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The land also rises (and falls)

Washington Sea Grant’s Ian Miller probes the complicated relationship between plate tectonics and sea-level rise on the Pacific Coast. What goes up may also come down – and coastal communities need to know.

By Eric Scigliano, Washington Sea Grant
Sea-level rise is a global phenomenon, but it’s also a very local one. “Relative sea-level rise,” meaning change that people can perceive such as flooding of their hometowns and water swallowing their shorelines, can vary widely across surprisingly short distances.  This is particularly true in Washington State, where geologist Ian Miller, Washington Sea Grant’s coastal hazards specialist, thinks sea level models and predictions have tended to pay too little attention to these differences and the terrestrial factors that drive them. This in turn encourages complacent assumptions that might someday endanger coastal communities.
Together with colleagues at the U.S. Geological Service, the University of Washington’s Climate Impacts Group and Adaptation International, Miller is working to refine these predictions down to the micro-local level and give specific communities the tools they actually need to plan for rising seas.
The essential point: Sea level rise isn’t just about the sea; it’s also about the land. Various forces cause terra firma to sink or rise on its own, in a process called vertical land movement (VLM).  In some Eastern states, settling sediments and groundwater extraction are making coastal lands subside, adding an estimated 1.3 to 1.9 millimeters a year to relative sea level rise on Maryland’s coast and .84 millimeters at Boston .
Washington’s story is very different. As late as the 1990s its shoreline was widely believed to be rising thanks to a phenomenon called glacial isostatic adjustment – the slow springing back of lands compressed by up to half a mile of ice during the last great glaciation. Closer observation has since shown that the picture is more complicated. This iostatic rebound has much less effect on the seismically active West Coast than another force, plate tectonics. Off the Oregon, Washington and far Northern California, the Pacific and North American plates collide, the former pushing the latter up. Along the San Andreas Fault, running under most of California’s length, the two plates slide past each other and don’t push up.
Because of these processes, the National Research Council, in a milestone 2012 report Sea Level Rise for the Coasts of California, Oregon, and Washington, concluded that north of Cape Mendocino the Pacific Coast is rising 1.5 to 3 millimeters per year, mitigating the effects of sea level rise; south of Mendocino it’s sinking 0.6 to 3.7 millimeters, amplifying that rise.
The NRC committee that produced the report included no members from Washington and, in Miller’s view, its broad-stroke formulations brushed over the much more varied and complex patterns of land movement. “In Washington, people seem to read the report and say, ‘Oh we’re going to have less sea level rise in the Pacific Northwest, so we don’t have to think about it,’” says Miller. “That isn’t helpful for local planners. You really need local assessments to generate the kind of discussion needed to plan for sea-level rise. Developing local projections really changes that view.”
Using tidal-gauge and GPS readings, Miller and his colleagues are attempting to develop better VLM estimates, and hence better assessments and projections of sea level rise. They found that vertical land movement “is insanely variable” along Washington’s Coast. For example, Neah Bay, at Washington’s far Northwest corner, is rising dramatically, pushed up by the offshore subduction fault. But the uplift falls off quickly, just 30 miles down the coast at the next settlement in LaPush, and 70 miles east along the Strait of Juan de Fuca at Port Angeles, where Miller is strategically based.
The effect is like pushing one coin under another; as one edge of the second coin tilts up, the other tilts down. Further east, along Puget Sound where most of Washington’s 7 million people live, the land is actually subsiding, amplifying the effects of rising seas. “For most of the country it’s probably not that big a deal, but if you span the [130-mile] distance from Neah Bay to Seattle, the difference amounts to about 4 millimeters of sea level rise a year, which equates to more than a foot by 2100. That’s a significant difference.”
Miller incorporated these findings into a comprehensive climate change assessment for the nearby Jamestown S’Klallam Tribe. “You could almost see the gears turning as they saw how they could adapt their planning to these projections,” he says.
He’s now doing the same, as part of a climate assessment for two sprawling Olympic Peninsula counties. Miller would like to apply the method he’s developed for estimating vertical land movement to all 17 tidal-gauge sites in Washington, and publish the results in a peer-reviewed journal. But he cautions that many qualifiers and uncertainties attend the projections. Data are fairly sparse: “We only have 20 years of GPS data. There’s a big data gap in Southwest Washington, but a suggestion that some areas, maybe Grays Harbor, may be subsiding.”Erosion, caused or amplified by changing climate and sea levels, may also alter land levels. And a big subduction-zone earthquake offshore could cataclysmically reverse the lifting trend, dropping Neah Bay down a meter or more. “If a big quake happens,” says Miller, “all bets are off.”

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