Waves are getting bigger and surf at least four metres high is becoming more common off California’s coast as the planet warms, according to innovative research that tracked the increasing height from historical data collected during the past 90 years.

Oceanographer Peter Bromirski at Scripps Institution of Oceanography used the unusual method of analysing seismic records dating back to 1931 to measure the change in wave height.

When waves ricochet off the shore, they collide with incoming waves and cause a ripple of energy through the sea floor that can be picked up by seismographs designed to detect earthquakes.

The greater the impact, the taller the wave is.

Until now, scientists relied on a network of National Oceanic and Atmospheric Administration buoys that collect data on wave height along US coasts, but that data along the California coast only went back to 1980.

“Until I stumbled upon this data set, it was almost impossible to make that comparison with any kind of reliability,” Dr Bromirski said.

To go back further, Dr Bromirski gathered a team of undergraduate students to analyse daily seismic readings covering decades of winters.

It was a slow, painstaking process that took years and involved digitising drums of paper records.

But he said it was important in learning how things have changed along California’s coast in the past century.

They found average winter wave heights have grown by as much as 30cm since 1970 when global warming was believed to have begun accelerating.

Swells of at least four metres are also happening a lot more often, occurring at least twice as often from 1996 to 2016 than from 1949 to 1969.

Dr Bromirski was also surprised to find extended periods of exceptionally low wave heights prior to about 1970 and none of those periods since.

“Erosion, coastal flooding, damage to coastal infrastructure is, you know, something that we’re seeing more frequently than in the past,” he said.

“Combined with sea level rise, bigger waves mean that is going to happen more often.”

Changes in waves are showing up in other ways, too.

“There’s about twice as many big wave events since 1970 as there was prior to 1970,” Dr Bromirski said.

The study, published on Tuesday in the Journal of Geophysical Research: Oceans, adds to the evidence climate change is causing massive shifts in the world’s oceans.

Other studies have shown waves are not only getting taller but also more powerful.

Damage from intense storms and massive surf is already playing out.

This winter, California’s severe storms and giant waves collapsed bluffs, damaged piers and flooded parts of the state’s picturesque Highway 1.

Dr Bromirski said that was a harbinger of the future.

Scientists say global warming might even be accelerating, ushering in even bigger waves.

Dr Bromirski said bigger waves would cause more flooding in coastal communities, erode away beaches, trigger landslides and destabilise remaining bluffs as sea levels rose and storms intensified.