The countryside has long been the summer refuge — leafy, shady, and often several degrees cooler than the city. But a pioneering modeling study suggests that as global warming continues, the rural gap in heat will narrow, eroding the countryside's role as a climate buffer in much of the world.
Cities heat up more than their surroundings because dark asphalt and concrete absorb the sun's rays — the well-known urban heat island effect. Whether that effect will strengthen or weaken as the climate warms has been hard to answer, because global climate models divide the planet into boxes 100 kilometers on a side, far too coarse to simulate weather at city scale. As part of the European Union's Destination Earth project, researchers at the European Centre for Medium-Range Weather Forecasts (ECMWF) have now built a global model fine enough to resolve cities — 9-kilometer resolution — and simulated a greenhouse-warmed world from 2020 to 2050.
The ECMWF team zoomed in on 200 cities across four climate regions — cold, arid, temperate and tropical. The results, published in June in Geophysical Research Letters, upend expectations: by 2050, cities on average will still be hotter than the nearby countryside, but "the rural surroundings of cities warmed up faster," says lead author Xabier Pedruzo-Bagazgoitia. Except in cold regions, the urban heat island effect is weakening.
The mechanism is a familiar one: as higher temperatures dry out the countryside, less evaporation means less cooling of the air, so rural warming accelerates and closes the gap with cities. Climate scientists called the work "highly valuable" — a 15-year-old modeling study had already predicted exactly this outcome. A separate regional-modeling collaboration using Paris as its testbed has likewise found the heat island effect will stay stable or weaken.
Caveats remain. Ashish Sharma of the University of Illinois Urbana-Champaign warns the result rests on single runs of a single model, and that it assumes cities will neither grow denser nor leafier over the next three decades — both of which would change the calculus. And the narrowing gap does not mean cities get spared, cautions Wageningen University climate scientist Gaby Langendijk: "Cities may still experience much higher absolute temperatures, more tropical nights, and greater heat stress under climate change." During a summer of heat and fire, that is a lesson Europe has learned all too well.
For researchers, the new city-resolving models open doors beyond temperature: studying how urban areas trigger storms and rainfall, alter wind patterns, and reshape the local climate of hundreds of cities worldwide.
