Where Will Wet-Bulb Temperatures Turn Unbearable?

A new study published in the Proceedings of the National Academy of Sciences has found that billions of people will be affected by heat unbearable for the human body if global temperature were to rise 2° C above pre-industrial levels. A great deal of these persons will be located in Northern India, Pakistan and Bangladesh as well as in Eastern Asia, the Arabian Gulf and the African Sahel Zone, according to the research.

The study by academics at U.S. institutions Penn State and Purdue University has tested and is applying a lower threshold of what level of humid heat humans can endure than previously used. Researchers had assumed that human physiology could endure wet-bulb temperatures - a measure taking into account heat and humidity - of 35° C. Lab tests, the first of their kind according to the study's authors, however found that at an average wet-bulb temperature of 30.55° C, young, healthy subjects could already not carry out simple tasks mimicking daily activities. These temperature could by extension be very dangerous for older people or those with preexisting health conditions if they don't have access to air-conditioned or otherwise cooled environments - not a given in the developing countries affected. Activities outside would become almost impossible under these conditions.

The human body is able to cool itself with evaporation of sweat, but this hinges on the humidity of the air and its ability to absorb more water vapor. This is why high temperature and high humidity pose a higher risk of overheating for humans.

The study's results show that at 2° C of global warming, Indian cities Kolkata and Delhi would experience between 30 and 40 annual hours of this dangerous and debilitating heat. In Lahore, Pakistan, this number would be even higher at almost 150 hours or more than a dozen 12-hour days. The highest result was calculated for Aden in Yemen at more than 200 such hours per year, or almost 17 days of 12 hours. At a global average warming of 3° C, the exposure to extreme heat would rise to between 157 and 171 hours in Kolkata and Delhi (13-14 days), 447 hours (37 days) in Lahore and 816 hours (68 days) in Aden. Mumbai and Chennai, only marginally affected under the 2° C warming scenario according to the survey, would start seeing unbearable heat for six to 10 hours a year. Other cities which would only really start to really see this effect at a 3° C warming are Shenzhen in China, Tokyo and Osaka in Japan, Lagos in Nigeria and New York and Chicago in the United States. At a warming of 4°C, the study calculates, even the aforementioned cities would have to endure a minimum of 23 (2 days) and a maximum of 115 hours (9.5 days) of heat beyond human endurance.