Tall buildings, including several shaped like triangles, are densely packed together, and the Sun appears on the horizon in the background.
Manama, the capital of Bahrain, holds the title of the world’s hottest city, new research reveals. Credit: iStock.com/typhoonski

Around the world, humans are exposed to a wide range of weather conditions. Now, researchers have found that Manama, Bahrain, and Yakutsk, Russia, top the list of cities having the most extreme temperatures. In an analysis of 13,000 cities around the globe, researchers furthermore found that smaller cities in lower- and middle-income countries were more likely to experience excessive heat and cold than larger urban areas in more affluent regions.

Looking Long Term

The highest temperature ever measured on Earth—a staggering 56.7°C—was noted a century ago in California’s Death Valley. Though it was extreme, single measurements such as this don’t capture long-term trends, Jiufeng Li, a geographer at Nanjing University in China, wrote in an email. Instead, he said, such measurements “are more likely to be triggered by occasional anomalous changes in atmospheric conditions.” Measurements made over longer time spans are potentially more meaningful, giving scientists an idea of how conditions are changing through time.

“There is a gap in our understanding of the hottest and coldest temperatures in urban areas.”

To that end, Li and his colleagues recently analyzed monthly averages of temperature and humidity in 13,135 urban clusters with populations ranging from roughly 50,000 to 40,000,000 people.

“There is a gap in our understanding of the hottest and coldest temperatures in urban areas,” Li explained. The researchers published their analyses in Bulletin of the American Meteorological Society.

Don’t Forget Humidity

Li and his collaborators considered three primary data sets: land surface temperature collected by the Aqua satellite’s Moderate Resolution Imaging Spectroradiometer (MODIS) instrument; approximately 2 million records of air temperature and humidity from individual weather stations on the ground; and global climate data pertaining to air temperatures, solar radiation levels, and dew point temperatures from the ERA5 reanalysis data set, a modeled hindcast of climate data created by scientists at the European Centre for Medium-Range Weather Forecasts.

Air temperature—modulated by humidity—was what the researchers sought to analyze for their sample of urban centers. However, more than half of the cities in the researchers’ sample lacked weather station measurements, so the researchers used machine learning to correlate their data sets. The researchers estimated a so-called thermal discomfort index using both temperature and humidity data. That metric most accurately reflects the conditions that humans experience, Li and his colleagues suggested in the study, because a humid environment can feel several degrees hotter than it really is. “The thermal discomfort index is a more accurate physiological indicator of human perception than air temperature,” Li wrote.

“These global analyses are really important to put the problems on a map.”

Using data spanning 2003–2019, the researchers estimated monthly averaged maximum and minimum thermal discomfort for each city. The 10 hottest cities were clustered across four countries: Bahrain, Pakistan, Qatar, and Saudi Arabia. The 10 coldest cities, on the other hand, spanned just two countries: China and Russia. Manama, Bahrain (population: 1,250,000), and Yakutsk, Russia (population: 216,000), took the honors of being the world’s hottest and coldest cities, respectively.

The 30,000-foot view afforded by these findings is valuable, said Cascade Tuholske, a geographer at Montana State University in Bozeman who was not involved in the research. “These global analyses are really important to put the problems on a map so that we can help those in cities who are most vulnerable to extreme temperatures.”

An Unequal Burden

Smaller cities tended to be overrepresented among the hottest and coldest locales, Li and his colleagues noted. When the researchers examined the top 20% of hottest cities and the top 20% of coldest cities, more than 80% had fewer than 300,000 people. Furthermore, cities that experienced extreme conditions were predominantly located in lower- and middle-income regions: Of the 2,627 cities in the hottest 20%, for example, 2,128 (81%) were situated in lower- and middle-income regions. For comparison, 42% of cities worldwide are in lower- and middle-income regions.

These results highlight that people living in smaller cities in lower-income regions face a greater risk of experiencing extreme temperatures, the researchers suggested. And that’s inequitable, the team maintained, because such cities are less likely to possess the infrastructure necessary to deal with an extreme climate. “These regions not only lack access to essential air conditioning and heating systems, but also struggle with unreliable energy and commodity networks,” Li wrote.

But these results shouldn’t trigger a sense of hopelessness, Tuholske was quick to point out. Research has shown that people living in smaller cities, which can be conducive to tighter social networks and stronger familial ties, are sometimes well poised to enact positive change. “There’s some compelling research showing that small cities may actually be the best opportunity for sustainable development,” Tuholske said.

—Katherine Kornei (@KatherineKornei), Contributing Writer

Citation: Kornei, K. (2023), Temperature extremes hit lower- and middle-income countries hardest, Eos, 104, https://doi.org/10.1029/2023EO230338. Published on 8 September 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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