constrained, largely due to sparse data and inadequate models. Each process likely has a significant but opposite effect on sea-level change, on the order of 0.5 mm per year.

SEA-LEVEL RISE OFF CALIFORNIA, OREGON, AND WASHINGTON

The sea level at any particular place along the coast is commonly measured using tide gages, which record the height of the sea surface with respect to the land surface, both of which may change over time. Relative sea level will rise if ocean levels rise and/or land levels fall. Records from 12 west coast tide gages indicate local variability in sea-level change along the coast, although most of the gages north of Cape Mendocino, California, show that relative sea level has been falling over the past 6–10 decades, and most of the gages south of Cape Mendocino show that relative sea level has been rising.

Factors That Affect Northeast Pacific Ocean Levels

Along the west coast of the United States, climate patterns such as the El Niño-Southern Oscillation and, to a lesser extent, the Pacific Decadal Oscillation, affect winds and ocean circulation, raising local sea level during warm phases (e.g., El Niño) and lowering sea level during cool phases (e.g., La Niña). Large El Niño events can raise coastal sea levels by 10 to 30 cm for several winter months.

The large mass of glaciers and ice sheets exerts a gravitational pull that draws ocean water closer. As the ice melts, the gravitational pull decreases, ice melt enters the ocean, and the land and ocean basins both deform as a result of this loss of land ice mass. These gravitational and deformational effects produce a spatial pattern of regional sea-level change called a sea-level fingerprint. Melting from Alaska and, to a lesser extent, Greenland, causes relative sea level to fall at decreasing rates from northern Washington to southern California, whereas melting from Antarctica causes relative sea level to rise along all three states. The net effect is a reduction in the contribution of the three ice sources to relative sea-level rise by 42 percent along the north coast (Neah Bay), 24 percent along the central coast (Eureka), and 14 percent along the south coast (Santa Barbara) for 1992–2008.

Factors That Affect Land Elevation in California, Oregon, and Washington

Although modern melting of land ice has a significant effect on sea-surface heights in the northeast Pacific Ocean, the melting and eventual disappearance of North American ice sheets that began more than 20,000 years ago has a significant effect on land levels in California, Oregon, and Washington. The massive loss of ice from the ancient ice sheets continues to cause uplift of about 1 mm per year in northernmost Washington, which had been covered by an ice sheet, and subsidence of about 1–2 mm per year in areas at the ice margin and beyond, which includes the rest of Washington, Oregon, and California.

Tectonics causes substantial regional uplift along much of the Washington, Oregon, and northernmost California coast, where ocean plates are descending below North America at the Cascadia Subduction Zone. South of Cape Mendocino, California, the Pacific and North American plates are sliding past one another along the San Andreas Fault Zone, creating relatively little vertical land motion along the coast. Local tectonics, as well as compaction of sediments, pumping of water or hydrocarbons from subsurface reservoirs, and fluid recharge can produce locally high rates of land subsidence or uplift. Water or hydrocarbon extraction, which can lower surface elevations up to tens of centimeters per year if fluids are not returned to the subsurface, is most important in California.

The total vertical land motion from all of these geological processes and human activities can be estimated from Global Positioning System (GPS) measurements, which show that much of the coast is rising about 1.5–3.0 mm per year north of Cape Mendocino. The coast south of Cape Mendocino is sinking at an average rate of about 1 mm per year, although GPS-measured rates vary widely (-3.7–0.6 mm per year).

PROJECTIONS OF SEA-LEVEL RISE FOR 2030, 2050, AND 2100

Global Projections

Projections of global sea-level rise are generally made using models of the ocean-atmosphere-climate system, extrapolations, or semi-empirical methods.



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