Heat stress may affect over 1.2bn people annually by 2100

Heat stress from extreme heat and humidity will annually affect areas now home to 1.2 billion people by 2100, assuming current greenhouse gas emissions, say researchers.

Rising global temperatures are increasing exposure to heat stress, which harms human health, agriculture, the economy and the environment, the study, published in the journal Environmental Research Letters, said.

"When we look at the risks of a warmer planet, we need to pay particular attention to combined extremes of heat and humidity, which are especially dangerous to human health," said study researcher Robert E. Kopp from Rutgers University in the US.

According to the researchers, heat stress is caused by the body's inability to cool down properly through sweating. Body temperature can rise rapidly, and high temperatures may damage the brain and other vital organs.

Heat stress ranges from milder conditions like heat rash and heat cramps to heat exhaustion, the most common type.

Heat stroke, the most serious heat-related illness, can kill or cause permanent disability without emergency treatment, according to the US Centres for Disease Control and Prevention.

For the findings, the study looked at how combined extremes of heat and humidity increase on a warming Earth, using 40 climate simulations to get statistics on rare events.

The study focused on a measure of heat stress that accounts for temperature, humidity and other environmental factors, including wind speed, sun angle and solar and infrared radiation.

Annual exposure to extreme heat and humidity in excess of safety guidelines is projected to affect areas currently home to about 500 million people if the planet warms by 1.5 degrees Celsius (2.7 degrees Fahrenheit) and nearly 800 million at 2 degrees Celsius (3.6 degrees Fahrenheit).

The planet has already warmed by about 1.2 degrees (2.2 degrees Fahrenheit) above late 19th century levels.

An estimated 1.2 billion people would be affected with 3 degrees Celsius (5.4 degrees Fahrenheit) of warming, as expected by the end of this century under current global policies, the study said.

"Every bit of global warming makes hot, humid days more frequent and intense. In New York City, for example, the hottest, most humid day in a typical year already occurs about 11 times more frequently than it would have in the 19th century," said lead author Dawei Li from University of Massachusetts.