Goodbye CA Coastal Fog, Goodbye Redwoods
Loss of fog, linked to urban heat islands, imperils coastal ecosystems
In Southern California they call it the “June Gloom” — the gray layer of heavy fog that drapes itself across much of Los Angeles in late spring. Here in the San Francisco Bay Area, we just call it “summer”: The months-long cycle of overcast mornings and chilly evenings that supposedly* prompted Mark Twain to complain that “The coldest winter I ever saw was the summer I spent in San Francisco.” While coastal California’s summer fog has long annoyed residents and tourists alike, the regular rush of cool, wet air helps sustain coastal ecosystems, including the state’s iconic redwoods. Now, thanks to human development, that weather phenomenon is at risk.
Photo by Don Graham
According to a story published last week in the journal Geophyiscal Research Letters, coastal fog in the Los Angeles region is on the decline. In the last 60 years, according to researchers, summer fog in the LA area has decreased by 63 percent. The culprit? The so-called “urban heat island effect” — a phenomenon in which the ambient temperature of cities is much higher (especially at night) than in surrounding undeveloped areas because of all the heat that builds up in our streetscapes of concrete and asphalt.
“We used cloud data from the last 67 years, and we can see that there have been huge declines in fog that have happened and that should continue happening,” says Park Williams, a researcher at Columbia University’s Lamont-Doherty Earth Observatory. Williams based his findings on detailed, sometimes hourly, weather readings from Southern California’s many airports, and matched that against census data on population density to chart development across the region. He and his colleagues were then able to demonstrate a link between the heat island effect and the diminishment of coastal fog. “This is a really solid process that is going on, and we have enough confidence to predict that it will continue.”
Beachgoers might be pleased by the findings. More sunny days, what’s not to like? But Williams and other scientists caution that most coastal California ecosystems — from chaparral slopes to the oak-studded prairie grasslands to the towering redwoods — have evolved to rely on water they get from coastal fog.
Todd Dawson, a biologist at University of California, Berkeley, says that many coastal plants — not just trees, but also the understory of shrubs and grass — depend on the water they receive from fog drip. Coastal redwoods, for example, get up to one-third of their total annual water needs from fog. “You think that all of this water is coming from winter rainfall, and of course a lot of it does,” Dawson says. “But a lot of it also comes from fog, and that fog comes during a really important time, during the summer, the longest and warmest days of the year. The fog subsidy is really important for their ecology and their physiology.”
Williams says that it would be useful to have a better understanding of the degree to which coastal fog sustains chaparral ecosystems, and whether a decrease in fog cover could be making such areas more flammable and prone to wildfires. “I don’t think that anyone is studying whether chaparral has become more flammable in the last 30 years,” he says, “but I would hypothesize that they have.” In short: less fog could mean more intense wildfire seasons.
Williams’ current study only looked at Southern California. He says that he’s interested in extending his research into the San Francisco Bay Area, where more than 6 million people live in close proximity to some of the world’s most iconic redwood groves, such as Muir Woods National Monument. Biologist Dawson says, “The open questions is: OK, this is happening in Southern California, but what’s happening here in Northern California, where the coastal redwoods live?”
* While this quip is frequently attributed to Twain, it might be apocryphal. See here.