Heat stress conditions in southern South America: characterization and long-term changes under observational uncertainty

Heat stress (HS) conditions are some of the most impactful health-related climate hazards, affecting human well-being and comfort, stressing public health and civil protection services, especially in developing countries. This work presents a comprehensive assessment of summer HS conditions across southern South America (SSA), based on five widely used indices derived from temperature and humidity variables, namely wet bulb temperature (wbt), simplified wet bulb globe temperature, heat index (hi), humidex and discomfort index (discomInd). Station data (STN) and five gridded datasets were considered, addressing the challenge of observational uncertainty in SSA. HS indices exhibit different sensitivities to input climate variables. The wbt is more responsive to changes in humidity than the other indices (swbt, hi, humidex, and discomInd), which distinguishes it in terms of spatial patterns, identification of high HS days and trends. Over the recent period (1981–2022), upward trends were observed in most indices, consistent with rising temperatures. A stronger signal of change was identified for hi, whereas a weaker, less robust signal was observed for wbt. The frequency of high HS days also increases across SSA with higher agreement among datasets and indices. Comparison between STN and datasets reveals larger biases in humidity variables, which were considerably reduced when computing the indices. Among the datasets, the finer-resolution ones performed best, highlighting MSWX and ERA5-Land over ERA5, depending on the analyzed aspect. Overall, this study lays the groundwork for robust HS climate assessments across the region, supporting the development of more tailored adaptation strategies in SSA.