The Radical Plan to Cool Down LA as the World Heats Up

By Nate Berg on at

It’s barely 10 a.m. on an August day in Hollywood, and the heat is already becoming oppressive. The temperature’s only 30℃, but in the direct sun it feels hotter—and it’s getting worse by the minute. Part of the reason is the ground. The black asphalt of this side street off Sunset Boulevard is sucking up the sun and radiating its heat back out. An infrared thermometer shows the surface temperature to be 44℃. By mid-day, it’ll rise above 65℃.

That’s why a crew of workmen are out here, giant squeegees in hand, spreading a thin coat of liquid over the asphalt. It’s an oil-based sealant, the kind that prevents roads from cracking and potholes from forming. But unlike most street sealants, this one has been specially formulated with a light coloured pigment, and within 20 minutes the crew has effectively turned the street from black to white. With the first coat barely dry, the surface temperature’s already dropped about 8℃.

This is what the city of Los Angeles is calling “cool pavement”—whitening blocks in each of LA’s 15 council districts to see how changing the colour of streets can bring down the overall temperature. The street conversions started in May, and the results so far are promising, with officials recording average temperature drops of at least 10 degrees F on the pavement itself.

The ambient effect is hard to really feel, and is probably small given the block-long installation is surrounded by other heat-sucking black streets. But city officials are hoping to make the case that this new approach to paving can have an impact. If they can be rolled out on a larger scale, cool pavements could play a key role in adapting the city to a warming climate.

nullWorkers whitening a street in LA as part of the city’s “cool pavement” programme to reduce the urban heat island effect. Image: Nate Berg

For all the existential threats L.A. faces from climate change—sea level rise, coastal erosion, increasing wildfires, decreasing water supplies—the one that will be most evident to the majority of residents is the heat.

“When we look at our vulnerabilities associated with climate change, we know that extreme heat is one of our top concerns,” says Lauren Faber, the city’s Chief Sustainability Officer. From public health risks to energy use spikes to sheer economic costs, hot days cause problems. And they’re on the rise. A 2015 study by UCLA researchers projects a dramatic increase in extreme heat in L.A. by mid-century. The annual number of days with highs above 35℃ could more than double in the already-hot San Gabriel Valley, and more than triple in Downtown L.A. These figures are based on current greenhouse gas emissions projections, but even if those emissions are slashed, the city’s going to get hotter. And that means a rise in heat exhaustion, heat stroke, and even heat-related death.

Part of the cause is climatic and part is geographic, but the city itself is also to blame. The hardscape of buildings, freeways and streets absorb the heat that would reflect, or otherwise dissipate, in less built-up places. This is known as the urban heat island effect, and though it’s not unique to L.A., it’s especially pernicious here. A 2014 study from the Yale-NUIST Center on Atmospheric Environment found greater Los Angeles to be roughly six degrees warmer than its rural hinterlands, bumping up energy and air conditioner usage, which cause even more of the greenhouse gas emissions that have caused the climate to change in the first place. And as climatic warming progresses, L.A.’s extra six degrees will only make the associated problems worse.

nullTaking the temperature of a recently-whitened pavement in LA. Image: Nate Berg

To slow this feedback loop, L.A. is trying to dial down its urban heat island effect. That’s one of the key goals of the “Sustainable City Plan,” a 2015 report from Faber’s office outlining dozens of targets the city should be working towards to improve its environmental sustainability and help it adapt to climate change. The plan specifically calls for the city to reduce its urban-rural temperature differential three degrees by 2035.

To get there, L.A. is pushing on multiple fronts, all of which are aimed at creating a reflective shield over the city. The first, most natural strategy is to increase the urban tree canopy. Through shade and evapotranspiration, trees are “natural air conditioners,” says Elizabeth Skrzat. She’s the policy director of City Plants, which works with the city’s Department of Water and Power to deliver and monitor the free shade trees it donates to residents as part of city cooling and energy saving efforts. They hold more than 100 tree adoption events a year, and gave away nearly 18,000 trees in 2016 alone. Though street trees can play a part in cooling the city, Skrzat says there’s much more potential in the front and back yards of L.A.’s low density residential areas. To encourage a bigger canopy in these privately owned swaths of the city, residents are eligible to receive up to seven trees, free of charge. “We’ll literally deliver them to their door,” she says.

