The city is expected to double the number of days it reaches 95F by 2050, but innovative materials can help beat the heat
Katharine Gammon in Los Angeles
Tue Jan 3, 2023 6:00 AM EST14
It may look like an ordinary roof, but the top of the Martin Luther King Jr. hospital near Watts, Los Angeles, gleams in the light. The roof is not made of ordinary materials: instead of absorbing heat, its paint-like coating reflects the sun's energy up. That benefits whoever works inside the building, officials say, because it cools and makes the interior more comfortable – as well as lowers energy bills.
This type of roof design comes at an urgent time for America's second-largest city. In Los Angeles, which has long been romanticized for its year-round sunshine and Mediterranean climate, the heat is a slow-moving disaster. In 2022, the city is grappling with a record-breaking September heatwave that brought triple-digit temperatures for days. A UCLA study predicts that by 2050 the number of days with a temperature of 95F (35C) or hotter will reach 22 in a year – more than double the number seen in the Los Angeles area today.
In a city where tree shade is unequally distributed and half of the surface is tarmac or dark concrete, the solution to increased 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 the high concentration of buildings, roads, and other development — of any city in California. And the extreme heat load often hits residents unequally – a recent county assessment found that Latinos make up 50% of the population of Los Angeles, but make up 67% of the population in communities with high vulnerability to extreme heat.
Design changes that can help cities absorb less heat are under way. These include installing cool roofs, lining roads with reflective materials (known as “cold pavements”) and increasing shade by planting more trees.
Nearly a decade ago, the LA mayor's office convened a scientist advisory panel. Led by researcher George Ban-Weiss of the University of Southern California (USC), the panel created a computer model of a city's environment and microclimate, allowing it to recommend where cool roofs and walkways and shady trees could most effectively be added. Since then, the city and county have lined more than 150 miles of city streets with cool sidewalks.
Craig Shaw, who manages the Cool Streets program, part of LA's Green New Deal that launched in 2019, says Cool Pavement was first used in Los Angeles in 2015, with an installation in Balboa Park. He said each street can be cooled down to 10-12 degrees Fahrenheit once the reflective coating is applied. "It's very effective for us," he said.
The city has converted 5.3 square feet of sidewalks in 2022 alone – some 92 miles of lanes – in the city's hottest areas. There's also a focus on newly paved roads, says Shaw, with the hope that the reflective coating will last as long as traditional asphalt: 20-30 years. Monthly temperature readings from the city's public services division compile data on how colder a treated street is than a nearby untreated road. In one test, cold pavement was measured to be 25 degrees cooler than nearby untreated asphalt.
Cool roofs, cool sidewalks – and natural shaders too
Ban-Weiss, the USC scientist who served on the task force to study urban heat in Los Angeles in 2017, died last year. Several 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 sidewalk project in Covina, California. He said his research showed that cold pavement lowered air temperatures by 1 degree Celsius, but the solution was not without flaws. Coatings used on cold pavement increase reflected shortwave radiation, he says, which can make people uncomfortable – think standing on a very shiny surface and being hit by sunlight reflecting off the ground.
And for cold pavement to have an important impact on air temperature, it must be used more widely. “The pilot project is an exciting opportunity to study real-world impacts on a smaller scale before we rush out and deploy them across the LA basin,” he said.
As well as changing the sidewalks, the Cool Streets program is also planting trees – 2,000 so far – which Shaw, program manager, says help keep temperatures down.
Trees seem like a non-controversial option: they provide shade and can cool nearby areas by more than 20 degrees Fahrenheit through evapotranspiration — the process of transferring water from the ground to the air via plants — according to the Environmental Protection Agency. Researchers have been working to determine which areas need trees most in Los Angeles, especially dense areas with lots of concrete and few parks.
But planting trees is no panacea, says Hannah Schlaerth, a doctoral student at USC who studies urban greening. That's because trees can, conversely, complicate urban heat mitigation in a number of ways. For example, trees emit 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 exacerbating air pollution, especially from certain species (palms emit large amounts of VOCs, while elms and oaks do not). Trees also block the wind from carrying pollutants. "Imagine the wind blowing through a canyon and then in a place where there are a lot of elements in the way," explains Schlaerth.
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 decreased cooling at night.
Not that Schlaerth overlooked the importance of trees in city temperature management — but more research needed to be done, he said, adding that he had met with Los Angeles urban planners who accepted his job.
Cold roofs, however, have been scientifically proven to be effective at reducing 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 "cool roofing." ". rating”, in which the city awards rebates.
Ko, of USC, has also researched other topics: the heat generated by cars in traffic, bodies in roads, and pipes in buildings. "That's actually one of the big contributions to urban heat islands that people sometimes don't realize – because it's not something you can see."
For example, studies in Tokyo have shown that citywide, anthropogenic heat alone can increase citywide average temperatures by one to two degrees Celsius, Ko said. "So in a given hot spot, anthropogenic heat" — heat from a running car engine, or a blast from an exhaust pipe behind a building — "can raise the temperature even further — and you can feel it."
Ko hopes his research can help other urban areas - such as emerging megacities in Africa and India - avoid falling into the design trap that more developed cities are now trying to fix. "Here, it's kind of hard to do it retroactively," he said. "But if you're developing, it's easier to implement while you're developing."
The road to 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 cities," said Shaw. "So this is the first step... We will continue the program as long as the heat continues to rise."