Abstract
Humidity is a significant factor contributing to heat stress, but without enough consideration in studies of quantifying heat hazard or heat risk assessment. Here, the simplified wet-bulb globe temperature (WBGT) considering joint effects of temperature and humidity was utilized as a heat index and the number of annual total heat wave days (HWDs) was employed to quantify heat hazard. In order to evaluate the humidity effects on heat waves, we quantified the difference in the number of HWDs over global land based on air temperature and WBGT. Spatial and temporal changes in surface air temperature, relative humidity, WBGT, and the difference in HWDs were analyzed using multi-model simulations for the reference period (1986–2005) and different greenhouse gas emission scenarios. Our analysis suggests that annual mean WBGT has been increasing since 1986, which is consistent with the rising trend in surface air temperature despite a slight decrease in relative humidity. Additionally, changes in annual mean WBGT are smaller and more spatially uniform than those in annual mean air temperature as a cancelation effect between temperature and water vapor. Results show that there is an underestimation of around 40–140 days in the number of HWDs per year in most regions within 15° latitude of the equator (the humid and warm tropics) during 2076–2095 without considering humidity effects. However, the estimation of HWDs has limited distinction between using WBGT and temperature alone in arid or cold regions.
Highlights
The probability and intensity of extreme heat waves have been increasing over many parts of the world owing to climate change [1]
Annual mean W over global land has been increasing since 1986 (Supplementary Figure S2a), which is consistent with the rising trend in surface air temperature (Supplementary Figure S2b) despite a slight decrease in relative humidity (Supplementary Figure S2c)
Our study underscores the consideration of humidity effects in defining heat waves and quantifying heat wave hazard, especially in the humid and warm tropics
Summary
The probability and intensity of extreme heat waves have been increasing over many parts of the world owing to climate change [1]. There is much more previous work on heat waves focusing on surface air temperature alone [2,3,4]. In addition to extreme high temperatures, surface relative humidity is an important factor in defining heat waves as it is directly related to human body heat exchange either [5,6]. A resting human body generates about 100 W of metabolic heat (in addition to any absorbed solar heating) and cannot dissipate heat if the ambient temperature is higher than the optimum body core temperature (near 37 ◦ C) owing to the second law of thermodynamics [7]. Public Health 2019, 16, 1513; doi:10.3390/ijerph16091513 www.mdpi.com/journal/ijerph
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