Study: Urban extreme heat exposure has increased by 200% since the 1980s

• 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.

• The researchers looked at population data during the same period in order to come up with a count of person-days in such extreme conditions. They also used population data to determine how much of an increase in heat exposure was due to population growth versus climate change-related trends.

What they found: According to the study, the number of person-days in which urban residents were exposed to extreme heat skyrocketed from 40 billion person-days in 1983 to 119 billion in 2016 — a threefold increase.

• The city that fared the worst in terms of a sharp uptick in person-days was Dhaka, Bangladesh, which saw an increase of 575 million person-days of extreme heat over the study period, though much of that increase was due to population growth.
• Cities that saw climate change drive much of the increase in exposure include Baghdad (Iraq), Lagos (Nigeria), Kolkata (India) and Mumbai (India).
• The study period did not include the record-shattering heat waves that struck North America this year, killing hundreds in the U.S. and Canada.

• The study repeatedly emphasizes the unequal burdens of extreme heat, with poorer residents often suffering the most due to lack of access to air conditioning, among other factors.
• The research calls into question whether the urban poor will be able to rise to greater levels of prosperity, since extreme heat has been tied to drops in economic output.

What they're saying: "The most surprising thing our study found was the sheer scale of change in urban extreme heat exposure globally," study lead author Cascade Tuholske of Columbia University told Axios.

• "Billions of people across thousands of urban settlements worldwide face increasing exposure since the 1980s. This isn't a problem of the future. Rather, urban residents, largely in Africa and Southern Asia, have had to deal with increasingly dangerous heat for decades and the problem is just getting worse."
• Tuholske views this as hopeful research, since known strategies, from early warning systems to green roofs and planting trees in urban areas can all help reduce the effects of urban heat islands and cut down on any heat-related fatalities.
• "Nearly half of all of the cities in this study experienced significant increases in heat exposure through climate warming or through rapidly growing populations, or both," Jeremy Hoffman, a climate scientist at the Science Museum of Virginia, told Axios. Hoffman was not involved in the new research.
• "We know that heat kills, and it disproportionately affects residents suffering from social isolation, chronic illnesses, advanced age, and exposure to other urban-amplified threats like flooding and poorer air quality," he said.