Last year's unusually potent El Nino produced monster waves that carved away record-breaking swathes of the US West Coast's beaches, said a study Tuesday, warning that recovery may take years.
Erosion at 29 beaches from Washington to southern California during the winter of 2015-16 was 76 percent more than usual, by far the highest rate ever recorded, according to the study in the journal Nature Communications.
Most Californian beaches lost more sand than historical extremes,
researchers said, and they warned that such events would become more regular as climate change exacerbates El Nino's effects.
"If severe El Nino events such as this one become more common in future... this coastal region, home to more than 25 million people, will become increasingly vulnerable to coastal hazards," said a statement from the US Geological Survey (USGS), which contributed to the research.
The team found that West Coast beaches retreated 35 metres (115 feet) on average -- up to 55 metres at one beach in San Francisco.
Typically, the average retreat is about 20 metres in winter. Sediment lost is usually replenished by mild waves moving sand onshore from the continental shelf, and more is deposited by rivers and through rock and cliff erosion.
"This El Nino produced waves that were, on average, about 50 percent larger than during a typical winter," Patrick Barnard of the USGS told AFP.
"Our largest peak waves are typically about six metres -- this winter the largest waves were around nine metres," he said by email.
Last year saw one of the three strongest El Nino events since record-keeping began in 1871. The others were in 1982-83 and 1997-98.
Worse to come
El Nino is a climate phenomenon that occurs every few years, alternating with La Nina to warm or cool parts of the Pacific Ocean and influence global rainfall patterns.
With El Nino causing "warmer sea surface temperatures in the eastern tropical Pacific, more evaporation and therefore more warm, moist air is transported to the mid-latitudes," Barnard said.
"Essentially this means more fuel for more frequent and powerful storms to develop in the eastern north Pacific."
Measured in terms of extreme wave energy, the 2015-16 El Nino was the strongest ever recorded on the US West Coast, Barnard said.
During such highly energetic events, sand can be transported so far offshore that it takes much longer than a season to migrate back, and can even be lost forever.
Most of the 29 beaches surveyed for the study have recovered poorly so far, Barnard said.
Most shorelines are still 10 to 20 metres more eroded than before El Nino, leaving beaches vulnerable to wave erosion and flooding.
"It may take years for some beaches to build back," said Peter Ruggiero of Oregon State University, a co-author of the study.
The situation was worsened by a long-running California drought, with little rain to bring sand to replenish the beach.
After the 1997-98 El Nino, it took some beaches a decade to recover, Ruggiero said.
"With more extensive droughts forecast for the US southwest due to climate change, (these) kinds of impacts will probably become more frequent," Barnard said.
Climate change is predicted to cause more droughts and more powerful El Ninos, as well as higher sea levels -- putting coastal communities doubly at risk.