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SAN FRANCISCO – As most of the U.S. sweltered in mid-July 2022 − when temperatures in many major cities reached the high 90s and even triple digits − a national weather map showed San Francisco topping out at 65 degrees.
It was just a typical foggy summer day in the city by the bay, which averaged 62 degrees that month, about the same as the next two Julys.
Now the advent of climate change raises the question of whether summertime visitors will stop rushing out to buy sweatshirts upon arrival and instead feel perfectly comfortable in shorts and T-shirts.
The future of San Francisco’s iconic fog has been debated in media stories during recent years, and some experts note a diminished cloud cover along the California coast that could lead to a warming trend.
But few if any detect signs that San Francisco’s summer chill is going away like the once-celebrated Fog City Diner, which shut down at the end of May.
“From the data, I can’t foresee it any time soon,’’ said Rachel Clemesha, a project scientist with the Scripps Institution of Oceanography at the University of California-San Diego who studies the state’s coastal climate. “There are years when there’s more or less cloud cover. The last couple of years have been within that range. It is a very foggy place, so it would be very dramatic to get you a fog-free city.’’
Data on decreasing fog along coast is ‘spotty’
Clemesha said some decrease has been confirmed in Southern California in what residents there call the marine layer, mostly in highly urbanized areas, but nothing that applies statewide.
Peter Weiss, a faculty researcher and lecturer at the UC-Santa Cruz department of environmental sciences, said that despite a growing narrative of waning fog along the California coast, the data to support it is “very spotty,’’ with few academic studies in the last decade.
The reasons include the fog’s unpredictability – Weiss calls it an “ephemeral phenomenon’’ – and the lack of a standard way to measure it. Some studies, such as the landmark analysis by James Johnstone and Todd Dawson in 2010, rely on airport visibility records. Others use satellite images to determine the extent of the cloud cover, and others yet believe water content is a more valuable gauge.
The airport records are the most extensive, going back to 1950, and Weiss said from that year until 2012 they revealed a 5% decrease in fogginess.
“Nobody’s quite sure why,’’ he said. “It probably has to do with the ocean’s sea-surface temperature, and that goes through various phases. Overall, there’s warming due to global warming, but it’s episodic. There appears to be some pattern with less fog after the warmer sea-surface temperatures, but this is still an area of research.’’
Many California residents, ecosystems would be affected
While the scientific community endeavors to figure out the long-range impact of climate change on California’s coastal fog, there’s a strong consensus that diminished cloud cover would have a harmful effect.
Nearly 27 million of California’s 39 million residents – close to 70% – live in coastal counties, by far the largest total in the nation, and they generate 80% of the state’s gross domestic product. Their lives are certain to be impacted, as would be the state’s powerhouse agriculture industry, which totals close to $60 billion a year in revenue.
Species such as the widely admired coastal redwoods, which get up to 40% of their yearly water intake from fog, could be threatened if that resource dwindled.
Daniel Fernandez, an environmental sciences professor at Cal State Monterey Bay, is part of a group seeking a grant from a private foundation to study how climate change may alter fog and affect various ecosystems.
“You could have significant die-off of species that are dependent on the fog at locations where it gets reduced,’’ he said. “It would also change how people live. When you look a fog zones, you don’t need air conditioning. There are a lot of things we don’t need that we take for granted. Those things could all change.’’
It can feel like living in a cloud
The fog, more prevalent in the summer, is created when warm and moist air sweeps over cold waters, which are churned off the California coast by strong winds in what’s known as upwelling. The marine cover can be light enough to simply cool down a warm day and thick enough to wet residents’ hair and obscure their eyeglasses, giving the impression they’re living in a cloud.
Some years the fog is thicker than others, but it tends to be more extensive in Northern California than the state’s southern coast because of the differences in their ocean temperature (colder in the north), latitude and topography.
Ian Faloona, a professor of land, air and water resources at UC Davis, said he and a colleague conducted a study using regional climate models and found a downward trend in cloudiness along the coast, but agrees the overall evidence “is not going to hit you over the head.’’
He compares that to the abundant data indicating California in general is warming quickly, about 1 degree Celsius – 1.8 degrees Fahrenheit – per decade. Even San Francisco is heating up a bit, though not nearly as fast: Its average summer temperature has risen by 1.6 degrees Fahrenheit since 1970, according to the independent research group Climate Central.
Two different perspectives on what the future holds
But climate scientists are split over whether the increased heat will lead to less fog because the air over the ocean won’t be cold enough to condense, or whether stronger winds will atone for that factor.
“Under climate change, we know the land is warming much faster than the ocean, so that temperature difference across the land and ocean interface is increasing, which could drive stronger winds, which could help preserve this cloudiness,’’ Faloona said. “So there are two arguments you could make about what we theoretically think should happen, and which one’s winning out I think is still an open scientific question.’’
Sara Baguskas, an assistant professor at San Francisco State University with a specialty in coastal fog, said the lack of conclusive evidence that it’s ebbing should not induce complacency but rather stimulate funding to study and predict its patterns.
She’s among the climate researchers who have heard from longtime coastal residents saying the marine layer has subsided over the years.
“So it’s not unreasonable to be concerned about coastal fog declining in the future, but it diminishing completely is unlikely,’’ she said. “No coastal fog in California is a scary thought for both people and ecosystems.’’
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