AMERICA HAS been baking this weekend. An estimated 128m people along the East Coast and in the Midwest were affected by excessive-heat warnings on July 20th. This particular heatwave is likely to fade quickly, but such events are becoming more common.
The US Global Change Research programme has projected that by mid-century, there may be 20-30 more days each year in most parts of America when maximum temperatures exceed 32°C. It is a similar story elsewhere. Researchers think that a record-breaking heatwave in France earlier this summer was five times more likely than it would have been without global warming. Europe is set to experience another bout of very hot weather this week.
The prospect of more frequent and intense heatwaves raises especially pointed questions for city officials, because surface air temperatures are higher in urban environments. This “urban heat-island effect” has several causes—including traffic and city layout. But its principal cause is simply that paved environments absorb more heat, whose release then warms the surrounding air. Daytime temperatures are 1°C-3°C higher in American cities than in surrounding rural areas; the differences are even starker at night.
This phenomenon was first documented in the early 19th century by a meteorological pioneer called Luke Howard. In his three-volume The Climate of London, Howard concluded that “the temperature of the city is not to be considered as that of the climate; it partakes too much of an artificial warmth, induced by its structure, by a crowded population, and the consumption of great quantities of fuel in fires.”
The stakes are far higher today. More people are living in cities—four in five Americans, according to the World Bank—and temperatures are rising. That increases the risks to human health: one study into a 2003 heatwave in the Midlands in Britain suggested that the heat-island effect was responsible for about half of the total heat-related mortality then experienced. Heat also worsens urban air quality by producing higher concentrations of ozone.
Heat islands also have profound effects on emissions, as higher temperatures outside increase demand for energy inside. Higher urban air temperatures are responsible for 5–10% of peak electricity demand for air conditioning in America, according to a study in
2005 by Hashem Akbari, then of the Heat Island Group at Lawrence Berkeley National Laboratory.
What, then, could bring urban temperatures down? The structure of cities matters. Street canyons, roads flanked on both sides by high buildings, create shade but also have less exposure to the sky and less chance for heat to be transferred away into the surrounding air. A study in 2018 from the Massachusetts Institute of Technology found that grid layouts, like those in New York and Chicago, are considerably hotter than those that are more chaotic, such as Boston or London. Researchers surmised that heat coming off one building is more likely to be absorbed by another one opposite in grid layouts.
Such lessons are more useful in places where cities are still being built out. Elsewhere, the emphasis is on changing the surfaces of cities. More vegetation is one obvious answer: in any heat map of Manhattan, for example, Central Park will show up as being considerably cooler than surrounding areas. Trees in particular offer lots of shade and, through a process called evapotranspiration, use energy from the sun to evaporate water within their leaves. That has a cooling effect; a study of street trees in California calculated that they were responsible for lower demand for air conditioning and net annual energy savings worth just over $100m. But trees take time to grow, and research is still needed on which varieties have the greatest cooling impact and grow best in cities.
Another answer is to increase the amount of sunlight reflected by artificial surfaces. Paved surfaces and roofs can make up as much as 60% of the surface of a typical America city; parking lots alone can account for 10-15% of the total. And many of these surfaces are made up of dark materials like asphalt and tar, which have a low “albedo” and thus absorb much more sunlight than they reflect.
Researchers proffer a variety of solutions to this problem, from the use of lightly coloured, more reflective aggregate in concrete, to permeable asphalt which is more likely to be cooled by the air or rainfall, to lime-based reflective paints for roofs and walls. A simulation carried out by Keith Oleson of the National Centre for Atmospheric Research in America found that installing white roofs globally would reduce the heat-island effect by a third. Real-world experiments, inevitably, have been somewhat smaller, but many of them, including pilot “cool-roof” programmes in the Indian cities of Ahmedabad and Hyderabad, have showed promise.
None of these ideas can solve the problem of global warming, of course. And there are trade-offs: permeable road surfaces require more maintenance, for example; reflective roofs might increase energy demand at colder times of the year. But two centuries after higher urban temperatures were first noted, the need to reduce them grows urgent.