This study assessed the association between average maximum temperature and suicide and investigated whether the number of monthly heat events affected suicide rates in California. This information will become increasingly important as climate change will continue to increase both the average temperature and number of heat events that occur. To our knowledge, this research is novel as it is the first to investigate the association between temperature and suicide rate in California, whereas other studies have focused on nation-wide temporal associations or geographic disparities. [7, 21]
A negative binomial regression suggested that maximum temperature was significantly related to suicide such that there was an increase of 6.1% in suicide rate per 10°F increase. Additionally, there was no significant evidence to support a relationship between heat events and suicide.
The results indicated seasonality of suicide incidence in California, which is similar to other studies that have found suicide rate increases with warmer temperatures. The American Psychological Association and ecoAmerica reported on the acute and chronic mental health impact of climate change which includes exacerbated stress, depression, anxiety, and violence. Extensive research on the causal relationship between heat and aggression reveal this may be due to an increase in arousal, negative and hostile thoughts, and reduced cognitive function.[23, 24] In addition to decreased self-regulation, social factors such as the beginning and end of the school year, agricultural cycles, and major holidays may also contribute to this seasonal trend.
The findings of this study suggest that the average temperature in California continues to rise due to climate change, and so does the rate of suicide. This study contributes to the literature that assesses the relationship between temperature and suicide and provides empirical evidence for the need for more research to evaluate the effects of climate change on mental health. Further investigation into environmental, biological and societal contexts that influence this relationship is imperative for the development and implementation of preventative suicide strategies and policies in this population.
There were several methodological limitations to this study due to its exploratory nature and restrictions in data availability. First, this analysis did not adjust for covariates such as gender and age. Similar research that has shown a significant association between temperature and suicide has demonstrated its relationship is influenced by gender, such that suicide is more common in males than females in warmer temperatures, and age, such that it is more common in elderly populations. A combination of biological and societal mechanisms are posited to be responsible for the gendered and age phenomenon. Social predictors such as socioeconomic status, mental health diagnoses, housing and employment opportunities, and weather-related variables such as humidity and altitude, were not accounted for and are known to be related to increased risk of suicide.[22, 26] Such variables were not included in this analysis as they were not readily accessible. Thus, the study can only offer a basic understanding of the role that temperature has on suicide incidence in California, while further research may address these mechanisms that remain unstudied.
Second, suicide surveillance has inherent limitations due to the sensitive nature of the topic. Stigma remains a major barrier to suicide prevention. As such, it is difficult to collect and access accurate suicide data; not all suicides are reported if there is inconclusive evidence, if the social and economic interests of the victim and their family need to be protected, or due to misclassification bias. As a result, it is challenging to achieve a holistic picture of the incidence of suicide. In an effort to decrease the likelihood of underreporting, California was purposefully selected for being a socially progressive state with a large, diverse population, and participating in the CDC National Violent Death Reporting System. Furthermore, the study used data from CDC Wonder, the primary data repository for health statistics in the US, which added reliability to our analyses. Similarly, a more contemporary time frame of 2008 to 2017 was selected to decrease the effect of stigma on reports of death by suicide.
A common pitfall in time series analysis is that secular trends can induce strong, but spurious, correlations. A time series decomposition method of analysis was employed demonstrating clear trend and seasonal components. The analysis provided evidence that seasonal variation in temperature may be a factor in observed patterns of suicide; however, it was insufficient on its own to establish a causal link between longitudinal changes in temperature (i.e., climate change) and suicide. While we cannot eliminate the possibility of a spurious relationship between temperature and suicide in its entirety, there remains a strong argument for the association between temperature and suicide.
This study attempted to conduct analyses on two different temperature variables to glean insight into its strength of association with suicide. However, it was identified that heat events were more likely to occur in the winter months and therefore did not establish a significant relationship between what was classified as a heat event and the suicide rates in California. This represents a greater distribution of temperatures in the winter months and faster warming in the winter months compared to the summer months over the 10-year period. It is possible that heat events were found to be less frequent in the summer months and most frequent in winter months due to the study’s operational definition of heat events being related to relative rather than absolute temperature. Heat events are traditionally defined as related to absolute temperature to account for high temperatures in the summer half-year whereas this study identified high temperature anomalies year round. Future research can investigate absolute temperatures to evaluate the relationship between heat events in the summer and suicide, and assess consistency with these results. Also, because this trend indicates that climate change will have the greatest impact on the severity and frequency of heat events in winter compared to summer, further analysis can determine the specific impacts on suicide rates and predict changes to suicide rates in the future.