Never mind the frigid mornings last week. Bay Area naturalists say the effects of climate change may be appearing in San Francisco as warmer, drier winters affect local plant and animal populations.
As dramatic as any of the changes is a flurry of anomalies in local bird sightings and distorted avian migratory patterns. Take the barn swallow. The seasonal visitor to San Francisco in the warmer months generally disappears in the winter, says Josiah Clarke, a Richmond District native who has been watching and studying local birds for 20 years.
But about five years ago, barn swallows began staying here for the winter.
"Barn swallows are absolutely a spring-summer phenomenon, but as of very recently they've been finding favorable winter conditions in San Francisco," he says.
And indeed, our winters are growing measurably warmer. Data collected by National Oceanic and Atmospheric Administration weather stations reveal an indisputable long-term increase in temperatures in the Bay Area. Since 1970, local wintertime lows have increased by a half degree to a degree Fahrenheit per decade, according to Eugene Cordero, a meteorologist at San Jose State University who is working to interpret the causes of this trend.
"That might not sound like much to some people, but in terms of accumulated weather change that's tremendous," says Cordero, who also notes that wintertime lows in the Mojave Desert are now more than four degrees warmer than they were 40 years ago.
Such desert warming, local bird experts say, may be the reason warm-weather raptors that migrate between South America and the desert Southwest — but usually no farther north — have made inaugural appearances in the Bay Area this decade. A zone-tailed hawk, for example, was seen in Cole Valley last spring — the first reported sighting in San Francisco.
"To go for 150 years with zero sightings north of San Luis Obispo and then have three in five or six years is a sign of something," says Keith Hansen, a professional bird illustrator in Bolinas, where the first zone-tailed hawk ever seen in the Bay Area flew overhead in 2001.
Just north of the Golden Gate Bridge last year, a birdwatcher reported seeing a gray hawk, a first for the Bay Area. Meanwhile, Hansen says the common black hawk, a tropical bird, has appeared at least twice this decade in Northern California.
Clarke observes that with many species, "where there was a pattern of no occurrence before, there is now a pattern of rare occurrence."
Plants also seem to be reacting to climate idiosyncrasies, says Dan Gluesenkamp, ecologist at the Audubon Canyon Ranch in west Marin. He has seen an invasion of tumbleweed along Highway 101 between Mill Valley and Novato. "That just didn't occur before," he says. "In the last two or three years, it's been all over along the highway. It's like Southern California creeping north."
Hey, we'll gladly take the warmer winters, the hawks, the weeds, and the swallows — but please spare us Los Angeles.