“Urban heat islands” are metropolitan areas that are significantly hotter than their rural surroundings, due to buildings, paved areas and human activities such as driving cars.
As a result, are intensified in urban areas, which are already home to half the world’s population. That number is expected to reach almost 70% by 2050
This so-called urban heat island effect can push temperatures up by 10 to 15 degrees Celsius (18 to 27 degrees Fahrenheit), vulnerable to extremely .
What causes urban heat islands?
Rural areas are usually covered in grass, crops or forests which helps cool the air, whereas the city’s dark concrete and .
by taking up water from the ground through their roots and then releasing it as vapor into the air. Impervious hard, dark surfaces such as sidewalks, parking lots and roads don’t allow water to trickle in and therefore can’t provide the same cooling effect.
Tall buildings and narrow streets can heat up air trapped between them. These “urban canyons” can block the that could otherwise help cool the area down.
Pollution from cars or the burning of fossil fuels can act as a miniature greenhouse layer over a city, keeping hot air trapped.
Heat islands often build throughout the day as sidewalks and roofs emit more of the sun’s heat — peaking at around three to five hours after sunset.
From sunrise until late afternoon, these surfaces are exposed to intense solar radiation and absorb heat through numerous layers. This stored heat is then slowly released after the sun has gone down.
Where is the urban heat island effect hitting hardest?
Bigger cities tend to store more heat than smaller ones. The urban centers of and Paris often record temperatures of around 4 degrees Celsius higher than rural surroundings at night.
The heat island effect is compounded by the rise in overall global temperatures.
2024 was the hottest year on record at about 1.55 degrees Celsius (2.79 degrees Fahrenheit) above pre-industrial levels. Current policies are projected to see that rise continue to 2.7 degrees Celsiusby the end of the century. This is caused by burning planet-heating fossil fuels that release greenhouse gas emissions into the atmosphere.
In turn, such heat islands could also be fueling as the demand for air conditioning powered by the burning of coal, oil and gas rises during heatwaves.
Are there solutions to cool down cities?
Yes — solutions include greening cities by adding more drought-resistant trees, shrubs and other green vegetation to urban centers, as well as fountains and ponds, green or “cool roofs” that absorb and transfer less heat from the sun to the building.
Such cool roofs reflect more sunlight than a conventional surface and therefore don’t heat up as much. White roofs stay coolest and can reflect about 60 – 90% of sunlight, but other reflective surfaces are also an option.
Cities such as started painting roofs white in 2009 to help curb the heat island effect. Cooler rooftops can also reduce internal building temperatures by up to 30% and lower air pollution and greenhouse gas emissions by reducing demand for power to cool the building.
At ground level, some countries spray water on sidewalks for cooling. In , it’s a centuries-old traditional practice in Japan called “uchimizu.”
Others are mixing residential, commercial, and recreational spaces or installing “cool pavements” that use permeable materials to reflect more of the sun’s radiation and enhance water evaporation.
Megacities like Los Angeles and Tokyo have laid such cool pavements. A study in one of Los Angeles’ hottest neighborhoods found that reflective pavement coating could reduce the heat island effect.
The city government of Tokyo has rolled out around 200 kilometers (about 124 miles) of such sidewalks so far, prioritizing areas in the city center. By 2030, Tokyo aims to cover 245 kilometers of metropolitan roads.
The small Asian city state of , meanwhile, has become one of the greenest cities in the world. It boasts more than 40% green cover with space given over to nature reserves and parks, gardens and vegetation.
By 2030, the city plans to give each citizen access to a park within a ten-minute walk. Singapore also strictly limits the number of cars on its roads through an expensive bidding system with a capped quota for the number of vehicles that can be registered.
Edited by: Tamsin Walker
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