• ENVIRONMENT

How cities are fighting back against the 'heat island effect'

As temperatures rise, researchers are rethinking how cities measure and respond to urban heat. But experts say successful solutions require finding solutions tailored to local conditions rather than relying on one-size-fits-all fixes.

Record-breaking heat and humidity are already upon us at the start of what is predicted to be a scorcher of a summer. 

Our cities, with their dark, asphalt surfaces, exhaust-emitting vehicles, and heat-trapping buildings, feel these temperatures the most. Many residents experience at least 8 degrees of additional heat just because of their neighborhood layout, according to Climate Central, which assessed heat patterns in 65 metropolitan areas.  

These high temperatures radiate starkly in contrast to surrounding suburbs on a thermal satellite map, earning the phenomenon its name: the Heat Island Effect. It’s been a guiding metric in heat mitigation since the field became widely studied over a decade ago. 

The Heat Island Effect can be a helpful—if at times overly simplistic—reference point, scientists say. It raises awareness about the severity of urban heat but experts say the phenomenon can put too much emphasis on the temperature of the ground, rather than the ways those temperatures affect people. High temperatures have been linked to cardiovascular deaths, chronic kidney disease mortality and respiratory failure. 

(Extreme heat can be deadly–here’s how to know if you’re at risk.)

Scientists and architects have begun to search for protections from these extreme urban heat waves. What’s emerged in some places is a patchwork of green spaces, architectural innovations, and technological advancement, that might help cities find relief from searing temperatures.

These efforts require surgical precision or they can backfire. No two cities are the same, and their reasons for absorbing heat are also not the same, says Mat Santamouris, a distinguished professor of high-performance architecture at the University of New South Wales. Heat poses different problems for different cities: In some it’s a chronic condition, while in others it’s a rare but severe disaster that requires emergency preparedness. Adopting the wrong strategies can be wasteful and even detrimental. 

“Every mitigation project in a city should be designed by experts and tested,” says Santamouris. “Or we risk having very poor results, spending a tremendous budget, and increasing temperature in the cities.”

The delicate art of building a heat-proof city

Each city is susceptible to heat in its own way. 

Sydney and Dubai overheat because they are close to the desert. In these contexts, reflective surfaces can repel solar radiation, lowering temperatures and shrinking the hot air column above the cities. 

By contrast, Athens’s heat comes from its dense planning and the blockage of airflow from the surrounding mountain range. In Athens the priority is ventilation: cooling rooftops to below the ambient air temperature with greenery and reflective materials. That pulls heavier, denser air down into the streets, drawing sea breezes through the city and flushing the heat out. 

Other cities have adopted these practices. Singapore, which is warming roughly twice as fast as the global average, has been a leader in both architectural and technological innovations when it comes to cooling, investing in green spaces, shade, and underground cooling pipes. 

(Why the heat index matters more than just the temperature)

Reflective surfaces on pavements or rooftops have become more common—and controversial—in the fight to beat the heat. Cities have used a variety of materials, from reflective paint to lighter-colored asphalt alternatives to cool surfaces. Reflective pavements were installed in one of Los Angeles’s hottest neighborhoods and during a 2022 heat wave saw ambient air temperatures as much as 3.4 degrees Fahrenheit cooler than those in neighboring areas, according to a 2024 study in in Environmental Research Communications. 

But reflective surfaces have also become a lightning rod for dispute in the field.

“I'm a big fan of white paint but up high,” says Vivek Shandas, a professor at Portland State University who focuses on developing strategies for addressing the implications of climate change on urban areas. “I like coatings that don't necessarily reflect it right into the places people are walking.” 

Others feel focusing on cooler surfaces misses the point.  

"You’re making communities cooler, but you're not making people cooler necessarily," says V. Kelly Turner, a professor of urban planning at the University of California Los Angeles who directs the Heat Research Program at the Luskin Center for Innovation. 

Planting the right trees in the right places

Another way to cool down a city? Introduce plants.

Medellin, Colombia, invested millions of dollars in “green corridors,” lush, connected thoroughfares across the city aimed at providing cool spaces and shade for pedestrians.

Santiago, Chile, pledged to plant 30,000 new trees in its metro area to provide shade and cooling for approximately a third of the country’s population. 

Tree planting pledges have long been touted as ambitious fixes to environmental strife. But applied irresponsibly, they can do more harm than good, says Santamouris. “Humidity, increases in temperature.” In rare cases tree canopies can trap heat instead of shielding from it.  

Considering the species of tree before planting is vital, testing if it can withstand the climate it is meant to protect from, how much water it needs, and where its shade would be most effective. For example, a tree planted in a parking lot is much less valuable to a community than one planted in a playground, says Ladd Keith, director of the Heat Resilience Initiative at the University of Arizona.

But be it greenery, architecture or reflective surfaces, these changes come at a cost, one many of the world's most heat-exposed countries simply can't afford, says Santamouris.

The future of fighting heat