CA, USA — Add mudslides to the mix of global-warming worries.
With California’s fire season in full swing, residents in charred areas such as the one around this bucolic resort face the prospect of catastrophic slides on newly denuded hills when winter rains return.
Mudslides often follow big fires as part of a natural process that has gone on for millennia in the West. But forestry experts are increasingly concerned about a noticeable upswing in torrential downpours in California that threatens to send even more mud and debris cascading into downstream communities.
The most likely culprit, scientists and other experts say, is climate change. Increased downpours are consistent with the predicted impacts of global warming, as higher temperatures cause more ocean water to evaporate, making for stronger storms.
“We know fires are going to continue in that Mediterranean climate,” says Wally Covington, a forestry professor at Northern Arizona University. “The question is, is there likely to be an intensification of rain, which will mean a river of mudslides.”
The evidence, so far, suggests that the answer is yes. From 1948 to 2006, storms measured at extreme precipitation increased 18% in the Pacific coastal states, including a 26% jump in California, according to a 2007 analysis of federal climate data by Environment California, an environmental group based in Los Angeles. In the Los Angeles area, data show a 58% increase in torrential storms.
The torrents of mud may be the latest in a series of stealth impacts of climate change suggesting a complex chain reaction. Bark beetle infestations have killed millions of coniferous trees from New Mexico to Alaska as drought and warmer temperatures — which many experts attribute to climate change — have made the trees more susceptible to attack. Scientists have been surprised by a rash of polar bear drownings in the Arctic, as sea ice has retreated too far in places for the bears to swim to safety.
Others say the impacts of global warming are overblown and that fires and other phenomena are unrelated to one another or are part of a natural process. Mudslides are likely to prove another point of contention.
In July, an unusually robust thunderstorm over the Sierra Nevada dumped as much as three inches of rain on fire-scorched hillsides in two hours, unleashing a wall of mud three feet high into communities around Lake Isabella below. About 50 homes suffered damage, highways were closed and dozens of people were trapped — including 60 firefighters battling a wildfire.
“You have increasing frequency of all the key conditions to mudslides: bare hills and heavy rains,” says Elizabeth Ridlinger, policy analyst at Environment California. “And clearly the increase in heavy rains can be tied to climate change.”
Also, more of California’s precipitation is falling as rain instead of snow. According to an August report from the University of California at Davis, the amount of precipitation that fell as snow at Lake Tahoe decreased to 34% in 2007 from 52% in 1910.
Researchers on the Environment California report say it is too early to tell whether mudslides have actually increased in tandem with the more-frequent downpours, since their report, one of the first on the subject, focused only on the precipitation. But mudslides are considered so dangerous — 16 campers died when slides from fire-charred slopes in Southern California engulfed them on Christmas Day 2003 — that state and federal teams are swarming across burn zones in the state to come up with contingency plans. With the worst fire season on record in California, emergency crews expect trouble.
“Once the brush is burned, the stage is set,” says Kevin Cooper, a wildlife biologist for the Los Padres National Forest surrounding Big Sur. “There is very little we can do effectively if it rains really hard, except to get out of the way.”
Mr. Cooper is part of a federal team that has just completed its mudslide assessment for the so-called Basin Complex fire, which burned 240,000 acres of brush and trees in the Santa Lucia Mountains overlooking Big Sur. The team’s preliminary conclusion: A 40-mile stretch of the California coast, including Big Sur, is at an “extremely high” risk of mudslides this winter.
On a recent inspection of Julia Pfeiffer Burns State Park, associate civil engineer Joan Carpenter, who works for the California Department of Parks and Recreation, stopped her truck at a narrow culvert beneath California Highway 1 where she said mud and debris from the blackened slopes above would likely clog and then overflow in a heavy rain. “It’s going to be ugly here,” Ms. Carpenter said.
At Big Sur, a community of about 1,500 situated at the base of mountain peaks stripped of much of the vegetation that held back rocks, boulders and other debris, some longtime residents are comparing the threat of mudslides to 1972, when a river of debris poured into town following wildfires.
Don McQueen, 79 years old, whose family owns property along a creek where officials say more slides are likely, recalls: “The mud came out of the canyon 30 miles per hour, carrying boulders as big as a car.”