The city also has some involuntary programmes to try to bring temperatures down. In 2014, L.A. enacted an ordinance that requires most new and renovated buildings to install lighter coloured roofs with high solar reflectance ratings. About 12,000 of these cool roofs have been installed so far. As new buildings rise and as older roofs require replacement, the city will gradually see all of its rooftops made from more reflective material.

But when it comes to surfaces that are particularly in need of intervention, it’s the city’s thousands of miles of blacktopped roads that are the next frontier.

“More than ten per cent of the land area in L.A. is asphalt, whether it’s streets or parking lots,” says Greg Spotts, assistant director of the city’s Bureau of Street Services, which is leading the cool pavements project, a pilot with a $150,000 ( £116,000) budget. “At scale, this could potentially move the needle on the heat island effect.”

nullA tree-lined boulevard in Beverly Hills. Image: Lady Madonna / Flickr Creative Commons

But for a city with 28,000 lane miles of roads, scaling up isn’t trivial. Because the cool pavement sealant is so new, costs are hard to predict, but they would undoubtedly be higher than traditional street renovations. (For the pilot project, the Bureau of Street Services has budgeted $10,000 ( £7,800) per installation. So far, all installations have come in under budget.) What’s more, not all of LA’s roads will necessarily benefit from a cool pavement approach, says David Fink, policy director of the nonprofit Climate Resolve. The urban heat island effect, he says, is not evenly distributed across the city. “People typically think of it as this dome of heat that sits over our cities, but when you get down into the cities, it’s more nuanced,” he says. “In L.A., there are hotspots around the city, parts that are dramatic urban heat islands, and parts of the city where the urban heat island is minimal.”

If the city is going to build more cool pavements, it will want to figure out the locations where they’ll do the most to reduce heat absorption. George Ban-Weiss, an engineering professor at the University of Southern California, has been leading various mapping efforts in L.A. to better understand how and where these types of efforts should be implemented. His team has a high-accuracy temperature monitor mounted to the roof of a car, and they’ve been driving around the city to try to understand how the heat island varies by neighbourhood and land use.

“The goal is to say where are people most vulnerable to extreme heat, and in those regions where the people are most vulnerable, what are the mitigating strategies that should be most pursued that would help the most,” he says.

A former member of a Lawrence Berkeley Lab group focused specifically on the heat island effect, Ban-Weiss has modelled how large-scale roll-outs of cool roofs and cool pavements could affect cities. He says that, in combination with efforts like tree canopy improvements, broad but targeted installations of cool surfaces can have a big impact. L.A. is the first city to take this approach to its streets, and if the cool pavement program is expanded along with other measures, it could do enough to reach the city’s target. “That three-degree goal is attainable,” he says.


For now, the city is moving ahead a few thousand square feet of white at a time. As the crews in Hollywood spread the new sealant across the asphalt of its newest pilot street, the area’s city council member, David Ryu, stopped by to see the transformation taking place. He believes the city should be doing all it can to improve its environmental sustainability, and he’s hopeful his colleagues on the council will be able to dedicate more funding to expand the cool pavements pilot project to a larger scale. “We want the Bureau of Street Services to go even farther,” he says.

Lauren says if L.A. is able to build on these first cool pavement projects, it can lead the state and the country in rethinking roads. “This has the ability to really go beyond our jurisdiction,” she says.

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Image: Nate Berg

There’s still a lot to figure out before that can happen, from identifying proper locations to working with sealant manufacturers to improve what’s currently a niche product. While the city’s tests are being conducted in each of its 15 council districts, officials and environmental activists alike know that poorer neighbourhoods with smaller tree canopies would benefit more from these types of interventions. Whether or not cool pavements spread across the neighbourhoods most in need any time soon, they’re part of a new way of thinking in L.A. that argues the time for action is now.

“We definitely have the ability to change the urban temperature in the city,” Fink says. “And I think we will.”

But there is still a lot of uncertainty about how that may happen. Even if the city decides to budget more money to expand the cool pavements programme, the cost of going from a few small streets to swaths of the city is unknown. And even if every homeowner adopted the seven free trees L.A. is offering, another drought could be right around the corner. Much can change—economically, politically, socially—between now and 2035. What’s certain is that as temperatures rise and extreme heat days add up, achieving that three degree drop will only become harder.


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