California winter waves may be boosted by climate change, imperiling fragile coast

Winter waves along California’s shoreline may be growing in height due to the impacts of climate change — posing a potential threat to an already vulnerable coastal ecosystem, a new study has found.

Probing nearly a century’s worth of seismic data, a University of California oceanographer has determined that from 1970 and on, California’s winter waves have grown by an average of 13 percent — or 1 foot — in comparison to those from 1931 to 1969.

Further such growth, driven by a warming climate, could amplify the impacts of sea-level rise and wreak havoc on the coastline, according to the study, published on Tuesday in the Journal of Geophysical Research – Oceans.

“After 1970, there is a consistently higher rate of large wave events,” said Peter Bromirski, a researcher emeritus at UC San Diego Scripps Institution of Oceanography, in a statement. “It’s not uncommon to have a winter with high wave activity, but those winters occurred less frequently prior to 1970.”

Bromirski was already well aware of the connection between wave height and seismic activity, having published a paper showing how to derive the former from the latter in 1999.

When waves reach shallow coastal waters, some of their energy is reflected back to sea — colliding with approaching waves along the way, Bromirski explained. This interaction creates a downward pressure signal that is then transformed into seismic energy at the seafloor, which is detectable by seismographs.

Because the strength of that signal is directly related to wave height, Bromirski was able to calculate one from the other — while filtering out for earthquake “noise,” which he said is much shorter in duration than waves generated by storms.

Bromirkski’s study joins a growing body of research suggesting storm activity in the North Pacific Ocean has risen amid changing climate conditions.

“Now, there are few winters with particularly low wave activity,” he said. “And the fact that this change coincides with the acceleration of global warming near 1970 is consistent with increased storm activity over the North Pacific resulting from climate change.”

To draw his conclusions, Bromirski needed to develop a novel way of chronicling trends in wave heights, since doing so would require access to decades of data. Meanwhile, buoys along the California coast had only been collecting such information since 1980.

While exploring how wave heights were connected to seismic records in 1999, Bromirski said he came across a trove of data at the University of California Berkeley that dated nearly 70 years.

But those records were analog — meaning he and multiple undergraduate students needed to invest multiple years of intermittent effort in scanning piles of seismograms from 1931-1992.

Once they accomplished the full digitization process, the researchers were able to transform the seismic data into wave heights and search for patterns.

They were able to pinpoint the 13 percent increase in wave height, and they found that between 1996 and 2016, there were about twice as many storm events that produced waves taller than 13 feet compared to those that occurred from 1949 to 1969.

“Waves ride on top of the sea level, which is rising due to climate change,” Bromirski said. “When sea levels are elevated even further during storms, more wave energy can potentially reach vulnerable sea cliffs, flood low-lying regions, or damage coastal infrastructure.”

Just looking at the past winter, in which California saw unprecedented rain and snow, Bromirski and his colleagues warned “the 2022-2023 winter may be a harbinger of climate change-induced heightened wave activity along the Pacific Coast.”

“If Pacific storms and the waves they produce keep intensifying as climate change progresses and sea-level rises, it creates a new dimension that needs to be considered in terms of trying to anticipate coastal impacts in California,” he said.