Distinct Favored Regions for Historical Record-Setting and Future Record-Breaking Humid Heat

Recent studies have revealed strong trends in humid heat, including the nearing of human physiological limits in some regions. Understanding of past extremes and their meaningfulness for contextualizing future possibilities, especially in the near-term, is limited by the absence of a global analysis focused on the most extreme humid-heat-anomaly events. Here we identify record-setting humid-heat days for 216 global regions and assess the likelihood of these records being broken even under present-day climate forcing. We use several reanalyses as a historical catalogue, and large climate-model ensembles to represent other statistically plausible events. Unlike the spatial pattern of large temperature anomalies, we find that humid-heat anomalies are most intense, and most seasonally and interannually concentrated, in the deep tropics and arid subtropics. Many top events have attracted little if any prior attention. The eastern United States is especially susceptible to record-breaking humid heat due to modest current records (>1% inferred annual exceedance probability) contrasting with numerous simulated large-anomaly days. Australia and eastern China are also prone to locally exceptional episodes, with >40% of ensemble members simulating events exceeding the ERA5-based distribution maximum. Model biases for key characteristics, together with the observed record-setting day affecting its estimated return period by >2.5x in half of regions, underline several valuable aspects of a joint observation/model perspective on humid heat. This approach aids in evaluating the plausibility of as-yet-unseen extremes; identifying regions of concern that might otherwise be overlooked and underprepared; and gauging regionally specific correlations between event magnitudes and societal impacts.