A study by University of California San Diego Scripps Institution of Oceanography researcher emeritus Peter Bromirski uses records dating back to 1931 to show that the average heights of winter waves along the California coast have increased along with climate change.
The study, published in the Journal of Geophysical Research – Oceans, achieved its extraordinarily long-time series by using seismic records to infer wave height, a method developed by Bromirski in 1999. The results join a growing body of research that suggests storm activity in the North Pacific Ocean has increased under climate change.
When waves reach shallow coastal waters, some of their energy is reflected back out to sea, according to Bromirski. When this wave energy collides with waves approaching the shoreline, their interaction creates downward pressure that is converted into seismic energy at the seafloor, which can be detected by seismographs. The strength of this seismic signal is directly related to wave height.
Bromirski developed this novel way to calculate wave heights because the buoys that directly measure wave heights along the California coast have only been collecting data since around 1980.
To work on the many decades of seismic records held at UC Berkeley, Bromirski needed to digitize the reams of analog seismograms spanning 1931 to 1992 so that he could analyze the data set as a whole. The process required the enthusiasm of multiple undergraduate students, a special flatbed scanner, and multiple years of intermittent effort to complete. Finally, with the digitized seismic data spanning 1931-2021 in hand, Bromirski was able to transform the data into wave heights and begin to look for patterns.
The analysis revealed that since 1970 California’s average winter wave height has increased by 13%, or about 0.3m, compared with 1931 to 1969. Between 1996 and 2016 there were also about twice as many storm events that produced waves greater than 4m along the California coast compared with 1949 to 1969.
The results echo an increase in wave height in the North Atlantic tied to global warming reported by a 2000 study.