UC Santa Barbara

Abstract Farmworkers in California’s Imperial Valley face a disproportionate risk of heat stress from prolonged exposure to extreme temperatures. However, heat stress is more than just extreme temperature and depends on humidity, winds, and sun exposure. These conditions can be aggregated into the wet-bulb globe temperature (WBGT), a metric endorsed by the U.S. Occupational Safety and Health Administration (OSHA) for monitoring workplace heat. To better inform heat mitigation strategies, we construct a climatology of summertime WBGT in Imperial Valley for 30 years during 1991–2020. The climatology is constructed from a dynamical downscaling of ERA5 reanalysis to 1.5 km using the Weather Research and Forecasting (WRF) Model, using quantile mapping to bias correct WBGT inputs against regional weather station observations. We find that WBGT is the highest in urban areas, where summertime daily maxima average 34°C and in rural croplands below mean sea level. National Weather Service (NWS) thresholds of WBGT that portend elevated heat stress are frequently exceeded, with 60%–70% of summer days exceeding the highest threshold of 32.2°C. WBGT also shows statistically significant warming trends over populated areas, and the onset of consecutive extreme WBGT days is occurring earlier in summer. Furthermore, breaks in the North American monsoon result in lower WBGTs, suggesting a source of intraseasonal WBGT predictability. These results underscore the need to downscale climate projections for the near future while bolstering heat mitigation and adaptation plans for vulnerable communities. Significance Statement Farmworkers in California’s Imperial Valley are at heightened risk of heat stress due to their prolonged heat exposure and unique social vulnerabilities. This heat stress can be proxied by the wet-bulb globe temperature (WBGT), which aggregates temperature, humidity, winds, and sun exposure. Using high-resolution climate modeling, we characterize summertime WBGTs in the Imperial Valley during 1991–2020. We find that the WBGT is the highest in urban areas and where croplands are below mean sea level. In fact, the average summertime WBGT is frequently above National Weather Service thresholds that indicate when rest periods should be taken. Furthermore, long-term warming trends and an earlier onset of extreme WBGT days put farmworkers increasingly at risk to heat stress. Additionally, we note that periods of relief from heat stress tend to occur during breaks in the North American monsoon, offering a route to improving forecasts of heat comfort. These results underscore the need for localized WBGT climate projections while bolstering heat mitigation and adaptation plans for vulnerable farmworkers.