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.
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.