Boffins call it a heat sink—a passive exchanger designed to dissipate heat—but when the sink is an actual city, its concrete and asphalt sweltering in the heat, it feels more like an oven to those who live and work there. The phenomenon where cities are hotter than the surrounding countryside is actually known as an urban heat island, and while the effect exists year-round, it is most acutely felt at exactly the worst moment—a heatwave.
In the countryside, vegetation uses sunlight and water from the soil for photosynthesis, which in addition to converting carbon dioxide into oxygen, also releases water into the air. This helps disperse solar energy and cool the surrounding area.
Meanwhile, in cities, there is not nearly as much vegetation to disperse heat. Moreover, asphalt and cement absorb solar energy during the day and release it during the night. The result is the city is hotter than the surrounding countryside, as buildings and streets act as a giant heat sink, and this is most noticeable during heatwaves.
France’s national meteorological service has found an average annual difference between Paris and surrounding rural areas on the order of 2 to 3 degrees Celsius.
During a heatwave, the difference “can reach close to 10 degrees Celsius,” said Meteo-France.
During a 2003 heatwave, when the daytime temperature hit 40 degrees C it fell to between 23 and 26 degrees during the night in the areas of the city that are the greenest or got the most breeze. But in the city center it fell only to 28 degrees.
This urban microclimate “aggravates the effects felt, in particular during the night, a critical period when normally the human body recuperates,” said Aude Lemonsu, who heads up Meteo-France’s research center. These urban heat islands thus magnify the effects of climate change, which is expected to increase the number of heatwaves, researchers warn.
To the list of factors making cities feel like ovens, there is another one which must be added: air conditioning. “The more you use air conditioning in buildings, the more you heat the outside air,” noted Lemonsu.
The vicious circle of air conditioning is abetted by the design of major cities.
A study published in a March 2018 issue of Physical Review Letters found that the more a city is designed into a square grid pattern, the more it traps heat. The orientation of buildings can also play a role—letting in more light lets in more heat.
While redesigning cities would be difficult and prohibitively costly, there are things that can be done. Planting more trees and plants can help cool the air locally.
“You can even create vegetal walls and roofs to reduce the heat created by buildings,” said Amandine Crambes, an urban engineer and planner at France’s environment and energy management agency, ADEME.
When buildings are built and renovated the choices made can have considerable consequences. If the outside temperature is 26 degrees C, the surface of a dark colored roof can reach 80 degrees, said Crambes.
Meanwhile, the surface of a light colored roof will be around 45 degrees C as it reflects more sunlight and absorbs less heat, and a roof covered with plants won’t rise above 29 C, she said.
Such cool roofs are gaining traction, and some cities like Los Angeles have even experimented with painting streets white.
Another possibility that can be explored: water. We all know from experience that a shower provides relief from heat. The city of Paris has been experimenting in recent years to see if watering streets during extreme heat can help reduce temperatures.
“The question of urban resilience is being taken into account more and more,” said Crambes, noting that authorities face difficult choices constrained by costs and the interest of different stakeholders, including property owners.
For cities, it is fast becoming imperative to make the most of the cool islands within the heat islands. Paris has opened up public parks normally closed at night to allow residents to sleep in the open air when a heatwave makes it difficult to spend a restful night in an apartment without air conditioning.
Paris and the central French city of Lyon have created maps showing where people can escape the heat, like air-conditioned museums and pools.
The city of light has also been working to adapt school courtyards to climate change. Out are stark asphalt squares and in are trees and water fountains. These courtyards may even be opened to seniors and others at risk to extreme heat if needed.