Co-occurring climate hazards and compound health effects in California: a scoping review

Abstract Compound climate events capture the overlap of multiple climate hazards in space, time, or both, which can amplify adverse health outcomes. Despite a strong commitment to climate policy and action, the state of California faces a broad array of these compound climate hazards, and existing adaptation approaches do not yet consider a compound framework for exposures. California is also home to a diverse population with many underserved communities that are particularly vulnerable to the effects of climate events. This scoping review is the first to comprehensively synthesize existing evidence on compound climate exposures and health in California, analyzing exposures co-occurring in the same place at the same time. We searched the Web of Science and PubMed databases and identified 20 articles analyzing the compound effects of climate stressors including heat, air pollution, wildfire smoke, meteorology, and microclimate factors such as green space. The strongest evidence emerged for the co-occurring effects of heat and air pollution—including wildfire smoke—on various health outcomes, including mortality, hospitalizations, and birth outcomes. Several studies also demonstrated spatial variability in these compounded effects at the neighborhood scale. We found heterogeneity in both exposure assessment techniques for characterizing climate extremes, as well as methods to evaluate effects on the additive or multiplicative scale, limiting comparability across studies. Several studies analyzed equity impacts, providing limited evidence that disadvantaged populations are disproportionately vulnerable to compound health effects. Key gaps remain, however, in evaluating the full extent of environmental justice implications, as well as regional effects. Despite these limitations, current evidence underscores the urgency of preparing California populations, particularly vulnerable communities, with resilience strategies to reduce risks from increasingly frequent and severe co-exposures during compound climate events.