Over the past 40 years, as climate change leaped into global awareness, exposure to extreme heat jumped by close to 200 percent in more than 10,000 of the world’s biggest urban areas, according to a study published in October in the Proceedings of the National Academy of Sciences.

Increases in dangerously high temperature and humidity were responsible for roughly a third of the global boost in exposure, while increased population accounted for the rest.

The study adds vivid context to the threats posed by a human-warmed planet, and to the challenges facing delegates at the United Nations climate summit taking place in Glasgow, Scotland.

Led by Cascade Tuholske, a postdoctoral researcher at Columbia University’s Earth Institute, the study used new fine-grained data sets to analyze the geographic overlap between urban growth and dangerous combinations of temperature and humidity.

“Many of the fastest-warming cities are in the humid tropics,” said Tuholske in an email.

The study analyzed 13,115 urban areas over the period from 1983 to 2016. Collective heat exposure was assessed in terms of person-days, or the number of days above a particular threshold in each city multiplied by the number of people affected.

The extreme heat was assessed by using day-to-day peaks in wet bulb globe temperature, or WBGT (max), a metric that takes into account humidity, sunlight and wind, in addition to temperature. The measure is considered particularly dangerous when it exceeds 86 degrees.

Using the 86-degree WBGT (max) threshold, the authors found that collective exposure to extreme heat and humidity across the cities studied soared from around 40 billion person-days in 1983 to 119 billion in 2016. Close to half of the cities studied showed increases in exposure that were statistically significant.

Higher WBGT (max) readings were the main culprit behind the exposure increase in many areas, including much of India. In a few other parts of the world, including East Africa, exposure climbed mainly due to population.
From 1983 to 2016, thousands of cities saw a significant increase in collective exposure to extreme heat and humidity. In some cases, the increased exposure was mainly due to population increase (green), while in others the increase was largely because of changes in weather (tan). Cities in white reflect roughly equal contributions from population and weather. (Cascade Tuholske)
Evidence piles up on climate change and heat

The new study by Tuholske and colleagues joins a rapidly growing body of work on the impact of extreme heat and how climate change will exacerbate the problem. The World Weather Attribution project concluded that July’s deadly, record-smashing heat wave over the U.S. Pacific Northwest and southwest Canada would have been “virtually impossible without human-caused climate change.”

There is high confidence that heat waves over land have become more intense and frequent across most of the world, according to the latest Working Group I assessment, released in August by the Intergovernmental Panel on Climate Change (IPCC).