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

Abstract 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 large ensembles of dynamically downscaled projections to estimate historical and end-of-century upper bounds for surface air temperatures. A number of physical mechanisms can influence upper bounds, and at the end of the 21st century, estimates based on mechanisms that yield more moderate upper-bounds produce values around <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>60</mml:mn> <mml:msup> <mml:mrow> <mml:mstyle scriptlevel="0"/> </mml:mrow> <mml:mo>∘</mml:mo> </mml:msup> </mml:mrow> </mml:math> C for much of the Western United States and in excess of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>80</mml:mn> <mml:msup> <mml:mrow> <mml:mstyle scriptlevel="0"/> </mml:mrow> <mml:mo>∘</mml:mo> </mml:msup> </mml:mrow> </mml:math> C for the hottest parts of the domain. Even cooler high-altitude locations have end-of-century upper bounds over <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>50</mml:mn> <mml:msup> <mml:mrow> <mml:mstyle scriptlevel="0"/> </mml:mrow> <mml:mo>∘</mml:mo> </mml:msup> </mml:mrow> </mml:math> C. Although these upper-bound estimates might seem implausibly large, increases in the upper bounds over the 21st century are similar to increases in dynamically downscaled peak surface temperatures after adjusting those downscaled temperatures to eliminate the possibly biased model trends in surface specific humidity. While upper bound estimates are high relative to historical observations, they nonetheless suggest that heatwave intensity risk is bounded, with uncertainty dominated by projections of surface and upper-level humidity.