Intensified Humid Heat Events Under Global Warming

Based on the Coupled Model Intercomparison Project Phase 5 multimodel simulations, this paper investigates the future changes in extreme wet-bulb temperatures (TWs) and extreme dry-bulb temperatures (Ts) globally under both low-emission and high-emission scenarios (Representative Concentration Pathways of 4.5 [RCP4.5] and 8.5 [RCP8.5]). Under a warming climate, T and TW exhibit a higher absolute increase in their mean values over the mid-high latitudes while they show higher relative increases in their mean values and variability over tropics. Humid and dry heat events which consist of consecutive extreme TWs and Ts show higher occurrences over tropics, governed by the joint increases in mean values and variability of TW and T therein. Humid heat events show intensifications to dry heat events with higher frequency, duration, and intensity, driven by the lower variability of TW than T. In the mid-high latitudes, heat events will start earlier and last longer in warm season under RCP8.5 than RCP4.5, motivating the need for global warming mitigation. Plain Language Summary The adverse impacts of high temperatures and high humidity on health and economics are worse than extreme temperatures alone. Wet-bulb temperature (TW) combining temperature (T) and humidity is a useful index for humid heat. We investigate future changes in extreme TWs and Ts based on a set of simulations under low-emission and high-emission scenarios. In a warmer climate, both extreme TWs and Ts show apparent absolute increases over the mid-high latitudes but higher increases in mean values and variability in relative terms over tropics. Governed by the joint impacts of mean values and variability of TWs and Ts, humid and dry heat events featuring consecutive days of extreme TWs and Ts show higher occurrences in the tropics. Future humid heat events are more frequent, intense, and longer lasting than dry heat events as TW is more sensitive to global warming than T. Both humid and dry heat events start earlier and last longer under the high-emission than low-emission scenario. Key Points . Extreme high wet-bulb temperature is more sensitive to the changing climate than extreme high dry-bulb temperature Future heat events have relatively higher occurrences in the tropics, and humid heat events are substantially intensified than dry heat The intensification of humid heat events is associated with the projected increases in specific humidity in the warmer climate

Automated summary

This study investigates the future changes in extreme wet-bulb temperatures and extreme dry-bulb temperatures globally, finding that under a warming climate, there are higher absolute increases over mid-high latitudes and higher relative increases over tropics. Humid and dry heat events are more frequent in tropics, with humid heat events intensifying more than dry heat events.