The Elusive Future of Coastal Fog
SAN FRANCISCO, California — It was the first morning of summer, the start of fog season. But the sky above the Golden Gate Bridge remained clear and blue.
Chris Dzierman, a bridge painter and foreperson, looked to the west. Near the horizon, a thick fog bank lurked. He wondered if and when it would roll in, smothering the bridge and beyond in wind and whiteness.
“It could last three minutes or three hours,” Mr. Dzierman said. “It’s fog. It’s got a mind of its own.”
Every summer, fog breathes life into the San Francisco Bay Area. But people who pay attention to its finer points, from scientists to real estate agents to bridge painters, debate whether there is less fog than there used to be, as science and general sentiment suggest.
The ecological, economic and social effects of fog are profound, perhaps no more so than in Northern California. Changes would be life-altering. But understanding fog is one of science’s toughest tricks. Quantifying the changes and determining possible causes, including global warming, is like chasing ghosts.
While coastal fog is not unique to California, few places in the world are so deeply associated with it. Fog pours through the Golden Gate and crawls up and down the hills of the city and the coast.
Fog is why one neighborhood is chilly, another is surprisingly sunny and the airport is where it is. It is why real estate agents talk about neighborhood fog patterns as much as home size and schools.
Summer fog is why the mighty coastal redwoods grow where they do, surviving California’s dry season thanks
to refreshing gulps of cold, wet air. It is why, until recently, few people worried about wildfires along the coast.
In June, July and August, the average daily high temperature in San Francisco is about 21 degrees Celsius, coolest of any major city in the continental United States.
Fog nourishes the natural world. It enriches the area’s cultural identity. It might even be a resource in California’s growing anxiety over water.
Which is why a decrease in California’s coastal fog, or the prospect of it disappearing entirely one day, is not a sunny proposition, particularly in and around San Francisco Bay.
The general consensus among the small cadre of scientists who study coastal fog is that it is decreasing, not just in California, but around the world. However, the reasons are not clear.
Early on a late spring morning, Peter Weiss, a scientist at the University of California, Santa Cruz, and three of his students were erecting fog catchers.
The idea is to harvest water from passing fog. A fog catcher is a stretched piece of mesh that, when it works, becomes saturated by fog moisture, dripping into a gutter, leading to a cistern.
Fog from the ocean is a dependable feature in several places around the globe, mostly on the west coasts of major continents. Villages in places like Peru and Chile, sometimes with almost no rain throughout the year, have for centuries sustained themselves largely on fog water. Its use is growing in places like Morocco.
The question in a changing climate is whether fog water is a viable resource, if not a solution, for places expecting a drier future.
“I would tend to think of it as a small drop in the bucket,” said Dan Fernandez, a professor in the Department of Applied Environmental Science at California State University, Monterey Bay, and a leading researcher in fog-catching. “But we need a lot of small drops in the bucket to deal with what we have coming.”
Todd Dawson, a professor of integrative biology at the University of California, Berkeley, has found that 30 to 40 percent of redwoods’ annual moisture arrives in the form of fog. In 2010, Dr. Dawson and a graduate student, James Johnstone, published a study that used observational data at airports in the coastal redwood region — from California’s central coast to its northern border, including the Bay Area. The researchers found that the frequency of fog, measured by fog hours per day, had dropped 33 percent since the middle of the 20th century.
The ramifications extend far beyond a species of tree. Does the longterm survival of redwoods and their ecosystems depend on fog? What about fog-cooled agricultural areas, like the Salinas Valley to the south of the Bay Area?
What would happen to water use, power grids, solar farms and wind
turbines? How would the future migrations of human populations be affected? And would San Francisco be, well, San Francisco without fog?
“Less fog is a game changer for a lot of things,” Dr. Dawson said.
For fog catchers, results are highly variable. The most that Dr. Fernandez has collected from a single catcher was 37 liters of water in a day. Any location that dependably captures an average of more than a liter a day during the summer, he said, might be worth expanding.
“Fog is great,” Dr. Fernandez said. “It’s not rain. But it’s something.”
Maybe fog will never be enough to satisfy the needs of city dwellers. But could a farm that gets no rain in the summer, and uses no outside irrigation, survive on fog? Maybe, depending on the crop and the fickleness of the area’s fog.
Someday, Dr. Fernandez and Dr. Weiss say, the foggy parts of California might be covered in fog farms, not unlike the solar farms found in sunnier spots. That depends on many factors, not least the future existence of fog.
California’s coastal fog is the result of sprawling atmospheric and oceanic phenomena, a delicate balance of powerful forces. As the world warms, this balance could be at risk, with unpredictable consequences.
“What I want to know is, do we expect more or less clouds with climate change?” said Rachel Clemesha, who studies coastal clouds at Scripps Institution of Oceanography at the University of California, San Diego. “I still don’t know enough to say confidently. It’s complicated.”
Otto Klemm, a professor of climatology at the University of Münster in Germany, is more certain. He has studied data from airports all over the world.
“Fog has decreased, more or less everywhere,” he said, attributing the link to climate change and to lower levels of air pollution, as water droplets have fewer particulates to cling to. “Of about 1,000 stations, 600 or 700 show a statistically significant decrease. All over Europe, all over North America, South America — everywhere.”
As for California, research can be contradictory or counterintuitive. A 2017 study using observational records of ships off the California coast suggests that fog is heavier than it used to be. So maybe it is just not coming onshore the way it once was.
In Southern California, during the periods colloquially called “May
Gray” and “June Gloom,” research shows an increase in cloud-base height — low clouds persist, but are now less likely to be in the form of ground-touching fog — because of the urban heat-island effect. Some experts surmise something similar is happening in San Francisco.
Travis O’Brien, an assistant professor of earth and atmospheric sciences at Indiana University, updated data from California coastal airport observations and found that the declining fog levels in the second half of the 20th century have appeared to stabilize. Other researchers, using satellite technology, concluded recently “that the number of foggy days fluctuates considerably yearto-year with no discernible positive or negative trend occurring between 2000 and 2020.”
Few people have spent more time living and working in the fog than Toby Kanzawa, who is a gardener at Golden Gate Park. He firmly believes there is less fog now.
“It’s something we always talk about,” he said. “We’ve noticed it out here.”
Golden Gate Park stretches almost five kilometers from the Pacific Ocean to the central part of San Francisco. Signs of fog’s influence are sprinkled around the park, even when the fog is nowhere to be seen. The Kwanzan cherry tree in front of the Japanese Tea Garden is twisted and tilted from the wind and fog that whips from the west. The azaleas, rhododendrons and magnolias sometimes emerge from foggy summer and bloom again in San Francisco’s sunny fall.
Just as the gardeners see the silent signs of fog, some see signs of it dissipating. Mr. Kanzawa sees it in the once-empty, fog-soaked portions of the park now crowded with picnickers. In the westside roads that were dangerous, if not impassable, on summer nights.
“It would get super thick,” Mr. Kanzawa said. “And now that’s a rarity. I haven’t seen it like that in a long time.”