Coastal Fog — Or The Lack Of It — Could Be A Wildfire Risk

Just before lightning caused two huge wildfires to burn more than a million acres in Northern California, Rachel Lazzeri-Aerts noticed conditions were ripe for a blaze. The San Jose State environmental studies professor says vegetation was dry and temperatures were hot. 

“We had a couple days of no fog,” she remembered about the mid-August weather in the South Bay. 

“You got the dry lightning and it's kind of a recipe for what we're seeing right now,” she said about the LNU and SNU complex fires on the periphery of the San Francisco Bay Area. 

Lazzeri-Aerts is just one of many researchers who says climate change is decreasing coastal fog, and that could be leading to increased fire risk. 

Fog moistens grasses and trees and is key to the health of coastal forests, especially during summer months, says UC Berkeley Plant Biologist Todd Dawson.

But according to Dawson, coastal fog has declined by 33% over the past century is partially the result of the warming climate, and that creates more terrestrial conditions that are prone to megafires.

“We've lost a good three hours per day of fog input along coastal California,” said Dawson. “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 … and it continues as the globe warms up.”

Dawson says when fog isn’t present the golden hills in the coastal range dry out faster and make them “more vulnerable to wildfires.”

Although he doesn’t attribute the current wildfires to the lack of fog, he says the link needs to be studied further. 

Fewer foggy days, combined with drought, have in some cases suppressed redwood tree growth in the eastern and southern parts of the state, said Kristen Shive, director of science for the advocacy group Save the Redwoods League.

She says the sites are already on the margins of the redwood range so “it’s not surprising” that’s the first place they’re seeing lower growth rates. 

Overall, the trees won’t go extinct in California, Shive says. Redwoods are resilient with thick bark and the ability to regrow from hollowed out stumps. But she says the lack of growth could be an early sign of what could come for the trees as the climate warms and fog disappears. 

“To me that's a signal of where losses could occur,” Shive said. 

Lazzeri-Aerts says the redwoods that burned in the recent fires could be the very redwoods that are compromised by climate change before their northern cousins.

“If we continue to see these warming trends, this Southern extent of the redwoods from Monterey to Santa Cruz is going to be the most impacted in terms of suitable habitat,” she said.

But the lack of fog isn’t all bad news for coastal redwoods. Trees in wetter climates — such as in northern California and parts of Oregon — benefit from extra sunlight when fog isn’t present, Dawson said. 

“Many of the trees seem to be growing faster in the northern part of the [coastal] range right now, probably because they were light-limited before,” he said. 

Despite the complex relationship the trees have with coastal fog, overall, scientists say, if the trend of less fog continues it could eventually spell bad news for the forests.

“If we lose fog all together, certainly there will be a negative effect because the trees definitely benefit by having it during the summer growing season .. but right now in the northern part of the range we don't see that,” Dawson said.