Temperature and humidity based projections of a rapid rise in global heat stress exposure during the 21st century. Ethan D Coffel. Radley M Horton, and Alex de Sherbinin. Published 22 December 2017. Environmental Research Letters, Volume 13, Number 1.

“As a result of global increases in both temperature and specific humidity, heat stress is projected to intensify throughout the 21st century. Some of the regions most susceptible to dangerous heat and humidity combinations are also among the most densely populated. Consequently, there is the potential for widespread exposure to wet bulb temperatures that approach and in some cases exceed postulated theoretical limits of human tolerance by mid- to late-century. We project that by 2080 the relative frequency of present-day extreme wet bulb temperature events could rise by a factor of 100–250 (approximately double the frequency change projected for temperature alone) in the tropics and parts of the mid-latitudes, areas which are projected to contain approximately half the world’s population…”