Battered West Coast a lesson on warming, study finds

Image: Erosion along San Francisco beach
Battering waves beat up this stretch of road along Ocean Beach in San Francisco, Calif., within three weeks in January 2010.Jeff Hansen / USGS
/ Source: msnbc.com

Severe erosion along the West Coast during the winter of 2009-2010 offers a look at, and lessons for, a warming world with rising sea levels, a new study finds.

A natural El Nino cycle that warms the Pacific Ocean produced those severe conditions, but computer models suggest that similar damage could come from sea level rise tied to human-caused greenhouse gases.

"If these trends continue," U.S. government and academic experts wrote in their study, "the combination of large waves and higher water levels, particularly when enhanced by El Ninos, can be expected to be more frequent in the future, resulting in greater risk of coastal erosion, flooding, and cliff failures."

Lead author Patrick Barnard, a coastal geologist with the U.S. Geological Survey, told msnbc.com that the study serves as a platform "to understand the broad coastal impact of conditions we are likely to experience more frequently in the future."

In California, the researchers found that winter wave energy was 20 percent above average for the years dating back to 1997, resulting in shoreline erosion that exceeded the average by 36 percent.

"The stormy conditions of the 2009-10 El Nino winter eroded the beaches to often unprecedented levels at sites throughout California," Barnard said in a USGS statement released with the study.

San Francisco's Ocean Beach saw some of the most severe erosion. Its shoreline retreated 184 feet — 75 percent more than in a typical winter — taking out a lane of a major roadway. Sites in San Diego, Santa Barbara and Ventura also saw significant erosion.

In its statement, the USGS warned of worsening erosion as warming melts ice sheets and glaciers, and potentially creates more severe storms.

"When combined with still higher sea levels expected due to global warming, and potentially even stronger winter storms," it stated, "these factors are likely to contribute to increased rates of beach and bluff erosion along much of the U.S. west coast, producing regional, large-scale coastal changes."

Barnard noted that El Ninos can raise winter water levels by 4-8 inches along the West Coast. Given the projected acceleration of global sea level rise that has been around a tenth of an inch annually, he said, "it is likely we will hit that level in the next few decades" along the coast due to global warming.

The study was published in the peer-reviewed journal Geophysical Research Letters.