New Study Addresses Growing Threat of Extreme Heat in Cities Worldwide

Rising air temperatures associated with climate change are a threat to cities throughout the world, but especially to the urban poor. The poor generally have fewer adaptive resources and less protective shelter; they have greater health vulnerability to extreme heat, and lower ability to evacuate. These conditions can be exacerbated by the urban heat island effect, where closely spaced structures with lots of pavement and limited green space, common to poorer neighborhoods worldwide, retain heat more readily and for a longer duration. Better understanding of patterns of local exposure to extreme heat is critically needed to design adaptive measures and improve health outcomes. However, until now, global, fine-resolution data on the intersection of extreme heat and population distribution in urban settings have been limited.

A new study “Global Urban Population Exposure to Extreme Heat,” published in the prestigious journal Proceedings of the National Academy of Sciences (PNAS), is the first to combine temperature, relative humidity, and population data to provide a more detailed and comprehensive view of how extreme heat exposure is likely to impact many more people in cities around the world in the coming decades. Lead author is Earth Institute Fellow Cascade Tuholske, based at Columbia University's Center for International Earth Science Information Network (CIESIN), with co-authors Kelly Caylor, Chris Funk, Andrew Verdin, Stuart Sweeney, Kathryn Grace, Pete Peterson, and Tom Evans. The team used new, fine-resolution temperature, relative humidity, and population data provided by NASA's Socioeconomic Data and Applications Center (SEDAC) to assess urban extreme heat exposure in more than thirteen thousand cities, from 1983 to 2016.

Using a daily maximum wet bulb globe temperature threshold of 30°C (86°F)—which accounts for a combined impact of both temperature and humidity on human health and wellbeing—global exposure was seen to increase nearly 200% from 1983 to 2016. Total urban warming elevated the annual increase in exposure by approximately 50% compared to urban population growth alone. Exposure increased for nearly half of urban settlements worldwide, which in 2016 comprised 1.7 billion people.

The authors also found that how total urban warming and population growth drove the trajectory of exposures was not evenly distributed, thus reinforcing the importance of crafting adaptation measures that address local needs. Their findings further suggest that previous research has underestimated extreme heat exposure, underscoring the necessity for improved data to support the development of targeted adaptions such as early warning systems to reduce harmful effects, especially on the urban poor.

Read more about extreme heat and its effect on communities: AP analysis: Exposure to extreme heat has tripled since 1983, published by the Associated Press, and Exposure to Deadly Urban Heat Worldwide Has Tripled in Recent Decades, Says Study, published by State of the Planet, a blog hosted by Columbia's Climate School.