A fifth of world population is affected; many U.S. cities on list

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A new study of more than 13,000 cities worldwide has found that the number of person-days in which inhabitants are exposed to extreme combinations of heat and humidity has tripled since the 1980s. The authors say the trend, which now affects nearly a quarter of the world’s population, is the combined result of both rising temperatures and booming urban population growth. The study was published today in the Proceedings of the National Academy of Sciences.

Over recent decades, hundreds of millions have moved from rural areas to cities, which now hold more than half the world’s population. There, temperatures are generally higher than in the countryside, because of sparse vegetation and abundant concrete, asphalt and other impermeable surfaces that tend to trap and concentrate heat—the so-called urban heat island effect.

“This has broad effects,” said the study’s lead author, Cascade Tuholske, a postdoctoral researcher at Columbia University’s Earth Institute. “It increases morbidity and mortality. It impacts people’s ability to work, and results in lower economic output. It exacerbates pre-existing health conditions.”

The researchers combined infrared satellite imagery and readings from thousands of ground instruments to determine maximum daily heat and humidity readings in 13,115 cities, from 1983 to 2016. They defined extreme heat as 30 degrees Centigrade on the so-called “wet-bulb globe temperature” scale, a measurement that takes into account the multiplier effect of high humidity on human physiology. A wet-bulb reading of 30 is the rough equivalent of 106 degrees Fahrenheit on the so-called “real feel” heat index—the point at which even most healthy people find it hard to function outside for long, and the unhealthy might become very ill or even die.

To come up with a measure of person-days spent in such conditions, the researchers matched up the weather data with statistics on the cities’ populations over the same time period. The population data was provided in part by Columbia’s Center for International Earth Science Information Network, where Tuholske is based.

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