Nonlinear increases in extreme temperatures paradoxically dampen increases in extreme humid-heat

Nonlinear increases in warm season temperatures are projected for many regions, a phenomenon we show to be associated with relative surface drying. However, negative human health impacts are physiologically linked to combinations of high temperatures and high humidity. Since the amplified warming and drying are concurrent, the net effect on humid-heat, as measured by the wet bulb temperature (T W), is uncertain. We demonstrate that globally, on the hottest days of the year, the positive effect of amplified warming on T W is counterbalanced by a larger negative effect resulting from drying. As a result, the largest increases in T W and T x do not occur on the same days. Compared to a world with linear temperature change, the drying associated with nonlinear warming dampens mid-latitude T W increases by up to 0.5 °C, and also dampens the rise in frequency of dangerous humid-heat (T W > 27 °C) by up to 5 d per year in parts of North America and Europe. Our results highlight the opposing interactions among temperature and humidity changes and their effects on T W, and point to the importance of constraining uncertainty in hydrological and warm season humidity changes to best position the management of future humid-heat risks.