Significance Wellbeing falters without sound mental health. Scholars have recently indicated that the impacts of climate change are likely to undermine mental health through a variety of direct and indirect mechanisms. Using daily meteorological data coupled with information from nearly 2 million randomly sampled US residents across a decade of data collection, we find that experience with hotter temperatures and added precipitation each worsen mental health, that multiyear warming associates with an increased prevalence of mental health issues, and that exposure to tropical cyclones, likely to increase in frequency and intensity in the future, is linked to worsened mental health. These results provide added large-scale evidence to the growing literature linking climate change and mental health.

Abstract Sound mental health—a critical facet of human wellbeing—has the potential to be undermined by climate change. Few large-scale studies have empirically examined this hypothesis. Here, we show that short-term exposure to more extreme weather, multiyear warming, and tropical cyclone exposure each associate with worsened mental health. To do so, we couple meteorological and climatic data with reported mental health difficulties drawn from nearly 2 million randomly sampled US residents between 2002 and 2012. We find that shifting from monthly temperatures between 25 °C and 30 °C to >30 °C increases the probability of mental health difficulties by 0.5% points, that 1°C of 5-year warming associates with a 2% point increase in the prevalence of mental health issues, and that exposure to Hurricane Katrina associates with a 4% point increase in this metric. Our analyses provide added quantitative support for the conclusion that environmental stressors produced by climate change pose threats to human mental health.

Social, economic, and physical systems are critical determinants of psychological wellbeing (1). By disrupting these systems, climate change is likely to exacerbate known risk factors for mental disorders (2). Mental health difficulties are already common and costly (3). Nearly one-half of Americans will experience mental illness in their lifetime (4), with anxiety, stress-related, and mood disorders comprising the majority of diagnoses (5). These psychological disorders worsen overall health (6, 7), diminish productivity (8), and reduce quality of life (3, 9). Even subclinical levels of distress can impair psychological (10, 11) and immunological (12) functioning, reducing the ability to cope with adversity.

Over the past decade, scholars have highlighted the direct and indirect threats that climate change poses to mental health (13, 14). Warming is likely to amplify the frequency and intensity of natural disasters (15), which often cause physical injury, psychological trauma, infrastructure damage, and societal disruption in affected regions. Gradual changes in climate are also expected to alter human systems in costly ways. Rising temperatures amplify risks to human physical health (16), harm economic activity (17), spur social conflict (18), and produce forced migration (19). Some communities are already experiencing disruption of livelihood and associated grief in the face of ecological losses (20). While the precise magnitude of these climate-induced adversities is difficult to estimate, the theoretical relationship between climate change and mental health risk is compelling (2, 21).

Empirical investigations of this relationship have primarily focused on discrete climatic events and weather extremes. For example, exposure to hurricanes and floods is associated with symptoms of acute depression as well as posttraumatic stress disorder (22⇓⇓⇓⇓–27). Furthermore, both heat and drought amplify the risk of suicide (28⇓–30), and psychiatric hospital visits increase during hotter temperatures (31⇓–33). Those with preexisting mental health conditions and lower socioeconomic status are among the most vulnerable to these adverse environmental conditions (21). However, population-level impacts have been difficult to characterize, as most investigations have been qualitative, local in scale, or limited to only the most severe mental health outcomes. Although we are beginning to understand the ways in which weather influences other psychological phenomena, such as cognition (34), emotional expression (35), and sleep (36), large-scale quantification of the mental health risks posed by climate change is lacking (2).

To begin to address this gap, we report on the relationship between historical climatic conditions and the mental health of 2 million randomly sampled US residents between 2002 and 2012 (study materials are available on Harvard’s Dataverse: https://doi.org/10.7910/DVN/OVQY76). Our measure of individuals’ reported mental health is drawn from the Center for Disease Control and Prevention’s Behavioral Risk Factor Surveillance System (BRFSS) between 2002 and 2012 (37). Respondents answered the following question: “Now thinking about your mental health, which includes stress, depression, and problems with emotions, for how many days during the past 30 d was your mental health not good?”

We code responses to this question as one if respondents indicate mental health difficulties over the period and zero otherwise (see Data). This measure has been shown to possess both psychometric test–retest reliability (38) and convergent validity with other standard measures of mental health status (39, 40). While not a direct measure of psychiatric disorders, this item has a number of strengths. First, it is to our knowledge the best large-scale, randomly sampled measure of individual mental health status in the United States. Second, the measure is spatially and temporally referenced in a manner that enables precise pairing with meteorological data. Third, it is likely able to capture both clinical and subclinical distress across a wide set of possible symptoms (5) and unlike measures of health care utilization, can account for the substantial portion of US adults who fail to seek treatment (41).

We combine our mental health data with meteorological data and empirical tools drawn from the climate econometrics literature to investigate the historical relationship between climatic variations and human mental health. These historical relationships can aid in estimating the magnitude of the risks that climate change poses to mental health. Following the theoretical framework of Bourque and Cunsolo Willox (14), we examine three selected types of environmental stressors likely to be produced by climate change: short-run meteorological exposure, multiyear warming, and acute exposure to natural disasters. We examine three questions.

