Greater Reduction in U.S. Heat Extreme Days in Overshoot Simulations with GFDL SPEAR

Heatwaves are increasing in both intensity and duration around the world, and they are expected to further intensify with a greater acceleration of climate change in the coming decades. However, it remains unclear to what degree extreme heat will change in response to reductions in greenhouse gases, such as through the introduction of possible future climate mitigation efforts. Assessing a wider range of 21st century climate change trajectories is therefore critical for improving societal and infrastructure planning for the risk assessment of extreme events. In this study, we consider a collection of overshoot scenarios using a high-resolution climate model (GFDL SPEAR) that includes rapid drawdowns of carbon dioxide and methane in the mid-21st century but differ in the onset of this mitigation by approximately a decade. Large ensembles are employed for each overshoot scenario to better account for the importance of internal variability and to sample a wider distribution of possible heat extremes. We primarily focus on changes in summertime heat across the contiguous United States and quantify these changes based on global warming levels before and after greenhouse gas reductions. The overshoot simulations reveal that heat extremes decline across most areas following a peak in greenhouse gas concentrations. However, we find that by implementing climate mitigation one decade earlier, there is a reduction in heat extremes equivalent to nearly two decades of fewer heat impacts across much of the United States. Mechanistically, this is linked to greater near-surface humidity and higher mean precipitation at equivalent global warming levels composited before and after mitigation. This climate response is further linked to changes in the average land-sea temperature contrast due to differences in the rate of surface cooling following a reduction of greenhouse gases. Our findings highlight the urgent need to understand the potential benefits, consequences, and surprises from plausible climate mitigation efforts on regional extreme events.