Climate Action Initiative

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.

ABSTRACT Compound precipitation‐wind extremes (CPWEs) pose significant socio‐economic challenges in urban areas, with the most severe impacts often felt by vulnerable populations. While CPWE studies have often focused on large‐scale assessments, localised urban contexts, at a scale of a city and its suburban surroundings, remain understudied, particularly in terms of how CPWEs might differentially impact different socio‐economic communities. This study aims to advance understanding of how CPWEs interact with social vulnerability at a localised scale, using the San Francisco Bay Area (SFBA)—a global hotspot for CPWEs with diverse climates, physiography, and socio‐economic factors—as a test case. Daily precipitation and wind speed data (~4 km) from the gridMET dataset (1982–2022) were used to identify CPWEs during the wet season (October–April). Patterns of CPWE event frequency and trends were then analysed. We found that the return period for CPWEs in the SFBA ranges from 0.5 to 2 years, suggesting nearly annual occurrences. Over recent decades, these events have become more frequent (an increase of two events per decade) and more intense, particularly in terms of daily precipitation trends (an increase of 0–5 mm/decade), across the SFBA. We further examined the connection between CPWEs and social vulnerability using the Social Vulnerability Index from the Centers for Disease Control and Prevention, considering factors such as socio‐economic status, household characteristics, racial and ethnic composition, housing conditions, and access to transportation. Key urban centres, such as San José and Oakland, emerged as hotspots where high CPWE frequency coincides with high levels of social vulnerability due to economic constraints, inadequate housing, and limited transportation access. These findings underscore the need for targeted adaptation measures to protect populations most at risk in the SFBA.

This is a case study of City Farm SLO.