Extreme heat is exposing people in big cities to potentially deadly temperatures three times more often than it did in the 1980s, according to a new analysis. Much of that increase is concentrated in South Asia, Africa, and the Middle East. Though climate change is a factor, heat stress is being exacerbated by poverty and the explosive population growth of cities in parts of the developing world.
The findings show policymakers need to adapt cities to extreme conditions, says Deepti Singh, a climate scientist at Washington State University, Vancouver, who studies heat waves and was not involved in the research. “Every single year we have so many deaths associated with extreme heat that to a large extent are avoidable.”
Some parts of the globe are already crossing temperature and humidity thresholds that can kill people in hours. In coastal India, West Africa, and the Persian Gulf, for example, heat waves reach deadly conditions every year, on average. And the pain is increasingly felt in cities, where populations are growing and heat-absorbing concrete and pavement compounds the problem.
But it has been a challenge to gauge the full impact these rising temperatures are having on urban dwellers around the world, says Cascade Tuholske, a geographer and postdoctoral scientist at Columbia University who led the research. Some parts of the world have spotty coverage from reliable weather stations, whereas satellite measurements of heat only cover larger stretches of land, obscuring conditions in individual cities.
To work around those gaps, Tuholske and fellow U.S. researchers turned to a data set recently developed by scientists at the University of California, Santa Barbara. The trove combines the global reach of satellites with the detailed observations of weather stations to create a portrait of daily maximum temperatures across the globe at a scale down to 25 square kilometers—an area about half the size of Manhattan.
The scientists then multiplied the number of days when heat conditions reached health-threatening levels by the number of people living in each of more than 13,000 cities, and tracked how that number changed each year. To gauge extreme heat exposure, they counted days when conditions hit a “wet bulb temperature” of 30°C. That thermometer reading might seem like a mild summer day, but the wet bulb measurement accounts for the way increased humidity makes heat feel more intense. That’s because sweat doesn’t evaporate as readily when it’s humid, depriving people of the cooling effect. At a wet bulb temperature of 30°C, a person would feel the equivalent of 41°C.
India alone accounts for more than half of the world’s increase in urban heat exposure—more than the other top 24 countries combined, the team reports today in the Proceedings of the National Academy of Sciences. The country is home to four cities in the top 10: New Delhi, Kolkata, Mumbai, and Chennai. Europe and North America have no cities in the top 50. An interactive map shows hot spots around the world.
The study’s reported increase in heat exposure in a city can come from two sources: rising temperatures, or a growing population that means more people are enduring the hot weather. Globally, each of those factors accounted for roughly half of the overall increase. The study doesn’t distinguish how much of the rising heat in a particular city is attributable to climate change versus the ways that cities trap heat more than surrounding rural areas.
The study’s approach highlights an interesting strategy for filling data gaps, says Singh, who has used satellite and weather station data to study extreme heat. But there is some uncertainty that comes with merging different data sets and trying to tease out something as precise as daily weather conditions over decades, she cautions.
The findings highlight the challenge of protecting people from heat waves in crowded cities in the developing world, where many people live in poverty and work at physically strenuous jobs, Singh says. “There are a lot of people in these places that are houseless, and their livelihoods are based on activity that exposes them to these conditions.”
Lubaina Rangwala sees the difficulty of addressing these threats firsthand as an urban planner in Mumbai working for the nonprofit World Resources Institute. Mumbai ranks fifth in the world for the greatest increase in heat exposure since 1983, with 46% coming from more days of extreme heat and 54% coming from increased population, according to the new research.
Mumbai’s government there has begun to take an interest in finding ways to blunt the impact of heat waves, Rangwala says. That can involve measures as simple as planting trees or painting roofs white to reflect sunlight.
But without careful planning, such work can be undermined, for example by efforts to build new housing for urban residents. High-rise buildings are “the model for redevelopment,” Rangwala says. But they can trap residents in sweltering apartments with little ventilation, “which is actually causing more harm than good in terms of its ability to deal with climate.”