Climate change and population growth have led to a worldwide surge in the number of people exposed to hazardous levels of heat, according to a sweeping study that examines 13,115 cities from 1983 to 2016.
Why it matters: Extreme heat is the top weather-related killer in the U.S. each year, and studies show that as the world continues to warm in response to greenhouse gas emissions, heat exposure will become so severe that it will reduce economic output in many regions.
- The researchers measured the average annual rate of increase in heat exposure in cities using a measure of person-days per year, and they found that increases in temperature were responsible for the majority of that trend. Person-days indicates how many people experience extreme heat and for how long when accounting for temperature, humidity and population.
- About 23%, or 1.7 billion, of the world's population saw their heat exposure increase in 2016, the study found.
The big picture: The new study, published Monday in Proceedings of the National Academy of Sciences, found that previous studies significantly underestimated extreme heat exposure, particularly in the tropics and rapidly growing parts of the world, such as sub-Saharan Africa, the Middle East and southern Asia.
What they did: The scientists had to overcome a paucity of data in parts of the globe, including India and sub-Saharan Africa. They used infrared satellite imagery along with ground instruments to find maximum daily heat and humidity levels in the cities they evaluated, and they defined extreme heat as having a wet bulb temperature of 30°C (86°F), which incorporates the effects of heat and humidity on the human body. ...