wet bulb temperature
“Wet bulb temperature” is a measured combination of heat and humidity. When the two combine together at about 35C, the human body can no longer cool itself even while resting in the shade. This represents a “drop dead” temperature that is already threatening millions. [175] [176]
a–f, Ensemble average of the 30-year maximum TWmax (a–c) and Tmax (d–f) temperatures for each GHG scenario: historical (a,d), RCP4.5 (b,e) and RCP8.5 (c,f). Averages for the domain excluding the buffer zone (DOM), land excluding the buffer zone (LND) and the Arabian Peninsula (AP) are indicated in each plot. TWmax and Tmax are the maximum daily values averaged over a 6-h window. [175]
The number of days per year which exceed the historical (1985–2005) mean annual maximum temperature (top row) and wet bulb temperature (bottom row) in 2060–2080. Maps show results under RCP 8.5 (see supplementary figure 8 for maps under RCP 4.5), and (b), (d) show the variation with latitude of the number of days per year under both RCP 4.5 and RCP 8.5, excluding water grid cells. Wet bulb temperatures exceed the historical mean annual maximum more frequently than air temperatures due to lower variability, especially in the tropics. [177]
By 2080, the frequency of wet bulb temperature events will increase by a factor of over 100. Population exposure could increase by a factor of 5 to 10, with 150-750 million person-days of exposure to wet bulb temperatures above those seen today. Under RCP 8.5, exposure to WBT over 35C could exceed a million person-days by 2080. Limiting emissions to a 4.5 RCP pathway would almost eliminate exposure to this threshold. [177]
The Heat Index is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature. [178]
Some of the most affected regions, especially Northeast India and coastal West Africa, currently have scarce cooling infrastructure, relatively low adaptive capacity, and rapidly growing populations.
In the coming decades heat stress may prove to be one of the most widely experienced and directly dangerous aspects of climate change, posing a severe threat to human health, energy infrastructure, and outdoor activities ranging from agricultural production to military training. [177]
Global risks of occupational heat exposure in the shade during the hottest part of the day, averaged over the hottest month. Results are illustrative from an average of four models calibrated to observations. For global temperature increases of 1·5°C and 2°C above pre-industrial levels, the temperature change for individual models is taken from the first decade at which they reach that temperature. For global temperature increases of 3°C, 2090–99 is taken (which is the last decade available). Low risk was defined as a WBGT of 25°C or less; moderate risk was a WBGT of 26–29°C; high risk was a WBGT of 30–33°C; and extreme risk was a WBGT of 34°C or more. Results of individual models, which also show individual climate variability, are shown in the appendix. WBGT=wet-bulb globe temperature. [179]