Upper bounds for 21st-century surface air temperatures in the Western United States

The last decade has seen a large number of severe heatwaves that were unprecedented in the observational record, highlighting challenges associated with observationally-based statistical quantification of the likelihood and magnitude of future extreme temperatures. An alternative to such probabilistic assessments is identification of upper bounds that quantify the hottest surface air temperatures that can possibly be achieved by the end of the 21st century. Theory, simulations, and observational analyses support the existence of a finite upper bound for surface air temperature; however, estimates for future upper-bound values that are realistic and usable for planning remain unavailable. Here, we combine atmospheric theory with ensembles of dynamically downscaled projections to estimate historical and end-of-century upper bounds for surface air temperatures. Our ensemble-based approach provides better tail sampling and therefore more robust assessments of the extent to which explicitly simulated surface temperature extremes are contained by theory-based thresholds. These data files provide upper bound estimates over the Western United States on the 9-km WRF grid.