L.A.’s mayor wants to lower the city’s temperature. These scientists are figuring out how to do it

oak up these rainy days, Southern California. They are not going to last forever.

Summer will be here before you know it, and if recent trends continue, it will likely be a hot one.

Globally, 2016 was the warmest year on record. Here in Los Angeles, temperature records were shattered last summer during scorching heat waves that saw highs of 100 degrees for five days straight.

If you think the city is too hot, you’ve got company at City Hall. Los Angeles Mayor Eric Garcetti agrees, and he wants to do something about it.

As part of a sweeping plan to help L.A. live within its environmental means, Garcetti has pledged to reduce the average temperature in the metropolis by 3 degrees over the next 20 years.

It’s a noble goal. Not only will it make you more comfortable, it will reduce energy consumption and improve air quality. It may even save lives — extreme heat kills more people each year than hurricanes, floods or tornadoes.

But how do you turn down the thermostat of an entire city in a warming world? And in a place as vast, sprawling and heterogeneous as Los Angeles, how do you measure success?

Fewer hot days

Professor George Ban-Weiss and his team at USC’s department of civil and environmental engineering performed model simulations that quantify how temperatures can vary across Southern California on a typical July day. The sea breeze drives temperature increases farther from the ocean, but localized heat and cool islands occur due to differences in land cover.

Los Angeles and neighboring cities

These questions have never been more relevant. L.A.’s heat problem is expected to worsen over the coming decades.

Climate models suggest that by 2050, the temperature in downtown L.A. will exceed 95 degrees 22 days per year. In 1990, only six days were that warm. The San Fernando Valley is expected to see 92 days of this  extreme heat per year, compared with 54 in 1990.

Climate change is primarily responsible for the warming trend, but it’s not the only force at work. Angelenos are also contending with an additional layer of misery caused by what’s known as the “urban heat island effect.” It means that cities — with their asphalt streets, dark roofs, sparse vegetation and car-clogged roads — are almost always a few degrees warmer than the more rural areas that surround them. 

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“ There is all this variation across the city. You can’t get a richer place to study climate and meteorology.

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— George Ban-Weiss, professor of environmental engineering at USC.

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The mayor’s plan to cool the region won’t compensate for all the effects of climate change.

“We can’t geoengineer the atmosphere,” said Matt Petersen, chief sustainability officer for the office of the mayor.

But Petersen believes we can do something about the way the city traps heat. By counteracting this heat island effect, he hopes to reduce the amount of warming L.A. will experience in the future. 

In early July, Petersen’s team convened a group of about 20 civil servants and university scientists to determine how to bring the city’s temperature more in line with what it would have been if Los Angeles had never been developed.

“What we are trying to do is create a research collective to help us reach our target,” Petersen said. “It’s a huge challenge.”

The city has already teamed up with USC environmental engineer George Ban-Weiss. A veteran of the Lawrence Berkeley National Laboratory’s Heat Island Group, he said there is no better place to test different ways of reducing urban heat than L.A. 

“There is all this variation across the city,” Ban-Weiss said. “You can’t get a richer place to study climate and meteorology.” 

The built environment is mostly responsible for the problem. More than half of city surfaces are covered by dark pavements and dark roofs. Traditional asphalt absorbs up to 90% of the sun’s radiation. As the asphalt gets hotter, it warms the air around it, adding to the overall heat. Even after the sun goes down, that accumulated heat lingersfor hours and continues to transfer warmth to the night air. 

One way to combat this heat sink is to replace the city’s streets and sidewalks with high-tech materials that reflect more sunlight and stay cooler during the day and at night. Some of these “cool pavements” reflect light only in the infrared part of the spectrum, which we cannot see. 

In the summer of 2015, the city’s Bureau of Street Surfaces tested one of these cool pavements at the Balboa Sports Complex parking lot in Encino. The new surface was approximately 11 degrees cooler than regular pavement in the mid-afternoon.

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