The number of person-days when city dwellers are exposed to extreme heat and humidity has tripled since the early 1980s, according to a new study of more than 13,000 cities worldwide.
Temperatures in cities are generally higher than in rural areas, owing to the lack of trees and the abundance of concrete. The dramatic growth in urban extreme heat exposure is a function of people moving from rural areas to warmer urban areas and of more severe heat as a result of climate change. Migration to cities accounted for two-thirds of the increased exposure, while climate change accounted for a third, the study found.
Scientists defined extreme heat as 106 degrees F on the wet-bulb scale, an indicator of how hot it feels given the temperature and humidity. Such severe heat can make being outside difficult for healthy people and can prove life-threatening for unhealthy people. In 1983, the number of person-days when people living in cities were exposed to extreme heat was 40 billion. By 2016, that figure had risen to 119 billion, according to the study, which was published in the Proceedings of the National Academy of Sciences.
“This has broad effects,” said Cascade Tuholske, a postdoctoral researcher at Columbia University and lead author of the study. “It increases morbidity and mortality. It impacts people’s ability to work, and results in lower economic output. It exacerbates pre-existing health conditions.”
Cities nearer the equator, such as Shanghai and Guangzhou in China, have been more severely affected. Dhaka, the capital of Bangladesh, the hardest-hit city, has seen an increase of 575 million person-days of extreme heat, the study found.
In the United States, cities in Texas and along the Gulf Coast have seen the largest growth in exposure to extreme heat. While population growth is driving the uptick in some places, such as Las Vegas, in others, climate change is the central factor. In Providence, Rhode Island, for instance, greater exposure to extreme heat is due almost exclusively to higher temperatures and humidity.
Future warming and urban growth could lead to a 400 percent increase in extreme heat exposure by 2050, with river valleys in South Asia and Africa — such as the Ganges, Indus, Nile, and Niger — being the most affected, the study’s authors wrote in The Conversation.
“Numerous studies have shown that extreme heat reduces labor productivity and economic output,” the authors wrote, adding that “the convergence of changes in extreme heat with large urban populations calls into question the conventional wisdom that urbanization uniformly reduces poverty.”