It's Not Just the Heat, It's the Humidity: Downscaled Wet-Bulb Temperature Projections and Implication for Future Summer Experiences from the American Climate Prospectus

Abstract:

The health impacts of extreme heat are significantly aggravated when combined with high humidity [1]. Wet-bulb temperature ($T_w$), measured by wrapping a thermometer in a wetted cloth and fully ventilating it, provides a physical metric of the combined effect of both heat and humidity. $T_w$ in excess of 30$^\circ$C is extremely dangerous and has been observed in the US only during the peak of the 1995 Midwest heat wave. Historically unprecedented $T_w$ in excess of 33$^\circ$C represents an extreme threat to human health, with heat stroke likely for fit individuals after less than one hour of shaded activity [2,3].

We present an empirical method for generating downscaled probability distributions of daily maximum $T_w$ conditional on dry-bulb temperature. The approach is based upon the statistical relationship between these two parameters, as estimated from reanalysis data. Using statistically downscaled temperature projections for Representative Concentration Pathways (RCPs) 8.5, 4.5 and 2.6, we project changes in $T_w$ for the next two centuries. We find that dangerously humid days ($T_w$ > 27$^\circ$C) will become increasingly common in the eastern U.S. under RCP 8.5, with the expected number of days per summer surpassing those of Louisiana today in Chicago in 25 years, Washington, D.C. in 30 years, New York City in 50 years and Portland in 60 years. By the end of the century under RCP 8.5, one extraordinarily dangerous ($T_w$ > 33$^\circ$C) day per year is expected in counties currently home to about one-third of the U.S. population. Mitigation can significantly the expected number of extreme wet-bulb temperature days, with only one-eighth of the U.S. population in counties with a 1-in-10 chance per year of an extraordinarily dangerous day by the end of the century under RCP 4.5.

References: [1] Liang et al. (2011), Building and Environment 46: 2472-2479, doi:10.1016/j.buildenv.2011.06.013. [2] T. Houser et al. (2014), American Climate Prospectus, www.climateprospectus.org, [3] Sherwood and Huber (2010), PNAS 107: 9552-9555, doi: 10.1073/pnas.0913352107