The Impact of Rising CO2on the Spatial Footprint of Extreme Heat Events

Changes to the spatial extent or footprint of extreme heat events will impact the number of people and/or natural systems that experience heat-related stresses at the same time. These changes in the footprint size of contiguous heat events could have wide-ranging effects on energy and water demands. In this study, we assess the ability of global climate models to simulate the geographic footprint of extreme heat events. We then use an ensemble of model simulations to examine the response of extreme heat spatial extents to rising atmospheric CO₂ concentrations. We identify and compare contiguous areas of extreme heat, including individual extreme heat days and heat waves, in model simulations with preindustrial CO₂ and 4x preindustrial CO₂ (4xCO₂) concentrations. We calculate extreme heat thresholds separately for the preindustrial and 4xCO₂ climate states to assess how the size of an extreme heat event relative to the 4xCO₂ climate compares with the size of an extreme heat event relative to the preindustrial climate. Overall, climate models simulate contiguous areas of heat extremes that are consistent with those in the observational record. In response to elevated CO₂, most land regions exhibit larger contiguous areas of extreme heat, suggesting that, in general, extreme heat events increase in size as CO₂ rises. Our analysis will explore the potential drivers of these spatial changes, including the contributions from atmospheric circulation and land surface fluxes.