A study published in the "Journal of Geophysical Research: Oceans" found that average winter waves have grown by as much as a foot since 1970; and surfs of at least 13 feet tall occurred at least twice as often between 1996 and 2016 than from 1949 to 1969.
The researchers aimed to find out how much wave heights had changed off California's coast during the winter months by analyzing seismic records over the past 90 years.
When waves ricochet off the coast, they send energy back toward the sea. When that energy hits incoming waves, it pushes energy downward, creating a seismic signal that can be detected.
Interpreting the seismic data was an important method to understand the impact of wave heights on California's coast, said Peter Bromirski, the study's author and researcher emeritus at the Scripps Institution of Oceanography.
Higher waves with higher sea levels allowed more wave energy to reach vulnerable sea cliffs and enhanced coastal flooding as well as damage to coastal infrastructure.
These issues are of particular concern off California's coast, where sea cliffs have already started crumbling, bringing down homes in recent years. During the past winter, severe storms and huge waves damaged piers and flooded parts of the scenic California Highway 1.
The study also added evidence to other research findings that ocean waves are becoming increasingly violent due to more extreme storms and wreaking havoc along coasts.
Because of rising sea level, the study warned that even moderate waves at the end of the 21st century might cause damage comparable to that of extreme weather events in the past.