Fog is a defining element of summer in Santa Cruz, obscuring the view of day trippers descending the hills to the coast and prompting kids to bundle up to hop on their bikes for summer adventures. Its fingerprints are visible in the vast coastal forests, even when it isn’t hanging in the air. The redwood trees towering in a clear blue sky soak up moisture from the fog on gray days. It is often their only source of water for months at a time.

Fog is essential for plants and animals, agriculture and human health, not only in California but in coastal zones around the world. But many scientists believe that fog is declining, another casualty of global warming.

Throughout California and the Western United States, some shifts driven by climate change are easy to see: increasing temperatures, shifting wind patterns and changing ocean currents. Fog trends, on the other hand, are as hard to grasp as the mist itself.  

“It’s an incredibly sensitive point between water being vapor, and water being liquid,” said Alicia Torregrosa, a project scientist at the U.S. Geological Survey and director of the Pacific Coastal Fog Project. “And that transformation, that transition, [is] driven by more things than you can even count.”

Fog’s most noticeable human impact is on transportation, because delays to shipping, road traffic, air travel and freight cost billions of dollars per year. Unusually persistent fog can blight important food crops, but it is beneficial to others, including the huge fields of artichokes and pumpkins along the Central Coast of California that thrive in just the right combination of light, temperature and moisture delivered, in large part, by coastal fog belts. And water harvested from fog is becoming increasingly important for people in arid, subtropical areas that are drying out with global warming.

Yet evidence is starting to mount supporting what has for years been anecdotally reported by shoreline residents and scientists—fog is vanishing from many shoreline zones, including the California coast and Santa Cruz region. While the reasons for the decline are hard to decipher, solving the mystery is crucial to understanding the future of fog and whether it will continue to grace our coastlines, nourish crops and sustain the redwoods. 

‘I’m Alarmed by What the Future Looks Like’

California has already warmed between 1 and 2 degrees Fahrenheit since 1900. Even if greenhouse gas emissions slow dramatically, the state’s most recent climate assessment projects that by 2080, Santa Cruz will have a climate like Goleta, 200 miles to the south, which is currently 2.3 degrees Fahrenheit warmer and 31 percent drier. 

In a 2010 paper, Todd Dawson, a professor of integrative biology at the University of California, Berkeley, and James A. Johnstone, a graduate student, collected airport flight-control records along the entire West Coast, from Baja to British Columbia, dating back to 1951. Their analysis, the most comprehensive data on fog patterns along the Pacific coast, showed that on average, fog has decreased about 30 percent in the past 60 years.

“In the ‘50s, coastal California, Santa Cruz included, was getting about 12 hours of fog every fog season, per day,” Dawson said. “Now it’s gone to about nine hours a day. So, shorter duration per day, and also per season—the fog season is actually shorter, too.” Historically, the fog season would last from June to October, but more recently, Dawson said, it often doesn’t start until July.

Dawson, whose research largely focuses on coastal redwoods, said he’s already seeing the impacts of lessened fog on the ancient giants. Their range will gradually shrink, he said, as areas on the edge of the misty zone, like Big Sur, become too warm and dry, and fog disappears. Small redwood trees are already dying there, and larger trees are showing signs of distress.

“Growth is compromised in some of the older trees,” he said. “They’re not growing as much each year, they’re thinning their canopies.” 

Dawson added, “That’s kind of a telltale sign that the southern end is drying out to a point where eventually the redwoods are not going to be able to sustain themselves; they’re just not going to make it.”

Whether that happens in 10 years or 100 years is uncertain, but the change is obvious. “I’ve been studying redwoods for 25 years, and I’ve seen it happen over the time that I’ve been studying them,” Dawson said. “I’m alarmed by what the future looks like.”

Not all scientists are convinced, however, that the 30 percent decrease Dawson documented will continue.  

“I think that the past may not be a really great predictor of the future,” Torregrosa said. “The 1951 data point is the highest level of fog, ever, in any of the measurements. And so if you [have] this outrageously high point, it’s going to change all your statistics.”

