Los Angeles is becoming too hot to bear. Can it design its way cooler? | Los Angeles

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Los Angeles is becoming too hot to bear. Can it design its way cooler? | Los Angeles

IIt may look like an ordinary roof, but the top of the Martin Luther King Jr. Hospital near the Los Angeles neighborhood of Watts glistens in the light. The roof is not made of a typical material: instead of absorbing heat, its paint-like layer reflects the sun’s energy upwards. This benefits anyone working inside the building, officials say, as it cools and makes the interior more comfortable – as well as lowering energy bills.

This type of roof design comes at an urgent time for America’s second largest city. In Los Angeles, long romanticized for its year-round sunshine and Mediterranean climate, heat is a slow-moving disaster. In 2022, the city faced a record-breaking September heat wave that brought days of triple-digit temperatures. A UCLA study predicts that by 2050, the number of days with temperatures of 95F (35C) or hotter will reach 22 per year – more than double the number the Los Angeles region is seeing now.

In a city where tree shade is unevenly distributed and half the surfaces are dark asphalt or concrete, the solution to rising temperatures may lie in design.

Los Angeles has the worst “urban heat island effect” – a phenomenon in which cities trap and retain heat due to their high concentration of buildings, roads and other developments – of any city in California. And the burden of extreme heat often falls unevenly on the population – a recent county assessment found that Latinos make up 50% of Los Angeles’ population, but make up 67% of the population in communities with high vulnerability to extreme heat.

Design changes that could help the city absorb less heat are underway. These include installing cool roofs, covering streets with reflective materials (known as “cool pavement”) and increasing shade by planting more trees.

A ‘cool street’ in Los Angeles is swapping traditional black asphalt pavement for a coating designed to reflect heat. Photo: MediaNews Group/Los Angeles Daily News/Getty Images/John McCoy

Almost a decade ago, the LA mayor’s office convened an advisory panel of scientists. Led by University of Southern California (USC) researcher George Ban-Weiss, the panel created a computer model of the city’s neighborhoods and microclimates, which allowed it to recommend where cool rooftops and sidewalks and shady trees can be added most effectively. Since then, the city and county have covered more than 150 avenue-miles of city streets with cool pavement.

Craig Shaw, who runs the Cool Streets program, part of LA’s Green New Deal launched in 2019, says cool pavement was first used in Los Angeles in 2015, with an installation at Balboa Park. He says individual streets can be cooled by 10-12 degrees Fahrenheit after the reflective coating is applied. “It was very effective for us,” he says.

In 2022 alone, the city converted 5.3 square meters of pavement – ​​about 92 lane miles – in the hottest areas of the city. There is also a focus on newly paved streets, says Shaw, with the expectation that the reflective layer will last as long as traditional asphalt: 20-30 years. A monthly temperature reading from the city’s general services department collects data on how much cooler the treated streets are than nearby untreated streets. In one test, the cool pavement was measured to be 25 degrees cooler than nearby untreated asphalt.

Cool roofs, cool paving – and natural shade makers too

Ban-Weiss, the USC scientist who served on the 2017 task force to study urban heat in Los Angeles, died last year. Some of his former students continue to unravel the science of heat in Los Angeles.

Joseph Ko, a doctoral candidate at USC, recently published a study on a cool paving project in Covina, California. He says his research shows that cool paving has reduced air temperatures by 1 degree Celsius, but the solution is not without flaws. The coating used in cool paving has increased reflected shortwave radiation, he says, which can make people uncomfortable – imagine standing on a very shiny surface and getting hit with sunlight reflecting off the ground.

And for cool pavement to have a significant impact on air temperature, it would need to be applied much more widely. “The pilot projects are an interesting opportunity to study the real-world impact on a smaller scale before we kind of rush in and implement it across the entire LA basin,” he says.

A man rests in the shade under a tree in Los Angeles during a record-breaking heat wave in September.
A man rests in the shade under a tree in Los Angeles during a record-breaking heat wave in September. Photo: Jae C Hong/AP

In addition to transforming sidewalks, the Cool Streets program also plants trees – 2,000 so far – which Shaw, the program’s manager, says helps keep temperatures down.

Trees seem like an uncontroversial choice: They provide shade and can cool the nearby area by more than 20 degrees Fahrenheit through evapotranspiration—the process of transferring water from the soil into the air through plants—according to the Environmental Protection Agency. Researchers worked to identify the areas in Los Angeles that need the most trees, especially dense areas with lots of concrete and few parks.

But planting trees isn’t a panacea, says Hannah Schlaerth, a doctoral student at USC who studies urban greening. This is because, counterintuitively, trees can complicate urban heat mitigation in several ways. For example, trees give off volatile organic compounds (VOCs), organic chemicals that easily turn into vapor (the smell of pine is one example). In the presence of sunlight, these compounds can form ozone – paradoxically worsening air pollution, especially from certain species (palms emit a large amount of VOCs, while elms and oaks do not). Trees also prevent wind from carrying away pollutants. “Think of the wind blowing through a canyon and then in a place where there are a lot of elements in the way,” explains Schlaerth.

Plant greenery also absorbs more heat than bare soil, which can increase temperatures above the tree canopy – Schlaerth found that with a 50% increase in greenery, there is higher daytime heating above the canopy and reduced cooling at night.

A sign shows the temperature on September 5, 2020 in Woodland Hills in Los Angeles County, California.
A sign shows the temperature on September 5, 2020 in Woodland Hills in Los Angeles County, California. Photo: Apu Gomes/AFP/Getty Images

Not that Schlaerth is discounting the importance of trees in temperature management of cities — but more research needs to be done, she says, adding that she has met with Los Angeles city planners who have been receptive to her work.

However, cool roofs have been scientifically proven to be effective in relieving heat. The Los Angeles City Council unanimously approved a cool roof requirement for residential buildings that went into effect in 2015, requiring homeowners and landlords installing new roofs to use light-colored materials such as shingles, tiles, spray coatings and membranes with a “cool roof” rating”, for which the city provides rebates.

Ko, of USC, is also researching another topic: the heat produced by cars in traffic, bodies on streets and pipes in buildings. “It’s actually one big contribution to the urban heat island that people are often not aware of – because it’s not something you can see.”

Studies from Tokyo, for example, have shown that anthropogenic heat alone can raise the citywide average temperature by one to two degrees Celsius, says Ko. “So in certain hotspots, anthropogenic heat”—heat from a running car, or the explosion of an exhaust pipe at the back of a building—“can increase their temperature by even more—and you can feel it. “

Ko hopes his research can help other urban areas – such as the megacities emerging in Africa and India – avoid falling into design traps that more developed cities are now trying to fix. “Here it’s quite difficult to do it retroactively,” he says. “But if you’re evolving, it’s easier to implement as you grow.”

The road for Los Angeles as temperatures rise remains long but crucial. “We will continue to develop new and better strategies to combat climate change and the warming of our city,” says Shaw. “So this is the first step … We will continue with the program as long as the heat continues to rise.”

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