Buildings and vast stretches of pavement in dense cities trap and generate heat, forming urban heat islands. Similarly, urban development can boost rainfall. Around the world, these so-called urban wet islands have seen precipitation almost double on average over the past 20 years, according to a study published in the Proceedings of the National Academy of Sciences of the United States of America. 

“What we knew up to now has been very focused on particular cities,” said Jorge González-Cruz, an urban climatologist at the University at Albany in New York who wasn’t involved with the work. Places such as Beijing and Houston have served as case studies showing that cities can influence temperature, rainfall, and storms. But the new study shows that the phenomenon occurs at a global scale. The analysis revealed certain factors that influence the wet island effect.

The researchers used daily satellite and radar observations from 2001 to 2020 to determine the difference in precipitation between cities (or the areas directly downwind) and their surrounding rural areas.

Of the 1,056 cities studied, 63% were found to be urban wet islands. “They’re pretty pervasive,” said study coauthor Marshall Shepherd, an atmospheric scientist at the University of Georgia in Athens. The difference between urban and rural precipitation—known as an urban precipitation anomaly—was most extreme in hot and humid places. Coastal cities were also more likely to experience a higher urban precipitation anomaly than inland cities.

Some cities were drier than surrounding rural areas, however. In these places, aspects of the environment often overshadowed the urban influence, said lead author Xinxin Sui, a Ph.D. student studying urban climate at the University of Texas at Austin. For instance, cities in mountainous regions tend to be built in valleys, whereas rain tends to fall in the surrounding higher altitudes.

To study the factors that drive cities’ precipitation, the researchers looked to urbanization and environmental factors including temperature, topography, and population. Cities with large populations—millions of people—tended to have increased precipitation relative to their surroundings. So did ones with high levels of aerosols, which can stem from pollution and help to seed clouds. Last, having a large heat island effect made cities more likely to have a greater precipitation anomaly.

Bigger, Wetter Cities

This information is important for urban planners, said urban climatologist Dev Niyogi, also of the University of Texas at Austin, who was part of the research team. Cities that are receiving more rain may need to plan for flooding, whereas places that are drier than surrounding areas may need to consider how precipitation impacts water resources.

Cities in general have been getting wetter. In 2001, the cities in the study got around 3.8 centimeters (1.5 inches) more rain than surrounding areas. In 2020, that number had risen to around 6.4 centimeters (2.5 inches). This is occurring because people have been building more and bigger cities, Niyogi said.

The urban heat island effect and climate change contribute to warmer cities with hotter air that can hold more moisture. That hot, moist air makes for an environment that’s more suitable for storms. “We have bigger storms, more frequent storms, and more ability for the city to interact with the storms just by the size that they have become,” Niyogi explained.

That has meant that urban-driven precipitation changes are compounding over time, he said. Places that are losing precipitation are tending to get drier, whereas those that have seen increased precipitation are getting wetter still, Niyogi said.

Current climate simulations don’t yet include the impacts of cities on precipitation. The models are too coarse, and the physics is limited to larger-scale processes, so researchers will need to develop ways to capture these effects, González-Cruz said. 

But that’s something that climate scientists are already considering, Niyogi said. One section of an upcoming report from the Intergovernmental Panel on Climate Change will address climate and cities, he noted. But there’s also an opportunity for those working at a local level to make changes to better equip their cities. But, he said, the dynamics of climate processes differ for each city. For instance, even though Houston and Austin are around 160 kilometers (100 miles) apart and experience the same climate, they show different rainfall trends. Houston is receiving an increasing amount of rain relative to nearby rural areas, whereas Austin is receiving a decreasing amount.

In Austin, where Niyogi is based, he and colleagues are teaming up with city planners on a pilot program to implement their research findings. “We can do something with our cities right now to make our near future better for us,” he said.

—Carolyn Wilke (@CarolynMWilke), Science Writer

Citation: Wilke, C. (2024), Many of the world’s cities have gotten wetter, Eos, 105, https://doi.org/10.1029/2024EO240455. Published on 17 October 2024.

Text © 2024. The authors. CC BY-NC-ND 3.0
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