Transient regional climate change: Analysis of the summer climate response in a high-resolution, century-scale ensemble experiment over the continental United States

Integrating the potential for climate change impacts on policy and planning decisions requires quantification of the emergence of subregional climate changes that could occur in response to transient changes in global radiative forcing. Here we report results from a high-resolution, century-scale ensemble simulation of climate in the United States, forced by atmospheric constituent concentrations from the Special Report on Emissions Scenarios A1B scenario. We find that 21st century summer warming permanently emerges beyond the baseline decadal-scale variability prior to 2020 over most areas of the continental United States. Permanent emergence beyond the baseline annual-scale variability shows much greater spatial heterogeneity, with emergence occurring prior to 2030 over areas of the southwestern United States but not prior to the end of the 21st century over much of the south central and southeastern United States. The pattern of emergence of robust summer warming contrasts with the pattern of summer warming magnitude, which is greatest over the central United States and smallest over the western United States. In addition to stronger warming, the central United States also exhibits stronger coupling of changes in surface air temperature, precipitation, and moisture and energy fluxes, along with changes in atmospheric circulation toward increased anticylonic anomalies in the midtroposphere and a poleward shift in the midlatitude jet aloft. However, as a fraction of the baseline variability, the transient warming over the central United States is smaller than the warming over the southwestern or northeastern United States, delaying the emergence of the warming signal over the central United States. Our comparisons with observations and the Coupled Model Intercomparison Project Phase 3 ensemble of global climate model experiments suggest that near-term global warming is likely to cause robust subregional-scale warming over areas that exhibit relatively little baseline variability. In contrast, where there is greater variability in the baseline climate dynamics there can be greater variability in the response to elevated greenhouse forcing, decreasing the robustness of the transient warming signal.