She added, “It’s not even clear that all the observed decrease was due to climate change.”

For example, fog decreasing over urban areas can sometimes be attributed to the warming effect of paved surfaces.

But a slight increase in California fog from 2012 through 2016 could be related to extreme heating of parched land areas near the ocean or increased pollution during that time, Dawson said.

Fog is “really difficult to predict for the very reason that climate is hard to predict; because it’s not just driven by one factor,” he said. Instead, it is “driven by all kinds of interacting factors—land surface, ocean surface, how much heating we get, what do the winds do, how fast is the land heating up compared to the ocean, is the cold water upwelling strong or weak.”

High Stakes for Santa Cruz

Redwood trees are perhaps the most famous fog-dependent Santa Cruz residents, but they’re far from the only ones. In the city’s 2019 integrated regional water management plan, officials acknowledged, “The future of coastal fog under climate change remains uncertain.” The mere inclusion of fog in the plan highlighted the importance of finding an answer.

The authors of the Santa Cruz report concluded that a decline in coastal fog would put additional demand on scarce water supplies in the region. Plants in the coastal zone can rely on fog for up to a third of their water supply, and it’s often the only source of water that prevents streams from drying up in the summer. 

Fog isn’t a matter of life and death for only plants. Losing fog can be deadly for people, too. If coastal fog continues to decrease dramatically, Torregrosa said, she is most worried about the impacts on human health, which were underlined for her during a severe heat wave in Santa Cruz and the Bay Area in 2006.

“There was about a week in the middle of the summer, when we usually get fog, where it just stopped,” she said. “It hit triple digits in most of the inland counties, and also got really, really hot in the coastal counties.” 

Although it didn’t get as hot on the coast, a subsequent study that analyzed excess heat deaths regionally showed a disproportionately high number in the coastal zones. 

“The speculation is that people didn’t know how to handle [the heat],” Torregrosa said. They went to the beach and sat in the sun, for example, or their homes didn’t have air conditioning and they didn’t know to take refuge in places like libraries or shopping malls.

Iconic Torrey Pines Also at Risk

The plight of Santa Cruz’s redwoods is echoed several hundred miles to the south with the rare Torrey pines of San Diego County. The area has warmed by about 3.4 degrees Fahrenheit since the late 1800s, with perhaps 5 more degrees predicted by 2100.

When fog rolls in, the trees’ long, fine needles trap some of the moisture, which drips down to moisten the ground, helping seedlings take root.

The long-term decline of fog at the Torrey Pines Natural Reserve seems obvious to environmental scientist Darren Smith, a California State Parks biologist who started working at the reserve 21 years ago.

“Fog is real important to their ecology,” he said. “I don’t think they’d do well with less fog. Fog is an essential contributor to the moisture the Torrey pines need for survival.”

He added that the trees are not completely dependent on fog, but “it is real critical in the climate, where rain amounts can be unpredictable and not always be enough to sustain the tree.”

Cleaner Air Reducing Fog

The context of pollution and increases in farming and development is crucial in examining changes in the formation and persistence of fog of all different types during the past 50 years, said Otto Klemm, with the Institute of Landscape Ecology at the University of Münster in Germany.

Klemm started studying fog in the late 1970s, near what was then the border between East and West Germany, and said he found “unbelievably high levels of pollution” that he attributed to factories and coal power plants in East Germany and Czechoslovakia emitting large amounts of sulphur dioxide.

By about 2009, data from weather stations around the world started suggesting a widespread fog decline, which Klemm said prompted him to do a global study. The results, published in 2016, showed a decrease of fog at hundreds of stations worldwide. 

Data from individual sites, including in São Paolo, Brazil, and Taiwan, showed that “the fog decline is correlated with both falling pollution levels and rising temperatures,” Klemm said. A temperature increase of 0.18 degrees Fahrenheit has about the same effect on fog as cutting aerosol pollution by 10 percent, the study showed.

