Scientists have long known that cities tend to be warmer than their surroundings—by 10° Fahrenheit (5.5° Celsius) on average in the US. The urban heat island effect, as it’s known, is produced by the abundance of heat-trapping asphalt and concrete, smog, and heat coming off buildings and vehicles.
Those urban areas are home to more than half the global population and counting—which means their high temperatures have an outsized impact on everything from heat-related illness to greenhouse gas emissions. “There’s a disproportionate relationship between the size of urban area and its impact on the human experience of climate change,” says Lei Zhao, a professor of civil and environmental engineering at the University of Illinois-Urbana Champagne.
To minimize the impacts of global warming, then, “we should plan based on urban-specific information, rather than regional projections,” says Zhao. But cities occupy just 3% of the Earth’s surface area—which means they usually get glossed over in global-scale climate models. “In the state-of-the-art global climate models, almost all of them don’t represent the urban areas because the fraction is so small and they won’t affect large-scale trends,” says Zhao.
City-specific temperature projections could be a useful tool for urban planners looking for ways to mitigate urban heat. So Zhao took a stab at building them in a Jan. 4 paper in Nature Climate Change. He ran geographic data on global urban areas—land covered by urban development, as opposed to cropland, forests, water, or other landscapes—through two dozen climate models to produce temperature projections for all the world’s urban landscapes.
On average, the models project urban areas to be 4.4°C (7.9°F) warmer by 2100 assuming a high volume of greenhouse gas emissions, and 1.9°C (3.4°F) warmer with mid-level emissions. Those numbers are a bit lower than the global average projections produced by the same models, Zhao says, because urban areas are starting from a warmer baseline—but they still end up warmer overall.
The models also project that urban humidity will fall in most places. That’s important because moisture released by trees and other vegetation has a greater local cooling effect in arid environments, meaning that building out more green spaces could be an effective solution to rising urban temperatures.
In addition to being uncomfortable, that increased urban heat causes all kinds of problems. It can be dangerous or deadly: Each year, about 700 people in the US die from heat-related illness, and globally the number of potentially lethal heatwaves is rising. High temperatures contribute to smog formation, and lead to increased greenhouse gas emissions from the power needed to run air conditioners. And they’re inequitably distributed: Lower-income neighborhoods tend to be hotter than leafier, wealthier ones.
Higher urban temperatures will hit certain parts of the world harder, too. Zhao’s projection paints a picture of where the greatest urban warming is likely to occur, including the US midwest, southern Europe, and the Levant. Some areas warm faster, Zhao says, both because they may be more densely developed and because climate warming itself is more pronounced at higher latitudes.
The projections still have limitations: Because the data are broken down into roughly 100-square-kilometer grid cells, it doesn’t correspond perfectly to individual cities. And it’s still not granular enough to be practically useful to urban planners, says Vivek Shandas, an urban planning professor with a focus on climate impacts at Portland State University.
“I’m not convinced that urban planners would use the results of this paper for making decisions about zoning, land uses, materials for development, or generally evaluating local built environments,” says Shandas. “Needed are approaches that provide intra-urban variation and take into account differences in the built environment and the complex atmospheric dynamics that amplify neighborhood heat.”
Still, Shandas says, any climate model that gets scientists closer to understanding urban warming trends is a step in the right direction—especially since neighboring cities will need to work together on solutions.
“That’s essential for effectively addressing environmental pressures that don’t stop at administrative boundaries,” he says.