California), and 14 percent along the south coast (Santa Barbara, California).

Vertical land motions along the west coast of the United States are caused by a complex combination of tectonics, glacial isostatic adjustment, sediment compaction, and fluid withdrawal and recharge. The area straddles two tectonic regimes: (1) the Cascadia Subduction Zone, where the buildup of interseismic strain is causing coastal uplift north of Cape Mendocino, California; and (2) the San Andreas Fault Zone, where the lateral motion of the lithospheric plates produces relatively little vertical land motion south of Cape Mendocino. Glacial isostatic adjustment is producing uplift in northernmost Washington, which had been covered by the former Laurentide Ice Sheet, and subsidence in areas peripheral to the center of the former ice mass, including the rest of Washington, Oregon, and California. Land levels in some areas also are rising or sinking because of local tectonics, compaction of wetland sediments, and/or fluid withdrawal or recharge. Continuous GPS measurements over the past two decades and leveling studies over the past eight or nine decades shows that the coast north of Cape Mendocino is rising at rates of 1.5–3.0 mm yr-1 and the coast south of Cape Mendocino is subsiding at a mean rate of about 1 mm yr-1, although with considerable spatial variability (-3.7–0.6 mm yr-1).

Tide gage records along the west coast of the United States indicate that relative sea-level change is variable along the coast. Most gages north of Cape Mendocino show relative sea-level fall for the past 6–10 decades, consistent with coastal uplift along the Cascadia Subduction Zone. Most gages south of Cape Mendocino show relative sea-level rise, consistent with land subsidence. When adjusted for vertical land motions and for atmospheric pressure effects, the rates of relative sea-level rise along the U.S. west coast are lower than the rate of global mean sea-level rise.

Although rates of sea-level rise are relatively low along the west coast of the United States, the combination of sea-level rise and winter storms increases the potential for significant coastal damage. Historically, most coastal damage has occurred when storm surges and large waves coincided with high astronomical tides and El Niños—a combination that can raise short-term sea level above sea levels projected for 2100. All climate models project ample winter storm activity, but a clear consensus has not yet emerged on whether storm frequency or intensity will change in the northeast Pacific. Several climate models predict a northward shift in the North Pacific storm track over the 21st century, and some observational studies report that a northward shift has been detected. However, most observational records are not long enough to determine whether a shift has begun.

Several observational studies have reported that high waves have been getting higher and that winds have been getting stronger in the northeastern Pacific over the past few decades. The magnitude and cause of these changes are under investigation; at least part of the observed increase likely reflects natural climate variability. But even if storminess does not increase in the future, sea-level rise will magnify the adverse impact of storm surges and high waves on the coast of California, Oregon, and Washington.



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