Fog is gradually disappearing along the coast due to climate change. Here’s why that matters

More hot days are surely still to come this summer and fall, parching yards and forests and drawing visitors out to the Pacific coast for relief.

By midsummer, nearly every available seaside camping space in Sonoma County had been reserved through August, and for good reason. On a 70-mile drive west to the Sonoma Coast at the height of summer, you might experience a temperature drop of 30 to 40 degrees. At Bodega Bay, summer overnight temperatures dip into the low 50s, with highs rarely out of the 60s.

While the state’s interior swelters, coastside summer visitors stay wrapped in hoodies and sweaters, bundled against chill winds and overcast mornings.

The reason for this extreme air conditioning can be traced to a natural, ancient feature, and its source comes swirling in every night from the ocean: dense gray summer sea fog.

Those drifting banks of damp gray are more than heat relief. They’re also a vital lifeline for millions of other California residents. For thirsty plants and animals all along the coast, they are a last reserve, the only source of water for up to seven months of the year.

But researchers have noted, over roughly the last decade, a gradual decrease in the amount of fog along the coast. What does that mean for these ecosystems, and for us?

(New York Times: What climate change could mean for fog in the Bay Area)

Grounded clouds

Walking in fog is really strolling through cloud.

The grounded cloud we call fog is made, like the puffy white ones higher up, of tiny water droplets, each on average about four times thinner than a human hair. As the density of droplets grows, visibility is reduced like an eraser. For ships at sea, that’s a mortal hazard. For nearly two centuries, shipwrecks and sinkings along California’s north coast were tragically common, as unwary captains sailed onto jagged shoreline rocks hidden in fog.

Today, the warning foghorn at the Bodega Bay jetty is loud enough to be heard miles offshore, and to the dismay of every Doran Beach camper, it does its job, blaring on and off the whole night long.

California’s dense sea fogs arise from a summertime combination of oceanic and atmospheric conditions. Steady stiff winds from the north stir the ocean and draw very cold water from the deeps to the surface near the coastline. When the sun-warmed moist air above contacts this much cooler surface, the moisture condenses into droplets, forming fog.

Trapped beneath a high-pressure layer overhead, a fog bank can be compressed into a solid overcast so thick it can take hours of sun or wind to disperse. In summer, these marine layers can linger for days but usually burn off by late morning.

What may not be obvious to the fogbound observer is that this fog, drifting inland through valleys and forests, carries a tremendous amount of water. Depending on its density, United States Geological Service studies suggest, a single cubic mile of California fog may contain tens of thousands of gallons of water.

On a summer day, millions of tons of suspended water can be drawn ashore around the Bay Area alone, a National Parks Conservancy article estimated.

Over millions of years, California’s diverse coastal life has adapted to, and now depends on, these seasonal fogs for life-giving summer moisture.

Water for trees, plants

UC Berkeley plant biologist Todd Dawson studies fog and how plants manage to wring water from it. Among the largest fog-dependent creatures he’s investigated, the giant sequoia sempervirens, or coast redwood, is a water lover, with each tree drinking as much as 160 gallons a day.

Studies have found that up to 40% of the water the redwoods need may come from fog. During the long months between rains, the shroud of moisture from the sea can be a redwood’s only water source.

Not coincidentally, coast redwood forests today only grow as far inland as the summer fogs can reach.

Unlike rain, the water in fog is sparsely distributed in midair, far from the redwoods’ roots. The trees have developed ingenious ways to take a drink. For example, the millions of flat waxy needles of a mature redwood can catch and condense the tiny droplets as they drift by, creating a steady drip — redwood rain — around the tree. The old needles that collect beneath the trees decay into duff, which is spongy and absorbent.

Recently, another researcher, Alana Chin with UC Davis Plant Sciences, confirmed a long-held suspicion that redwood trees might be able to obtain water directly from fog. Her group discovered coast redwoods grow a second type of leaf, made of tissue that absorbs water. The study found that a large redwood tree can absorb as much as 14 gallons of water in just an hour if its leaves are wet.

Not only redwoods, but other native species are dependent on sea fog. Some only spring to life when it appears.

Many of the trees that grow along Sonoma County’s fogbound coast, rivers and streams are heavily draped in summer with lacy strands of gray-green lichens, which take their water directly from the drifting fog banks. Other species of lichens have adapted to survive on bare exposed coastal rocks by raising tiny absorbent leaflike structures in the damp air to collect the windblown moisture.

Fog drip in California forests doesn’t just support trees. It also provides moisture to the ferns, huckleberries and other plants that grow in the coastal forest understory, chapparal and prairies. According to Dawson, 100% of their water may come this way, during summer.

Other California plants, like seaside lupines and grasses, also have developed specialized structures that can pull and condense droplets from the moist air, including fine hairs that cover leaves and stalks, or special leaf coatings. As local hikers know, a walk through knee-high coastal prairie on a foggy morning will make pants and shoes dripping wet.

Fog helps California ecosystems in another way. By keeping the air humid, fog reduces the amount of water plants lose through evaporation. With persistent fog, wild lands are less vulnerable to extreme wildfires, and fires move more slowly when they come.

In addition to summer heat relief, some of California’s prized agricultural bounty is also directly impacted by fog, including, of course, grapes. In our flagship industry, winemaking, we have several appellations in the fog belt. Certain prizewinning wine grape varieties, like pinot noir and chardonnay, excel with foggy nights.

Diminishing fog

Given its influence across the landscape, the study of fog has grown in importance. And recent work has found that the frequency of fog in California may be in decline.

When Dawson at UC Berkeley compiled evidence from the past century, he found coastal fog had declined here by as much as 33%.

“We’ve lost a good three hours per day of fog input along coastal California,” said Dawson, who attributes the change to the warming climate.

“That’s not just in the redwoods. That’s any coastal ecosystem, whether it’s a chaparral system, a grassland or the redwood forest itself.”

For now, the ethereal pleasure of watching a gray veil drift silently through a dimly lit redwood forest or a dense fog bank crest and overflow a seaside hill remains a treat for coastal visitors and a precious resource for California life.

Stephen Nett is a Bodega Bay-based Certified California Naturalist, writer and speaker. Contact him at snett@findingcalifornia.com