Recent Very Hot Summers in Northern Hemispheric Land Areas Measured by Wet Bulb Globe Temperature Will Be the Norm Within 20 Years

C. Li, X. Zhang, F. Zwiers, Y. Fang, and A.M. Michalak

Our body’s ability to deal with heat is a function not only of temperature, but also of ambient humidity. In this study, we looked both at historical heat waves and those projected for the future by climate models, using a metric that accounts for both heat and humidity. We found that the recent very hot summers that have occurred in the northern hemisphere will be the new normal within 20 years. By the 2030s, at least half of summers will be as hot and humid as the most extreme summers observed up to this point. This points to a severe human health risk and a massive adaptation challenge.

Figure: Anthropogenic influence has shifted summer wet bulb globe temperature (WBGT) toward higher levels. (a) Trend histograms of the observation-constrained 1973–2012 land average summer WBGT in a climate with (yellow) and without (green) anthropogenic influence. The observed trend is marked by a black point. (b) Histograms of the observation-constrained land average summer WBGT forced with only natural forcing in the climate around 1975 (represented by 1973–1977; green), with both natural and anthropogenic forcings in the climate forced around 1975 (orange) and around 2010 (represented by 2008–2012; red). The orange and red stick marks show the observations in the two periods, and the dashed vertical line shows the record land average summer WBGT experienced during 1973–2012, which occurred in 2010.


Wet bulb globe temperature (WBGT) accounts for the effect of environmental temperature and humidity on thermal comfort, and can be directly related to the ability of the human body to dissipate excess metabolic heat and thus avoid heat stress. Using WBGT as a measure of environmental conditions conducive to heat stress, we show that anthropogenic influence has very substantially increased the likelihood of extreme high summer mean WBGT in northern hemispheric land areas relative to the climate that would have prevailed in the absence of anthropogenic forcing. We estimate that the likelihood of summer mean WGBT exceeding the observed historical record value has increased by a factor of at least 70 at regional scales due to anthropogenic influence on the climate. We further estimate that, in most northern hemispheric regions, these changes in the likelihood of extreme summer mean WBGT are roughly an order of magnitude larger than the corresponding changes in the likelihood of extreme hot summers as simply measured by surface air temperature. Projections of future summer mean WBGT under the RCP8.5 emissions scenario that are constrained by observations indicate that by 2030s at least 50% of the summers will have mean WBGT higher than the observed historical record value in all the analyzed regions, and that this frequency of occurrence will increase to 95% by mid-century.

Li, C., X. Zhang, F. Zwiers, Y. Fang, A.M. Michalak (2017) "Recent very hot summers in northern hemispheric land areas measured by wet bulb globe temperature will be the norm within 20 years," Earth's Future, 5 (12), 1203-1216, doi:10.1002/2017EF000639.