Rising tide: Winter waves are getting much larger in California

Winter waves off the coast of California are growing in size – a phenomenon that can be linked to the ongoing progression of climate change. This is the conclusion of a meticulous study led by Dr. Peter Bromirski of the Scripps Institution of Oceanography at UC San Diego.

By leveraging 90 years of seismic data to deduce wave heights, the research has resulted in the most extensive record to date of the Pacific Ocean’s assault on California’s coastline. The study, published in the Journal of Geophysical Research – Oceans, was based on seismic records dating back to 1931. 

The analysis revealed that the average winter wave height in California has grown by a foot since 1970, when compared to the height of winter waves from 1931 to 1969.

The methodology used for the study, first developed by Dr. Bromirski in 1999, correlates seismic energy to wave height. The data adds to a rapidly growing collection of research suggesting that storm activity in the North Pacific Ocean has been ramping up as a result of climate change.

Focus of the study

The potential implications of this discovery are significant, as escalating winter wave heights could exacerbate the risk of flooding and coastal erosion, already under threat from accelerating sea-level rise due to global warming.

Dr. Bromirski explains the process behind his unique method: “When waves reach shallow coastal waters, some of their energy is reflected back out to sea. When this reflected wave energy collides with waves approaching the shoreline, their interaction creates a downward pressure signal that is converted into seismic energy at the seafloor.” 

This seismic energy then travels inland as seismic waves, detectable by seismographs. By correlating the strength of the seismic signal to wave height, Bromirski was able to infer one from the other.

Finding alternative data sources

Dr. Bromirski’s approach was not without challenges. For instance, he needed to isolate the seismic signals generated by wave action from those produced by earthquakes. However, he explained that this task was not as daunting as it may seem, due to the typically shorter duration of earthquakes compared to the ocean waves produced by storms.

The new technique was created to overcome the limitation of the introduction of buoys for direct wave height measurement along the California coast – which only began around 1980. 

Dr. Bromirski was interested in data from decades prior to the 1970s – when the acceleration of global warming was notably significant. He set out to find alternative data sources that could provide wave records stretching back further in time.

Seismic records 

Filling this data gap became possible thanks to seismic records held at UC Berkeley, which date back almost 70 years. This treasure trove of data was initially in an analog format – pages upon pages of seismograph readings. 

Dr. Bromirski spearheaded an ambitious effort to digitize these analog seismograms, spanning from 1931 to 1992, with the help of a dedicated team of undergraduate students, a specialized flatbed scanner, and several years of intermittent effort.

Upon successful digitization of the seismic data, Dr. Bromirski transformed these records into corresponding wave heights, allowing him to analyze the patterns within. 

What the study revealed

The detailed examination revealed a clear trend: since 1970, California’s average winter wave height has increased by 13 percent, approximately a foot, in comparison to the average wave height from 1931 to 1969. 

Furthermore, between 1996 and 2016, Dr. Bromirski found that the number of storm events generating waves over 13 feet tall along the California coast was roughly double the amount from the period spanning 1949 to 1969.

“After 1970, there is a consistently higher rate of large wave events,” Bromirski said. “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.”

Implications of the study 

The findings echo the results of a 2000 study, which reported an increase in wave height in the North Atlantic tied to global warming. If these escalating winter wave heights persist due to climate change, the impact on California’s coast could be dire.

“Waves ride on top of the sea level, which is rising due to climate change. 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,” said Dr. Bromirski.

To further validate the findings, Dr. Bromirski compared his results with atmospheric patterns over the North Pacific, particularly the Aleutian Low, a semi-permanent wintertime low-pressure system located near Alaska’s Aleutian Islands. He found that the intensity of the Aleutian Low has generally increased since 1970, a pattern that corresponds with increased storm activity and intensity.

The research offers sobering insights into the escalating challenges that California’s coastline may face with climate change, rising sea levels, and increasingly intense storm activity. The work highlights an urgent need to explore new dimensions of coastal impacts to better predict and mitigate the consequences of climate change in the years to come.

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