The storms that battered the West Coast during the winter of 2009-10 eroded record chunks of shoreline, and more will likely disappear as the changing climate brings more such powerful storm seasons, scientists warn in a new study.
Pacific waves were 20 percent stronger on average than any year since 1997 and higher-than-usual sea levels drove them further inland, tearing away on average one-third more land in California.
The state's beaches were "eroded to often unprecedented levels," said Patrick Barnard, a coastal geologist with the U.S. Geological Survey who led the research.
"It's the kind of winter we may experience more frequently" as global temperatures rise, he said.
Nowhere along the West Coast was erosion more pronounced than at Ocean Beach in San Francisco. That winter, the Pacific encroached 184 feet inland, 75 percent more than in a typical season.
Waves reaching 30 feet eroded bluffs and triggered the collapse of a section of Highway 1. It reopened with one of its two southbound lanes permanently closed. San Francisco built a 425-foot rock bulwark to protect the road and the wastewater treatment plant behind it.
The southern end of Ocean Beach "really is in a sad state," said Benjamin Grant, a consultant with the San Francisco Planning and Urban Research Association. He's leading the nonprofit's development of a voluntary master plan for the beach.
"Large piles of rubble make it very difficult
The raging storms also stripped several San Diego beaches, leaving nothing but "cobble, boulders and just rock," Barnard said.
Bluff, dune and cliff erosion, along with winter waves hauling off fine sand, is part of the natural dynamic along coastlines. Summer brings smaller waves carrying sand back on shore, replenishing beaches.
However, after the winter of 2009-10, there was less beach replenishment than usual. That leaves scoured beaches vulnerable to even worse erosion if one powerful winter is followed by another.
The force behind the damaging storms of 2009-10 was a different type of El Niño, a climate pattern that periodically brings wetter winters to the California coast.
Warmer-than-usual sea surface temperatures in the eastern equatorial region of the Pacific create the classic El Niño, which drenched California during the winters of 1982-83 and 1997-98.
But researchers observed another type of El Niño has become far more prevalent, the Central Pacific El Niño.
It's characterized by warm sea surface temperatures in the central Pacific, flanked on the east and west by cooler waters. It's also called El Niño Modoki; the last word is Japanese for "similar, but different."
The new study notes that the Modoki occurred more frequently during the past two decades than the classic El Niño. Climate change is expected to raise central Pacific water temperatures, increasing by as much as fivefold of El Niño Modoki frequency, according to a 2009 study in Nature.
The estimated frequency is based on varying projections for carbon dioxide emissions in coming decades.
Given the odds that the newly identified El Niño will continue its regular appearance, researchers decided to compare its effects to a typical El Niño, Barnard said.
They found that Modoki packs a punch when compared to a typical El Niño.
"Pretty much everywhere we surveyed, the erosion during the 2009-10 winter was comparable to or more severe" than the classic El Niño in 1997-98, Barnard said.
He acknowledged that the study encompassed five to 13 years of data, depending upon the beach. The researchers studied sections of the coastline, using GPS, buoys and airborne laser mapping between Seattle and San Diego.
But given what he called a startling lack of coastline studies, Barnard said the data they gathered "is the best we have. There's nothing like it, and it covers a really broad area."
Barnard was the lead author of the study, published by the American Geophysical Union on July 9 in Geophysical Research Letters. Researchers with five other institutions participated.
With little more than a decade of data, "we couldn't unequivocally" say this portends the future, he said.
"But there's no indication that there's a light at the end of the tunnel anytime soon, given the current trends that we're observing."
Suzanne Bohan covers science. Contact her at 510-262-2789.