The Bigger Kahuna: Are More Frequent and Higher Extreme Ocean Waves a By-Product of Global Warming?

Increasing maximum wave heights off the Pacific Northwest coast may pose a greater threat than rising sea levels

Erica Harris, Oregon State University

Armand Thibault looked out over the Pacific's rumbling winter waves from his balcony in Neskowin, Ore. "The predicted high tide today is a 10.1 [feet]," he relayed via YouTube on Friday, January 29. "I'm very glad we don't have a storm surge behind this one. Tomorrow is supposed to be a 10.2, so it should be interesting."

Fortunately, Neskowin didn't experience a storm surge on Saturday either. But like a growing number of seaside towns along the Pacific Northwest coast, it is only a matter of time before another storm blows extreme waves across their coastlines and adds to already massive flooding and erosion. Bigger and more frequent extreme surf, on top of rising sea levels, are the forces behind this damage, according to a study published January 18 in the journal Coastal Engineering. And, based on early results from other research, this phenomenon may not be limited to Oregon and Washington State.

"What is very interesting off of our coast, and not well known yet, is that increasing wave heights have been more important in terms of causing erosion and flooding over the last few decades than sea-level rise," says George Kaminsky, a coastal engineer with the Washington State Department of Ecology. "But because of the amount of attention going to climate change, which is synonymous with sea-level rise for people on the coast, [the latter] is much more on the radar screen."

Kaminsky is working to redirect the attention of local decision-makers toward the growing rollers and the risks they pose for everything from ecosystems and offshore energy structures to coastal development. That effort is made more urgent by the newly published results from Peter Ruggiero of Oregon State University and his colleagues: Whereas global sea-level rise creeps along at a fraction of an inch per year—providing relatively ample time for coastal managers to respond—annual maximum wave heights in the Pacific Northwest are rising at a rate of about 10 centimeters, and sending wave run-up increasingly higher onto beaches. As a result, according to a separate paper by Ruggiero that is currently under review, the contribution of waves to the total water level, or run-up plus tide, is rising an estimated three to 10 times faster than the sea level.

Thankfully, the Pacific Northwest coastline is also home to relatively few people. But could more densely developed regions see changes, too? In an upcoming paper to be published in the Journal of Coastal Research, Richard Seymour of Scripps Institution of Oceanography at the University of California, San Diego, shows wave heights are increasing down the coast through southern California. Peter Adams of the University of Florida reports a similar trend from North Carolina's Cape Hatteras to Florida's West Palm Beach, although his results also await publication. And some of the first evidence that breakers might be on the rise actually came from across the Atlantic: records of an annual increase of about 2.2 centimeters in average wave heights between 1962 and 1986 off England's southwestern tip.

These regions may soon look for lessons from Oregon and Washington, particularly from one troubled town: "Neskowin is on the bleeding edge," says Patrick Corcoran, a coastal hazards educator based in Astoria, Ore. "So there aren't canned things we know are going to work. It's a matter of learning our way through the problem."

Clues from 100-year waves
Here is what Ruggiero is learning: As a student in the mid-1990s, he and his advisor used records dating back to 1981 to project the height of a 100-year wave—a monster bore that appears about once a century—at 10 meters. But then from 1997 to 1999, they recorded at least five wave events higher than that mark. "When you have five or six events that are bigger than a 100-year event occurring over just three years," Ruggiero says, "you need to figure out what's going on."

The answer: Pacific Northwest waves were getting bigger. Based on sophisticated statistical analysis of deepwater buoy data now spanning 30 years, the new 100-year event estimate is 14 meters. Ruggiero even suggests the possibility of one towering higher than a five-story building.

It's too early to say how long the trend will last and how much higher the tallest waves will get. A better understanding of what's behind this phenomenon is needed first. Because the current phase of the Pacific Decadal Oscillation—an ocean temperature circulation pattern that causes regional weather fluctuations—covers most of Ruggiero's buoy data, natural cycles cannot be ruled out. At the same time, a study published in a 2001 issue of the Bulletin of the American Meteorological Society suggests a link between the increasing frequency and intensity of storms, which cause the extreme waves, and sea-surfacetemperatures. And Seymour is working on a paper that he thinks solidifies the connection between global climate change and the wave climate of the U.S. west coast. "I think I have found the smoking pistol," he says.

Layers of variables
But even if that mystery is solved, a confluence of other factors in the Pacific Northwest prohibits easy solutions. "We have sea-level rise on top of El Niño on top of increasing waves and increasing frequency," Kaminsky says. The region is also anticipating a major earthquake, which could instantly raise the region's sea level a meter or more. Not to mention that the Pacific Northwest is already home to some of the biggest offshore swells in the world, averaging about three meters in the winter. This is why the region is also one of the world leaders in ocean-based renewable energy speculation. (But don't start thinking that bigger extreme breakers mean more green energy. A device able to withstand such intense waves would have to be "so big and strong and cumbersome that it wouldn't do a good job of taking energy out of small waves," Seymour notes.)

One common tool used to protect shoreline is a man-made barrier of boulders, called riprap. A continuous line of thick riprap separates the oceanfront homes of Neskowin from the tumultuous tide. But as these rocks get knocked around and sink into the sand, they require frequent and expensive maintenance. Thibault actually features a backhoe excavator in his January 6 YouTube post. "Unfortunately, we're having some work done on our rock, which I was hoping we wouldn't have to do this year," he says. "But the ocean is giving us no alternative."

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