First, anthropogenic warming is likely to present humans with increasingly extreme meteorological conditions in any given year (42). Do recently experienced meteorological stressors affect individuals’ reported mental health? Additionally, are those who are most vulnerable to mental health challenges more affected by such meteorological stress (14)? In our sample, more vulnerable populations include those with lower incomes, those who experience a higher average burden of mental health problems (43), and those who may be less able to smooth adverse temperatures (44) as well as women, who are more susceptible to the mental health difficulties captured by our measure (4). To examine this set of questions, we use pooled cross-sectional analyses, leveraging exogenous meteorological variation to examine the effect of short-run (past 30 d) weather exposure on individuals’ mental health outcomes (Pooled Cross-Section and SI Appendix, Fig. S2) (45).

Second, climate change is also likely to increase the rates of year-over-year and decade-over-decade warming of local climates and the chronic stressors that such warming produces (14, 46). Does longer-term warming have detrimental impacts on individual mental health over time? To examine this question, we use the long-differences approach (47), examining the association between spatial variation in multiyear warming and longer-run changes in mental health (Long Differences and Fig. 3B).

Third, climate change is also likely to amplify the frequency and intensity of acute climatic events, like tropical cyclones (14, 15). Does direct exposure to costly tropical cyclones worsen individual mental health outcomes? To examine this question, we use a difference-in-differences approach, leveraging the landfall of Hurricane Katrina to examine the association between tropical cyclone exposure and our measure of reported mental health outcomes (Difference-in-Differences) (48).

Discussion By spatially and temporally linking individuals’ reports of mental health difficulties to the environmental exposures that they experienced, we show that short-term weather, multiyear warming, and tropical cyclone exposure each relate to worsened mental health outcomes. The effect of shifting from average monthly temperatures between 25 °C and 30 °C to averages greater than 30 °C on the probability of mental health difficulties (+0.5% points) is approximately one-quarter the size of the association between 1 °C of 5-y warming and the prevalence of mental health issues (+2% points), which is, in turn, one-half the size of the association between exposure to Hurricane Katrina and occurrence of mental health difficulties (+4% points). A number of considerations are important to the interpretation of our results. First, an optimal measure would contain added information on the nature and severity of each individual’s particular mental health symptoms. Future studies should investigate the risks posed by climate change to specific psychiatric and neurobehavioral symptoms, such as substance use and psychosis. Varied psychopathology, subclinical distress, and chronic stress each confer unique risks and treatment needs. Second, our data consist of a randomly sampled, pooled cross-section of respondents. An ideal source of data would track the same individuals over time to enable controlling for individual-specific characteristics and more precisely monitoring mental health responses to climatic factors over time. This limitation is notable in the case of our Hurricane Katrina analysis, where results may be partially attributable to selection out of affected areas. Third, we do not uncover the causally mediating factors underlying our results. Exposure to more extreme meteorological conditions may produce physiological stressors that precipitate poor mental health, such extremes may initiate inflammatory processes that worsen mental health, or the effects may run entirely through reductions in health maintenance behaviors, like exercise (50) and sleep (36). Future studies and advances in climate econometric methods (45) are needed to investigate causal mediation in this setting. Fourth, measurement error exists between the temperatures that we observe and the temperatures that respondents actually experienced, attenuating the magnitude of our estimates (48). This added measurement error suggests that our temperature-related estimates may represent a lower bound of the effects of temperature on mental health. Fifth, we observe that the mental health of low-income individuals may be most harmed by a changing climate. However, our data are from a wealthy country with a temperate climate. Regions with less-temperate climates, insufficient resources (51), and greater reliance on ecological systems may see more severe effects of climate change on mental health (28). Sixth, we examine three selected types of environmental adversity likely to be produced by climate change. As a result, our estimates only represent a sampling of the possible risks that climate change poses to mental health. Unfortunately, clear historical analogs for other climate-induced environmental stressors—like inundation from sea-level rise—are more difficult to measure. Nonetheless, it is vital for future large-scale empirical studies to investigate the many additional ways that climate change might harm mental health. Seventh, while robust to many tests (Methods), we cannot definitively rule out unobserved heterogeneity with respect to our long-differences and difference-in-differences estimation procedures. As a result, care is warranted with respect to the causal interpretation of these estimates. Eighth, we measure the direct effect of exposure to environmental stressors on self-reported mental health. Worry about climate change itself may exacerbate these environmental impacts on mental wellbeing (52, 53). Moreover, interactive effects between exposure to climatic stressors and other social stressors may modify these effects. Ninth, our observed effects may not persist into the future. Humans may adapt technologically and physiologically to warmer climates to minimize the impact of warming on mental health (44). Individuals may also adapt via psychological coping mechanisms, such as avoidance, seeking social support (54), or fostering mental preparedness (55). Ultimately, if observed relationships from the recent past persist, added climate change may amplify the society-wide mental health burden in the face of the acute environmental threats produced by warming in natural systems. 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.

Acknowledgments We thank our anonymous reviewers for their constructive feedback on this manuscript.

Footnotes Author contributions: N.O. and R.M. conceived of the research; N.O. designed research; N.O. performed research; N.O. analyzed data; and N.O., R.M., M.P.P., and I.R. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Data deposition: The study materials are available on Harvard’s Dataverse: https://doi.org/10.7910/DVN/OVQY76.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1801528115/-/DCSupplemental.