The projected values vary by latitude, with the highest sea levels expected off the coast south of Cape Mendocino (4–30 cm for 2030, 12–61 cm for 2050, and 42–167 cm for 2100, relative to 2000) and the lowest sea levels expected off the coast north of Cape Mendocino (-4–23 cm for 2030, -3–48 cm for 2050, and 10–143 cm for 2100). The lower sea levels projected for Washington, Oregon, and northernmost California reflect coastal uplift and gravitational and deformational effects, which lower the relative sea level. Major sources of uncertainties in the regional projections are related to assumptions about the rate of future ice losses and the constant rate of vertical land motion over the projection period. Uncertainties are larger for the regional projections than for the global projections because more components are considered and because uncertainties in the steric and ocean dynamic components are larger at a regional scale than at a global scale.

Extreme events can raise sea level much faster than the rates projected by the committee. For example, unusually high sea levels may occur temporarily when major storms coincide with high astronomical tides, and especially during years when regional sea levels are anomalously heightened during El Niño events. As mean sea level continues to rise, the number of extreme high water events and their duration are expected to increase. A simulation based on predicted tides, projected weather and El Niño conditions under a mid-range greenhouse gas emission scenario, and the committee’s projections of sea-level rise suggests that the incidence of extreme water heights in the San Francisco Bay area would increase from about 9 hours per decade for the recent historical period (1961–1999) to hundreds of hours per decade by 2050 and several thousand hours per decade by 2100. In addition, the duration of these extremes would lengthen from 1 or 2 hours in the historical period to about 6 hours by 2100.

The biggest game changer for future sea-level rise along the U.S. west coast would be a great earthquake (magnitude greater than 8) along the Cascadia Subduction Zone. Such earthquakes have occurred every several hundred to 1,000 years, with the most recent occurring in 1700. During a great earthquake, some land areas would immediately subside and relative sea level would suddenly rise, perhaps by 1 m or more. This earthquake-induced rise in sea level would be added to the projected rise in relative sea level (about 60 cm by 2100).

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