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How to Cool Down a City

Singapore’s prime minister has described climate change as “life and death.” He has reason to worry: Stifling temperatures and humidity already last all year, and the city-state has warmed at twice the global average over the past six decades.

Heat like this isn’t just uncomfortable. It can cause chronic illness and death, including heat exhaustion, kidney damage and even heart attacks. With two-thirds of the global population expected to live in urban areas by 2050, urban heat is an enormous global health challenge.

Rapid urbanization has made Singapore hotter. A big part of the problem is how almost every global city is built.

Cities cut down trees and remove plants that provide shade and naturally cool the air.

They cover large areas with concrete and asphalt, which absorb heat during the day and release it at night.

They densely pack skyscrapers into urban canyons that limit wind flow and trap pockets of heat.

And their residents expel waste heat from gas car exhausts and air conditioners, helping to transform a hot day into an unbearable one.

Preventing climate change is out of Singapore’s control: The city-state emits less than 0.1% of global carbon emissions. But there is a surefire way to limit city temperatures, researchers say: Revive the natural processes that cooled the land before urbanization.

Most cities do not have Singapore’s wealth and centralized political system, which allow it to move quickly to build new infrastructure. But while some of Singapore’s strategies to reduce excess heat are expensive, many of them are straightforward, and cheaper than planning for, say, floods or hurricanes.

As temperature records were shattered around the world this summer, Singapore’s blueprint for slowing the urban impacts of extreme heat is gaining urgency.

Researchers say that planting more trees is the most effective way to reduce a city’s temperature.

“If you wanted to invent the most effective kind of climate management technology from the ground up, you could spend a lot of time trying to do that. You would just engineer a tree,” said Brian Stone Jr., director of the Urban Climate Lab at the Georgia Institute of Technology.

The streets around the Khoo Teck Puat Hospital are lined with trees, and the central courtyard of the building is full of dense foliage. During the day, trees shield pedestrians from the beating sun and prevent the sun’s rays from warming the concrete sidewalk. At night, temperatures are lower, as there’s less heat released from the sidewalk.

In order to rely on trees to regulate climate stress, cities will need to treat them as infrastructure to ensure they are healthy and effective, according to Dr. Stone. That will come at a cost, but just a fraction of what cities spend on other environmental protections.

“It’s a real budget item, but it’s not out of proportion to what we already spend on environmental management in cities,” he said. “It’s less than 1 percent of what we spend maintaining storm sewers in L.A. every year.”

Singapore is also encouraging the integration of greenery directly into buildings by offering financial incentives for rooftop gardens and vertical green facades. The foliage works as natural blinds, shading the structure and insulating the building’s material from the heat, reducing the need for air conditioning.

Parkroyal on Pickering Hotel

Junpei Nicholas Lin/Skyscapist for The New York Times

Singapore has painted the roofs of some buildings with light-colored reflective paints, which absorb less heat and could reduce the ambient temperature around the buildings by up to 3.5 degrees Fahrenheit, initial studies suggest. A similar program in New York City has covered more than 10 million square feet of rooftops with reflective paints since 2009, reducing the need for air conditioning and the waste heat it generates.

Building without

cool paint

Building coated

with cool paint

Building without

cool paint

Building coated

with cool paint

Satellite image by Planet Labs

Simple design decisions can also have a big impact on a building’s temperature.

Buildings in the new Jurong Lake District avoid directly facing the sun, cooling indoor temperatures. And architects are designing buildings that encourage cross-ventilation, which allows air to move from one side of a building to another, pushing hotter air out and bringing cooler air in.

“You can have a big area that’s not energy dependent on churning out lots of air conditioning to make it comfortable,” said Richard Hassell, founding director of WOHA Architects, which designed the Parkroyal hotel. “You can make it passively comfortable.”

There are limits to what can be achieved by rethinking a city only one building at a time. Even the most cutting-edge buildings can negatively influence their surroundings in unforeseen ways.

A new residential development with all the “bells and whistles” of efficient urban design could still make the neighborhood hotter if it blocks wind flow, said Winston Chow, principal investigator at Cooling Singapore, a research group.

“You’ve got a new residential development that cancels everything out, so it’s a net loss,” he said.

To tackle the urban heat island effect, it’s vital to not only improve the design of each building, but to consider its relationship with the surrounding city.

One street in Singapore, known as “air conditioner alley,” demonstrates how a lot of poorly coordinated small decisions can compound to cause a big heat problem. Hundreds of air conditioning units all siphon hot air out of the apartments and businesses, onto the same narrow street.

“Air conditioner alley” in Singapore

Rebecca Toh for The New York Times

A similar dynamic plays out in most cities with hot climates. Buildings are cooled one at a time, lowering their temperature at the expense of making the environment hotter.

Phoenix, Ariz., recorded 17 days at or above 115 degrees Fahrenheit in July, which was the hottest month ever recorded globally. Water bottles warped, devoted runners strapped on headlamps to jog at 4 a.m. and dozens of people died in surrounding Maricopa County from heat-related causes.

The design of the city made the brutal summer worse. While Phoenix has planted some trees in shadeless neighborhoods, overall it has few trees and large stretches of sun-absorbing asphalt. More than 1.3 million residents live in areas where the urban heat island exceeds 8 degrees, according to research by Climate Central.

Instead of cooling small spaces individually, Singapore’s Marina Bay, which was centrally planned, cools down many buildings at once by running chilled water through a network of insulated pipes. The district cooling network is far more efficient than multiple small A.C. units, reducing both energy consumption and waste heat.

Other cities have similar systems, including Paris, and so do some American college campuses. But to work efficiently, district cooling often requires coordination between multiple landowners and developers, and retrofitting existing structures is expensive. Singapore, which can centrally plan a large-scale new development like Marina Bay, has an advantage.

Large green spaces like parks are a more widely practical intervention, cooling areas beyond their boundaries, researchers say. Air temperature measurements show that Singapore’s 155-acre Bishan Park can be up to about 3 degrees cooler than high-density residential blocks in the middle of the city.

“The larger the park space, the further it penetrates into the residential areas as well,” said Mr. Chow from Cooling Singapore.

But even parks have their limits. Singapore has built out a more systematic solution, a network of green corridors that connect green spaces together and allow cool air to flow throughout the city.

Network of

green corridors

Network of

green corridors

Network of

green corridors

Network of

green corridors

Source: The National Parks Board, Singapore

“A corridor can at least generate this kind of cool air circulation in a city. And the cool air can extend to the outside area, creating pockets of relief from the heat,” said Tamara Iungman, a researcher at the Barcelona Institute for Global Health.

Widely planting street-level trees along sidewalks across the city is the most effective solution to reduce temperature, according to researchers at the Urban Climate Lab.

“We can’t rely on a centralized, intense clustering of urban forests or microforest to provide cooling for the whole city. We really have to disperse,” said Dr. Stone from the Urban Climate Lab.

Singapore’s Rail Corridor

Junpei Nicholas Lin/Skyscapist for The New York Times

Can Singapore’s efforts to reduce urban heat islands actually outpace rising global temperatures? Probably not, local officials acknowledge. But holding temperatures steady would be a huge victory.

“I think we’re just trying to not see the increases that we anticipate if we don’t do anything,” said Adele Tan, deputy chief executive of Singapore’s Urban Redevelopment Authority.

Urban planners and policymakers are recognizing that inventions to cool down cities also help in other ways. Green corridors and large green spaces support biodiversity, provide recreational spaces for residents and aid flood prevention.

“It’s a pleasant surprise to be here at this moment in climate change, realizing that our number one intervention has all these other benefits,” Dr. Stone said.

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