Cities under the heat: What are urban heat islands?

"Urban heat islands" are areas in cities that are significantly warmer than rural surroundings due to buildings, paved surfaces, and human activities such as driving.

As a result, heat waves are intensifying in cities.

This so-called urban heat island effect can increase temperatures by 10 to 15 degrees Celsius.

With more than half of the world's population already living in urban areas – and that number is expected to reach almost 70 percent by 2050 – many people are suffocating from the heat.

What causes urban heat islands?

Rural areas are usually covered in grass, crops, or forests, which helps cool the air, while the dark concrete and asphalt of the city absorb heat.

Plants act as natural air conditioners by absorbing water from the soil through their roots and then releasing it into the air as water vapor. Impervious hard, dark surfaces, such as sidewalks, parking lots, and streets, do not allow water to penetrate and therefore do not provide the same cooling effect.

Some countries try to compensate for this by spraying water on sidewalks to cool them down. In Japan, it's an age-old traditional practice that even has its own name: "Uchimizu."

Tall buildings and narrow streets can trap and heat air. These "urban canyons" block the natural flow of wind that could otherwise help cool things down.

Pollution from cars or the burning of fossil fuels can create a kind of miniature greenhouse layer over the city, keeping hot air trapped.

Heat islands often form during the day, as sidewalks and roofs reflect more of the sun's heat. They peak 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. After sunset, this stored heat is then slowly released.

Where is the urban heat island effect strongest?

Larger cities tend to store more heat than smaller ones. Urban centers like London and Paris, as well as Belgrade, often record temperatures around four degrees Celsius higher than rural areas at night.

Urban areas are more "vulnerable" to heat, as the amount of global warming due to climate change is amplified by the heat island effect.

Last year, 2024, was the warmest year on record, at about 1,55 degrees Celsius above pre-industrial levels. At current rates, global temperatures are expected to rise by 2,7 degrees Celsius. This is caused by the burning of fossil fuels, which heats the planet and releases greenhouse gas emissions into the atmosphere.

The result is heat islands and they could also fuel climate change, as demand for air conditioning, driven by the burning of coal, oil and gas, increases during heat waves.

Are there solutions for cooling cities?

Yes – solutions include greening cities by planting more trees, shrubs and other drought-resistant green vegetation in urban centers, as well as more fountains and lakes, green or so-called "cool roofs" that absorb and transmit less solar heat to the building.

Such cool roofs reflect more sunlight than a conventional surface and do not heat up as much. White roofs are the coolest and can reflect 60 to 90 percent of sunlight, but other similar surfaces are also options for reflecting invisible radiation.

Cities like New York City began painting their roofs white in 2009 to help combat the urban heat island effect. Such cool roofs can reduce indoor temperatures in buildings by up to 30 percent and reduce air pollution and greenhouse gas emissions by reducing the energy demand for cooling the building.

Additional measures include installing "cool sidewalks" that use permeable materials to reflect more solar radiation and improve water evaporation, as well as mixing residential, commercial, and recreational spaces.

Megacities like Los Angeles and Tokyo already have such cool sidewalks in an attempt to further cool down urban environments. A study conducted in one of the hottest neighborhoods in Los Angeles found that a pavement coating that reflects solar radiation could reduce the heat island effect.

The Tokyo Metropolitan Government has so far installed about 200 kilometers of such sidewalks, with priority given to areas in the city center. By 2030, the Japanese capital aims to cover 245 kilometers of city streets.

The small Asian city-state of Singapore has become one of the greenest cities in the world. More than 40 percent of its land area is green. They have given space to nature reserves and parks, gardens and greenery. The city plans to have every household within a ten-minute walk of the nearest park by 2030. Singapore also strictly limits the number of cars on its streets through a system of limited and expensive quotas for the number of vehicles that can be registered.