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Blog Post · April 1, 2024

The Future of Fog

photo - Fog in Redwood Forest

Fog is central to life in California. That’s why the saying “The coldest winter I ever spent was a summer in San Francisco” (dubiously attributed to Mark Twain) persists. Heck, someone’s even created a social media account for San Francisco’s fog. But climate change is going to disrupt this quintessentially Californian weather experience. We asked Todd Dawson, a scientist who has long studied the relationship between fog and redwoods, to divine the future of fog for us.

Why does fog occur in California, and why is it so important to the state’s ecosystems?
photo - Todd Dawson

Fog forms on the western edges of most continents around the globe. When warm, moist air meets the cold water upwelling from the deep ocean, that creates most coastal fogs. On the West Coast of North America, fog occurs mostly from May to October every year, as the high-pressure system that drives winter precipitation moves south.

Our coastal climate would be hot and dry without fog. When the Central Valley heats up during each summer day and that hot air rises, it pulls the cooler, moist air onto land to replace it, which brings in the fog banks that usually live over the ocean. They move onshore, keeping coastal temperatures much milder than they would otherwise be. Fog also provides an enormous, critical water subsidy that sustains many coastal systems. Our coastal fog has a high water content, so when it strikes surfaces such as redwoods and grasses, it drips into the ecosystem. It represents anywhere from 30–40% of all the water coastal redwoods get each year.

What are climate models saying about the future of fog? Has California’s fog changed in recent decades?

In 2010, we started looking at airport records that measure fog for air traffic control. Records got robust and dependable in 1950, so we analyzed everything from 1950–2010, and our analysis showed that fog declined by 33% over that 60-year period.

We’re now seeing less fog on both a daily and an annual basis. In the 1950s, we had 12 hours a day of fog; now it’s about nine. Fog season used to start in early May and end in October; it now starts in late May/early June and ends in September. That’s all due to the warming planet.

Fog has continued to decline since 2010, but at a slower rate than it did over that 60-year period. We’re not sure exactly why. It may be because the ocean is now warming at a faster pace than the land, which is reducing the temperature difference between the coast and inland. All over the globe, especially along the western edges, it’s getting harder for meteorologists to model fog and cloud cover, because sea surface and land surface temperatures are changing rapidly and in unpredictable ways.

For a long time, ocean upwelling was very predictable during the summertime. Now it’s very unpredictable in one important way: the strength of the upwelling doesn’t seem to be changing, but there’s a lens of warm ocean water over the top of the upwelling zone. This is terrible for wildlife on the coast because upwelling is the basis of a productive near-shore food web. Upwelling brings nutrient-rich water to the surface, where sunlight allows algae to grow and start the food chain. A slower or higher-temperature upwelling will impact many species, including fish, birds, seals, sea lions, whales…everything that wants to eat.

How might these changes affect coastal redwoods, creeks, and agriculture?

As temperatures increase, the fog water subsidy is dropping, and that’s already having an effect in the southern and eastern parts of the coast redwood range: tree crowns are thinning out and we’re losing trees.

Redwoods are the great fog harvesters—preserving them, and the fogs they harvest, will help keep streams cooler and keep more runoff going into them, which affects stream life, like salmon, and other coastal wildlife. If we don’t manage these forests to some extent and instead get catastrophic fires, then those forests may be goners. Some harvesting and burning has to be part of the equation. The indigenous people used to do this all the time. Bring it back.

These impacts will play out for agriculture as well. Strawberries, artichokes, and many other cash crops live in cool coastal climates and do well under foggy conditions. San Francisco State University researcher Sara Baguskas has studied coastal fog and the strawberry industry, and she’s finding that strawberries are more water efficient during fog events. The same is true for the wine industry: white wine grapes prefer cooler climates and fog, and the flavors of California’s white wines are changing because there’s less fog in Sonoma and Napa. Also, farms will need to irrigate more, but temperatures may go too high to be productive.

Dan Fernandez at Cal State Monterey Bay is investigating fog harvesting to supplement the water supply. UC Santa Cruz is testing this out with a pilot project. In the Atacama Desert in Chile, small towns in the southern part of the desert capture fog by putting massive screens on the hills; troughs at the bottom funnel water into storage cisterns. The isotope composition of these fog waters matches the isotope composition found in villagers’ hair, proving that these villagers are getting all their water from fog. They are literally fog people.

What can California do to preserve its fog?

We have to curb global warming. Californians can’t do this by themselves. Everything we do in California has to happen everywhere—this is a local problem with a global solution. We’re all interconnected.


climate change Drought extreme weather Forests and Fires Freshwater Ecosystems Water Supply Water, Land & Air wildfires