Lots has been said about warming temperatures and rising sea levels, but a new study puts the spotlight on a more imminent threat to coastal communities: extreme waves that are growing taller in some parts of the world.

Data from buoys off the Pacific Northwest coast found that since the mid-1970s the height of the biggest waves has increased on average by nearly four inches a year. That's about 10 feet over that period.

"The waves are getting larger," said lead author Peter Ruggiero, an assistant geosciences professor at Oregon State University.

And that, he said, means "the rates of erosion and frequency of coastal flooding have increased over the last couple of decades and will almost certainly increase in the future."

In the study published in the journal Coastal Engineering, Ruggiero and his colleagues report that the reasons are not completely certain.

"Possible causes might be changes in storm tracks, higher winds, more intense winter storms, or other factors," Ruggiero said. "These probably are related to global warming, but could also be involved with periodic climate fluctuations such as the Pacific Decadal Oscillation, and our wave records are sufficiently short that we can’t be certain yet."

The team also looked at how high a "100-year event" might be, given that planners use those scenarios in approving development projects. Using the new data set, the researchers  estimated that the biggest waves could get up to 46 feet tall — a 40 percent increase from 1970s estimates of 33 feet.

Ruggiero said that the study reinforces earlier ones showing similar trends off some other coasts, among them the U.S. Southeast Atlantic, the Northeast Pacific and southwest England. On the other hand, areas like the North Sea and the Mediterranean have shown little to no increase.

'Double whammy'
Ruggiero said he's working on a publishing another study that shows the increase in Pacific Northwest wave heights over the last 30 years "has been significantly more important than sea level rise" in terms of flooding and erosion threats to the coast.

"The bottom line," Ruggiero said, "is that water levels have already increased in the Pacific Northwest due to wave heights and as sea level rise accelerates the region will experience a 'double whammy'. So it is critical for engineers and planners to consider both processes."

Both "winners and losers" are expected in terms of beach stability, with some areas gaining sand, but already some negative effects are visible in coastal towns like Neskowin, Ore.

"Neskowin is already having problems with high water levels and coastal erosion," Ruggiero said.
"Communities are going to have to plan for heavier wave impacts and erosion, and decide what amounts of risk they are willing to take, how coastal growth should be managed and what criteria to use for structures," he added.

Ruggiero emphasized that another factor for the Pacific Northwest is that a large earthquake could drop the shoreline by several feet, worsening the impact of extreme waves.

That proved to be the case in Sumatra, Indonesia, during the 2004 quake and tsumani, he said, and some of the shoreline there dropped by up to five feet.

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