The cause of these dramatically higher waves is not completely certain, but "likely due to Earth's changing climate," the authors say in a study just published online in the journal "Coastal Engineering."
Scientists from Oregon State University and the Oregon Department of Geology and Mineral Industries report that the 100-year wave height could actually exceed 55 feet, with impacts that would dwarf those expected from sea level rise in coming decades.
Increased coastal erosion, flooding, damage to ocean or coastal structures and changing shorelines are all possible, scientists say.
"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," said Peter Ruggiero, an assistant professor in the OSU Department of Geosciences. "The Pacific Northwest has one of the strongest wave climates in the world, and the data clearly show that it's getting even bigger."
Waves crash on the Oregon coast. (Photo by Anne Hornyak)
December and January are the months such waves are most likely to occur, although summer waves are also significantly higher, the scientists say.
"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. But what is clear is the waves are getting larger."
Ruggiero, together with co-authors Jonathan Allan with the Coastal Field Office of the Oregon Department of Geology and Mineral Industries, and Paul Komar in the OSU College of Oceanic and Atmospheric Sciences, has also authored a section in the 2009 "Handbook of Coastal and Ocean Engineering" that warns of the same increasing wave heights in both the North Atlantic and North Pacific oceans.
"Storm intensities and the heights of ocean waves are being affected by Earth's changing climate, including the extreme waves generated by both hurricanes and extratropical storms, attributed to global warming," states their contribution to the Handbook.
"The increases in wave heights have been documented by buoy data collected in the North Atlantic and North Pacific, with the rates of increase being important to engineering design and in the development of coastal-hazard zones," the three scientists write.
In the early 1990s, Ruggiero said, a fairly typical winter might have an offshore wave maximum of a little more than 25 feet. It was believed then, based on data from two offshore buoys, that 33 feet would be about as large as waves would ever get, even during a 100-year storm.
But then a major El Nino happened in 1997-98 and led to a string of 100-year wave events greater than 33 feet.
Using more sophisticated research methods, the researchers now believe that maximum 100-year waves in the Pacific Northwest could exceed 50 feet.
Waves on the beach at Neskowin, Oregon (Photo by RichPT)
If wave heights continue to increase, they may continue to dominate over the acceleration in sea level that is anticipated over the next couple of decades.
Previous concerns about what sea level rise could do is already a reality. If sea levels do rise in the future, that will add to the damage already being done by higher waves.
Some effects are already visible, Ruggiero said.
"Neskowin is already having problems with high water levels and coastal erosion," he said. "Some commercial structures there occasionally lose the use of their lower levels."
"Going to the future, 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 said.
The largest wave height increases have occurred off the Washington coast and northern Oregon, with less increase in southern Oregon and nothing of significance south of central California, the Oregon researchers found.
The study also noted that similar increases in wave heights have occurred in the North Atlantic Ocean, as well as the seasonal total power generated by hurricanes.
These issues do not consider the potential drop in land level that is expected to occur in this region with a subduction zone earthquake at some point in the future. Ruggiero noted that he did some research in Sumatra following the huge 2004 earthquake there – an area with geology very similar to that of the Pacific Northwest – and some of the shoreline had dropped from 1.5 to five feet. If and when that occurs, the impacts on shorelines could be enormous.
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