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Is LA losing its marine layer?

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New study finds that Southern California’s cloud cover has dissipated by as much as 50 percent since 1970

Stratus clouds are burning off an average of two hours earlier compared to 40 years ago.

There’s a new culprit in Southern California’s fire season: clouds. Or more precisely, the lack of them.

Clouds across Southern California’s coastal region are disappearing at an alarming rate, according to a new study published in the journal Geophysical Research Letters.

Since the 1970s, 25 to 50 percent of clouds have dispersed, the study found. With fewer clouds, comes drier land—and that could be a significant factor in Southern California’s extended fire season.

Park Williams, an associate research professor at Columbia University’s earth institute and the study’s lead author, says thinning cloud cover is a frontrunner in the factors causing wildfires during summer in Southern California.

“The reason is pretty straightforward,” he says. “The clouds provide shade during the day. A cloudier summer provides more shade and that allows [vegetation] to hold onto moisture for longer. If you reduce clouds, then you should be inherently increasing the potential for fire.”

The clouds in question are stratus clouds—what Southern California natives usually refer to as the marine layer, but which are actually just a symptom of the dense layer of cool air. They’re the quintessential clouds that roll in at night, blanket the basin at dawn, and then burn off later in the afternoon to reveal a bright blue LA sky.

There are other reasons why California is so susceptible to wildfires, including climate change and intensifying dry and wet season cycles. Williams’s study aimed to quantify how much blame can be placed on disappearing clouds.

It was successful in identifying that usually low-hanging stratus clouds have elevated an average of 150 to 300 feet since the 1970s due to rising ground temperatures.

For a cloud to form, temperatures need to be cool; the warmer the ground, the higher the clouds have to be off the ground to form. Because the clouds are higher up in the sky, they burn off an average of two hours earlier than 40 or so years ago, the study found.

But according to climate scientist Daniel Swain, there are many reasons for Southern California’s aggressive fire season, and many of those reasons are highly nuanced. While this latest study draws needed attention to the dispersal of cloud cover, Swain says it doesn’t directly link fewer clouds to more fires across all of California.

“I think it’s important to contextualize these phenomena. It’s affecting a pretty small zone—a strip along the coast that’s about 5 to 10 miles wide,” he says. “But in these very specific regions, it may be increasing early season fire risk to some extent, because these places are typically shrouded in fog most mornings.”

Without the fog, fire risk is undoubtedly higher. While Los Angeles has had a fairly heavy season of May Gray, it’s what happens when the gloom dissipates that matters.

“As I’m looking out my window right now, I see a giant plume of smoke,” Swain said Tuesday, referring to a brush fire that broke out that afternoon in Benedict Canyon. “Look, today’s one of the few days that we didn’t have [the marine layer], and look what’s happened.”

La Tuna Canyon Fire Prompts Evacuations in Burbank
Firefighters use hand tools in chaparral brush to fight the La Tuna Fire in September 2017. LA’s thinning cloud cover is one of the factors causing wildfires during the summer in Southern California.
Photo by David McNew/Getty Images

In the case of last summer, it wasn’t fewer clouds that led to the most devastating fire season in California’s history. Instead, it was a deadly combination of excess rain growing an abundance of vegetation, drought-like conditions drying that vegetation into perfect fuel, and powerful winds fanning the flames.

It’s specifically in the summer months that LA sees a more direct correlation between fewer clouds and more fires, largely because fewer clouds means more exposed land and drier vegetation.

It doesn’t help that LA is constantly expanding. The city’s sprawl raises temperatures across the region. This effect, in which urban areas see a 2 to 6 degree increase in temperature compared to rural surroundings during the day, and up to 22 degrees at night, is known as the heat island effect.

According to Williams, that effect largely irreversible.

“There will still be some cloudy years like 2010, but if 2010 had occured back in the 1960s, it would’ve been a monster year, and that’s because the entire baseline has shifted,” he says. “I don’t see any way to get back to that just because what’s built is built. It’s doesn’t seem like LA would go backwards on urbanizing.”

Things may never return to the dense cloud cover of the 1960s, but there are ways to lessen the effects of urbanization on Southern California’s climate.

For one, says Williams, as LA expands, developers can use reflective colors on the ground—materials that are not black like asphalt—to reflect sunlight and keep the city cooler. Painting rooftops white, keeping LA’s green spaces natural and development-free, and planting more trees will also promote cloud cover, and hopefully shorten LA’s fire season to a more manageable—and less deadly and less costly—length.