Estimating the effects of reduced sunlight due to solar geoengineering on suicide in the United States

BackgroundSolar geoengineering, whereby sunlight is reflected back into space at the outer atmosphere to reduce incoming sunlight, is increasingly considered a viable option to mitigate global warming, yet the health consequences of reducing incoming sunlight remain poorly quantified. ObjectivesThis study examines the effects of sunlight exposure on the rate of suicide across the United States over nearly three decades and projects the impact of geoengineering-induced reductions in sunlight on suicides by 2100. MethodsThe analysis relates sunlight exposure, as measured by solar insolation, to the suicide rate at the county-by-month level in the United States between 1979 and 2004 (N = 444,861), after adjusting for temperature, precipitation, county-by-month effects, and state-by-year effects. We project the excess suicides due to the negative radiative forcing required to keep the temperature rise below 1.5 {degrees}C by 2100. ResultsWe find that suicide rates increase by 6.99% (95% CI: 3.86, 10.13) as sunlight decreases by one standard deviation, which is almost equivalent to the difference in sunlight between the lowest (Vermont) and highest (Arizona) state-level averages. The effects are similar across an extensive set of county characteristics and over time, suggesting limited adaptation to sunlight exposure in suicidal behavior. We also find that insufficient sunlight exposure increases the searches containing depressive language on Google Trends. These estimates suggest that solar geoengineering could result in 1.26-3.18 thousand additional suicides by 2100 under the business-as-usual scenario, which could more than offset the suicides averted by temperature fall. DiscussionOur findings highlight the substantial benefits of sunlight exposure on the incidence of suicide and mental well-being, thus calling for climate policy to better balance the potential benefits and harms of solar geoengineering.