Around the same time that average global temperature started rising steeply in the 1980s and 1990s, many countries started dealing with a more obvious environmental problem, air pollution, by enforcing air quality laws. Pollution declined in many places, and so did fog.

Research out of UC Berkeley in 2019 showed that a 30-year decline of tule fog, which can close schools and stop traffic in California’s Central Valley during the winter, correlated with declines in pollution. There is no reason to think that pollution levels don’t also play a role in the formation and persistence of coastal fog formation in places like Santa Cruz, Klemm said. The trick is finding enough data to reveal the trend, which has proved difficult for a variety of reasons. 

“You would have to analyze those locations in a long time series, and good air pollution data records don’t go back very far, maybe to the 1980s,” he said. “We don’t have good data for the days of highest pollution levels, and measurements were taken differently then.” 

But the data that can be analyzed show some startling numbers. In São Paulo, the number of foggy days has halved—from more than 144 per year in the early 1950s to just over 70 today. In Los Angeles during the same period, the number of foggy days per year dropped from 200 to 30.

Based on the numbers, it’s possible to infer that human emissions made parts of the world, at least around industrial centers, foggier. Subsequent measures to clean up air pollution are now reducing fog. And since fog loaded with industrial pollution is more reflective than clean haze, the reduction in smog allows more warming, he said. That, in turn, can diminish fog even more.

Fog Connects Ecosystems

Long before scientists started tracking fog, many societies recognized its importance to ecosystems and humans, said Kathleen Weathers, an ecologist specializing in fog research with the Cary Institute of Ecosystem Studies in New York.

“There are fog belts around the world and many local cultures have names for fog and the fog season,” she said. “It is the reason, in many cases, that plants and animals can survive.” 

Icy fog that forms in some high valleys of the Western United States, for example, is known as pogonip, a word thought to originate with the Shoshone people who lived in the region. Pogonip—a word Santa Cruzans will recognize from the eponymous open space bordering UC Santa Cruz—forms only in conditions cold enough to threaten respiratory health, and was thus feared by Indigenous people and settlers.

Weathers’ research in the Namib Desert of Southwest Africa, where fog is the main source of moisture in some ecosystems, documents how some animals interact with fog.

“There are beetles standing on their head doing yoga in the morning to collect fog on their legs to have that drip down so they can drink it later,” she said. Engineers try to mimic the hydrophilic exoskeleton of the vapor-harvesting insects to build roofs that can collect water from fog for people to use. 

Fog that Weathers studied along Chile’s rugged coast might transport fertilizer from the ocean to forests, she said. Cold water from the deep ocean that rises to the surface in upwelling areas often carries nutrients to the foamy wave tops, where they form the nuclei for droplets of fog droplets to condense around before it floats over land.

“We always think about things flowing from land to estuary to ocean, and this is completely opposite,” she said. “That is one of those connections that is kind of mind-blowing. You can pose the question, ‘Is the ocean feeding the forest? And what’s the likelihood that there is anything living in the fog droplets?’”

Weathers pointed to ocean bacteria found in the mountains of Italy, and viruses and other organisms that are transported by fog droplets from the sea onto land. “It really is an exchange system,” she said.

The climate in places like California, Chile and the Namib Desert share traits that shape their coastal fog, like sharp contrasts between sea and land temperatures, strong coastal currents in the oceans and regional wind shifts. 

As greenhouse gases trap more of the sun’s heat on the planet’s surface, research shows that key ocean currents are shifting poleward and the tropics and subtropics are expanding, which shifts the position and direction of winds that affect fog formation and persistence. That combination of factors threatens ecosystems that are most dependent on fog, including the biodiverse mountain forests of Mexico, Costa Rica and Hawaii.

These forests might be the most vulnerable because rising temperatures could quickly push their fog belt upward, Klemm said, “exposing sensitive cloud forests with many species that can only survive in that zone. Some plant communities are so specialized they wouldn’t survive in their present form without fog.”

During the coming century, residents and visitors will see how much Santa Cruz’s iconic, towering redwoods fit into that category.