On average, countries closer to the equator have lower rates of suicide compared with countries in higher latitudes. The World Health Organization has found that many countries with higher average temperatures have lower prevalence of psychiatric disorders. In the United States the prevalence of many psychiatric disorders thought to result in part from serotonin dysregulation (for example, depression, anxiety, and PTSD) is lower in counties that are on average brighter and warmer, even after accounting for potential sociocultural and demographic confounders.

When we look closer at the effects of acute temperature fluctuations in psychiatric patients, we also see that acclimation may be at play. The peak incidence of depression and suicide generally do not occur during the highest temperatures of midsummer, but rather at times when sunshine and temperature increase most rapidly (that is, spring). Once people acclimate to the change in temperature, it appears that the effects of heat are greatly reduced. This is consistent with clinical trials that have effectively used repeated exposures to supplemental light and heat to treat nonseasonal depression, panic disorder, and even bulimia. Therefore, the impact of global warming on the serotonin system and the prevalence of many psychiatric disorders will likely be complex and will change with acute and chronic alterations in climate.

The impact of rising temperatures on the side effects of psychotropic medications is more clearly negative. Anti-dopaminergic activity of antipsychotics increases the temperature homeostasis set point. Anticholinergic inhibition of sweat secretion and pro-serotoninergic and antihistaminic reductions in heat elimination all contribute to hyperthermia. Even medications that do not directly impact thermoregulation, such as lithium and other mood stabilizers, have altered pharmacokinetics and toxicities in the context of dehydration. This is a likely explanation, in part, for why psychiatric patients have a threefold increased risk of mortality during heat waves compared with the general population.

For certain psychiatric patients, increased risk of heat stroke is likely due to more than iatrogenic effects. For example, patients with schizophrenia have endogenous difficulties maintaining temperature homeostasis. Studies from the 1950s, before the use of neuroleptics, show that patients with schizophrenia were at increased risk of heat-related deaths compared with the general population. Taken together, we should be aware that our psychiatric patients are at greater risk of hyperthermia-related fatalities as global warming progresses, and we will need to educate and provide resources for them accordingly.

During his remarks at the 2015 GLACIER Conference in Anchorage, Alaska, President Barak Obama said, “Climate change is no longer some far-off problem. It is happening here. It is happening now.” The same can be said for the effects of the climate crisis on mental health. Psychiatrists and and other health care professionals need to be aware of the impacts that rising temperatures are having on our patients’ psychological and biological well-being.

You can find all of the articles referenced in this piece on the Climate Psychiatry Alliance online library. If you are interested in learning more about what psychiatrists are already doing to address these issues, and to perhaps become more involved, please visit here. ■