Southern California is home to tens of millions of people, and Los Angeles, with a population of almost 4 million, is the second largest city in the U.S. A new study published in Geophysical Research Letters (GRL), a journal of the American Geophysical Union, has found that urbanization around coastal Southern California is affecting the region’s clouds and fog. This trend has important implications for ecosystems and cities.
As cities grow, adding roads, buildings, industry and people, temperatures rise relative to the outlying suburbs and rural areas, and create the “heat island effect.” Concrete and asphalt absorb heat during the day and retain it at night. As the cities of coastal southern California get warmer (At the most urbanized sites, nighttime temperatures have risen at a rate of nearly 1˚F hotter every decade.), they are driving fog away and causing the low stratus clouds, crucial for providing shade and moderating temperatures in summer, to rise.
Researchers at the Earth Institute’s Lamont-Doherty Earth Observatory, Scripps Institution of Oceanography, NASA Goddard Institute for Space Studies, Oregon State University and University of California at Santa Barbara studied the frequency of summer low clouds and fog from Santa Barbara to San Diego by evaluating records collected at 24 airfields from 1948 to 2014. They found that the frequency of fog in the Los Angeles Basin had decreased 63 percent, and that the altitude of cloud-bases had risen at airfields in the most urbanized areas, with L.A. and San Diego showing the most cloud-base rise and urban warming. Ontario, 40 miles east of L.A., had an 87 percent decrease in fog. Two Channel Islands, which are undeveloped, actually had more fog, which suggests that in the absence of urbanization, coastal southern California might have gotten foggier.
Park Williams, lead author of the study and assistant research professor at Lamont-Doherty, explained that low marine clouds around Los Angeles and San Diego form fog in the mountains. In summer, this fog is critical to the ecosystem because it delivers water and provides shade to an otherwise dry area. California is already in its fourth year of severe drought, which last year resulted in 1,000 more wildfires than usual. With less fog, the already flammable chaparral mountain ecosystems are likely to become even more combustible. The National Interagency Fire Center is already predicting above normal potential for significant wildfires across part of Southern California in May and June.
Fewer clouds overall also allow more sunlight to reach the earth, creating feedback as the surface warms and pushes clouds even higher. The increased sunlight makes cities hotter, creating more intense and longer heat waves and more demand for energy and water.
Clouds are usually closest to the ground at night and early morning, but the heat island effect, once a mainly nighttime phenomenon, now persists into early morning, driving clouds higher and reducing fog.
Since urban warming is happening globally, the new findings are relevant to other cities as well. “Clouds are important globally for local climate. Low marine clouds sit over the eastern portions of subtropical ocean areas around the world,” said Williams. “So cities on the South American west coast, the Mediterranean coast, the South African west coast and the Australian west coast will likely be affected by this trend too.”
The findings are good news for people who dislike clouds in the summer and find fog a nuisance. Fog often makes driving difficult and delays air traffic, so transportation will be safer with less of it. Fewer clouds will also make solar energy more efficient.
But wildfires and heat waves may spur people to try to slow the effects of urbanization and the decline in cloudiness. “Planting urban trees and creating green roofs could help prevent the urban heat island effect from growing,” said Williams. “And in the long term, keeping areas unurbanized, such as Oxnard, Ventura and Camarillo between Santa Barbara and Los Angeles, or Camp Pendleton south of Los Angeles, where the urban effect hasn’t yet taken hold, would help.”
Williams, who attended school in Southern California, is saddened by the loss of fog. “I always loved these clouds—I spent time in the pine forests on the Channel Islands. In summer, the forest would be dripping wet because of the fog. I liked the eeriness of the light and the scenery when the fog set in.” he said.
“When everything was cooler and wetter, the tree species in the sparse forests on the southern California coast used to live up and down the coast to Baja. They all died off except in these little forest pockets, which suffer when drought occurs. Today they are on the edge of survival—the fog is allowing them to hang on.”
With ongoing global warming and a growing population, urbanization and the decrease in fog and cloudiness in the southern California region will likely continue in coming decades.