In a burning hot city in the middle of the summer, when you can practically see the heat rising off the asphalt roads, an air-conditioned building can feel like an oasis.

But the trouble is, the cool interior that air-conditioning units offer can make the streets outside hotter. To make cities cooler overall, researchers believe that buildings should adopt more passive cooling techniques that lower temperatures without electricity, reducing the workload of air-conditioning units.

If buildings adopted special coatings, improved ventilation, ceiling fans, and other technologies, Mat Santamouris, a physicist at the University of New South Wales in Sydney, Australia, told Discover that cities could reduce the average temperature by several degrees, thereby reducing the overall need for air-conditioning across the board.

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Why Should We Reduce Air-Conditioning?

Air-conditioning is a great technology, reducing the smoldering temperatures in the summer — especially in hotter climates. But air-con comes at a high cost—in single-family homes, the units alone cost several thousand dollars, to say nothing of the monthly utility bills. Figures published several years ago by the U.S. Energy Information Administration estimated that air-conditioning accounted for about 12 percent of total energy consumption per home.

Aside from the cash cost, all this energy takes a toll on the environment. Some research estimates that air conditioning accounts for about 3 percent of global greenhouse gas emissions, and demand is set to increase as poorer countries develop, according to Our World in Data. The increase in greenhouse gas emissions raises average global temperatures — so, in a vicious cycle, air-conditioning contributes to global warming, which in turn means more air-conditioning will be needed.

Santamouris argues that achieving zero-carbon goals will be extremely difficult if we continue to rely on air-conditioning.

Aside from the emissions caused by their energy use, air-conditioning units generate heat directly. While they may cool the air inside your home or building, air conditioners expel heat into the surrounding outdoor air. This means that air conditioners in every building make the streets and the buildings around them a little hotter.

Some cities can manage some of this extra heat by planting more trees and creating more parks, which provide shade, but Santamouris explained that it isn’t enough.

“Now we need much more radical solutions,” he said.

But solving this problem isn’t as simple as turning the units off. As the world continues to warm, some regions are experiencing more frequent heat waves.

“Heat waves have a tremendous impact on the economy; they have a tremendous impact on health,” Santamouris added.

How Can We Reduce the Need for Air-Conditioning?

In research recently published in Nature Reviews Clean Technology, Santamouris and his colleagues examined various passive cooling technologies that help keep temperatures down without relying on air-conditioning units.

Examples of effective technologies include radiative coatings. The latter types of paints reflect most sunlight rather than absorbing it — when used on rooftops, they can redirect this heat upward through the atmosphere and into space. Meanwhile, evaporative coating is relatively new and uses a hydrogel-based cooling system. This polymer paint basically sucks up moisture at night when temperatures are cooler, then releases it when temperatures heat up.

Shading systems on the outside of buildings and improved ventilation inside can help block sunlight and redistribute heat more efficiently. Advanced ceiling fans can increase comfort in higher temperatures and cool rooms, Santamouris said.

Furthermore, green roofs feature gardens or other vegetation, contributing to passive cooling. Cities can also be cooled by using reflective pavement for sidewalks and streets,

Livia Fritz, a geographer at the University of Geneva in Switzerland who was not involved in Santamouris’ recent review, said his study is timely and agrees that passive cooling should take center stage, with mechanical cooling, such as air conditioning, used only as a “backup or short-term response rather than the default.”

Many passive cooling technologies can be retrofitted into existing buildings, though Santamouris acknowledges that older heritage buildings may have more trouble making some of these changes. But once implemented, many of them, like coats and ventilation, cost little to run. Fans, meanwhile, consume less energy than air conditioners.

Meanwhile, new buildings should plan to include such technologies — especially since they will still be standing in the climate realities of decades to come, Santamouris said.

How Will Passive Cooling Reduce City Temperatures?

The use of multiple passive cooling technologies, when combined with efficient building design, can reduce the need for air-conditioning by up to 80 percent. And it doesn’t come at the cost of reduced sunlight in buildings — Santamouris said we can have excellent light without getting the heat. He cites the example of tropical Singapore — a city whose average yearly daytime highs range from 87 to 91 degrees Fahrenheit — which now has buildings that don’t even use air conditioning.

The impact is cumulative with regard to reducing city temperatures. The more buildings that use passive cooling, the cooler a city will become overall.

“Cooling needs to be part of a systemic approach to urban adaptation, not simply a set of technical fixes for individual buildings,” Fritz told Discover. “How cities are planned and governed shapes people’s exposure to heat and how vulnerabilities are distributed across different population groups.”

Passive cooling isn’t just about reducing the need for air-conditioning in the developed world. Much of the globe’s population lacks the resources to even equip their homes with air-conditioners. Passive cooling can provide a cheaper alternative to expensive, electricity-powered air conditioning.

“This is inherently a question of justice, since not everyone has the same capacity to protect themselves from heat,” Fritz said.