It's Not the Heat, It's the Humidity: The Changing Nature of Summer Hot Days

An increase in warm season heat extremes is a well-understood and predicted consequence of climate change. The impact of these extremes, however, is additionally dependent on humidity: human perception of heat increases with increased humidity, whereas wildfire risk and crop losses are increased under high heat, low humidity conditions. In a stationary climate, temperature and humidity exhibit spatially-variable covariance during the warm season, ranging from close to a one-to-one relationship in humid regions to a negative covariance in monsoon regions. Here, we quantify the observed changes in the distribution of humidity for the hottest days with a focus on trends indicative of a changing temperature-humidity covariance structure. We identify coherent regions where changes in moisture availability are altering the characteristics of high heat days. The observed results are contextualized using the CESM1 Large Ensemble to determine the potential contribution of internal variability. Finally, we use reanalyses to examine the changes in the vertical structure of the atmosphere related to trends in near-surface moisture, with implications for the probability of convective activity and extreme precipitation.