Rising global temperatures are increasing exposure to extreme heat and humidity. Assuming current greenhouse gas emissions, the resulting heat stress could annually affect areas that will be home to 1.2 billion people by 2100, according to a recent Rutgers study.
An estimated 1.2 billion people would be affected with 3°C of warming, as expected by the end of this century, under current global policies. Image: Alona Stepaniuk/123rf
The research, published in the journal Environmental Research Letters says that the number is more than four times the number of people affected today.
Heat stress can harm human health, agriculture, the economy and the environment. Most climate studies on projected heat stress have focused on heat extremes but have not considered the role of humidity, another key driver, according to the researchers.
“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,” says senior author Robert E Kopp, director of the Rutgers Institute of Earth, Ocean, and Atmospheric Sciences.
“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, a former Rutgers post-doctoral associate, now at the University of Massachusetts.
A scorching heat wave led to temperature records in at least seven countries in Europe on July 25, 2019. Credit: NASA Earth Observatory
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 Centers for Disease Control and Prevention.
The study looked at how combined extremes of heat and humidity increase on a warming Earth, using 40 climate simulations to obtain 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 are projected to affect areas that are currently home to about 500 million people if the planet warms by 1.5°C and nearly 800 million at 2°C. The planet has already warmed by about 1.2°C above late 19th century levels.
In New York City, extreme heat and humidity, comparable with the worst day in a typical year today, is projected to occur on four days in a typical year with global warming of 1.5°C and about eight days per year with warming of 2°C. With 3°C of warming, extreme heat and humidity are projected to occur for about 24 days in a typical year.