Large reductions in United States heat extremes found in overshoot simulations with SPEAR

Zachary Michael Labe , Thomas L. Delworth, Nathaniel C. Johnson, Bor-Ting Jong, Colleen E. McHugh, William F. Cooke, Liwei Jia

Increases in the intensity and frequency of heatwaves are already evident in the observational record, and these increases are expected to be further amplified in future climate projections with greater radiative forcing. However, it is unclear how temperature extremes will respond regionally to emissions reductions and declines of greenhouse gases later in the 21st century, such as through the implementation of global climate mitigation efforts. Here, we evaluate a set of large ensemble experiments that simulate hypothetical 21st century overshoot scenarios using the GFDL SPEAR climate model. While the two overshoot scenarios include a similar evolution of greenhouse gas reductions, they differ in the timing of this drawdown by about a decade. For this study, we then assess whether differences in the timing of starting climate mitigation influences summertime heat extremes across the contiguous United States (CONUS). By quantifying changes in extreme heat relative to the global mean surface temperature before and after the peak in greenhouse gas concentrations, we find significant decreases in the number of CONUS heat extreme days in response to mitigation. This is further amplified for the earlier overshoot scenario, which suggests a greater benefit (i.e., the time below an extreme temperature threshold) in reducing heat impacts by starting climate change mitigation even in as little as a decade sooner. The reductions in heat extremes are consistent with greater mean precipitation and humidity across most of CONUS for equivalent global warming levels. Changes to the global mean land-sea contrast are also found arising from differences in the rate of surface cooling following the greenhouse gas drawdowns. Our results also emphasize the importance of conducting more coordinated large ensemble modeling experiments to understand the range of possible effects of global climate mitigation efforts on changes to regional extreme events.