during subsidence (e.g., Yamaguchi et al., 1997), and Japanese historical records of a tsunami from a distant source. Modeling of the tsunami waveform (Satake et al., 1996) and estimates of coastal subsidence based on detailed microfossil studies (Hawkes et al., 2011) suggest an earthquake magnitude of 8.8 to 9.2. The coastal subsidence and associated sea-level rise were spatially variable, with the largest rise in sea level (1–2 m) occurring in northern Oregon and southern Washington, where the plate boundary forms a wide, shallow arch (Leonard et al., 2004, 2010; Hawkes et al., 2011). Other sections of the margin subsided <1 m and the southernmost part of the subduction zone was uplifted (Leonard et al., 2004, 2010; Hawkes et al., 2011).

Discussion

Changes in regional meteorological and climate patterns, including El Niños, coupled with rising sea level, are predicted to result in increasing extremes in sea levels. Models suggest that sea-level extremes will become more common by the end of the 21st century. Waves riding on these higher water levels will cause increased coastal damage and erosion—more than that expected by sea-level rise alone.

The biggest game changer for future sea level along the west coast of the United States is a great Cascadia earthquake. The related coastal subsidence of such an earthquake would, in a matter of minutes, produce significantly higher sea levels off the Cascadia coast than 100 years of climate-driven sea-level rise. A great earthquake could cause 1–2 m of sea-level rise in some areas, which is significantly higher than the committee’s projection for Cascadia in 2100 (0.6 m). Further, the earthquake-induced sea-level rise would be an addition to the expected global warming-related sea-level rise.

CONCLUSIONS

Global projections are commonly made using ocean-atmosphere GCMs, which provide a reasonable representation of the steric contribution to global sea-level rise, but do not yet fully capture the cryospheric contribution. The IPCC (2007) projections made using this method are likely too low, even with an added ice dynamic component. Some studies project the cryospheric contribution by extrapolating current observations into the future, but the results depend on assumptions about the future behavior of the system. Semi-empirical methods avoid these difficulties by projecting global sea-level rise based on the observed relationship between sea-level change and global temperature. However, the highest projections made using this method (e.g., Grinsted et al., 2009) require unrealistically rapid acceleration of glaciological processes.

Given the strengths and weaknesses of the different projection approaches and the resource constraints of an NRC study, the committee chose to use GCMs developed for the IPCC Fourth Assessment Report to estimate the steric contribution and extrapolation techniques to estimate the cryospheric contribution. The contributions were then summed. The land hydrology component was assumed to be near zero and was not factored into the projection. The committee’s global projections for 2100 are substantially higher than the IPCC’s (2007) projection, mainly because of a faster growing cryosphere component, and are somewhat lower than the Vermeer and Rahmstorf (2009) projections. The committee estimates that global sea level will rise 8–23 cm by 2030, 18–48 cm by 2050, and 50–140 cm by 2100, relative to 2000 levels. As the projection horizon lengthens, the uncertainties grow, and hence the ranges widen. The major sources of uncertainty in the global projection are related to assumptions about the increase in rapid ice dynamics and the growth of future greenhouse gas emissions.

Formal projections of future sea-level rise along the west coast of the United States have not been made, although a few studies have presented ranges of possible outcomes for California and Washington. Methods vary but usually involve a combination of global models and local information. The committee’s projections account for factors that affect sea level in the area, including local steric variations; wind-driven differences in ocean heights; the gravitational and deformational effects associated with melting of Alaska, Greenland, and Antarctic glaciers; and vertical land motions along the coast. The local steric and wind-driven components were estimated by extracting northeast Pacific data from the same ocean models used for the global projections. The cryosphere component was adjusted for gravitational and deformational effects and then extrapolated forward. Vertical land motion was estimated using continuous GPS measurements.



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