of Cape Mendocino is experiencing mean subsidence, so vertical land motion contributes positively to relative sea-level rise.

Figure 5.9 shows the total regional sea level projected for the years 2030, 2050, and 2100, relative to year 2000, for a transect along the west coast. The shape of the curve is dominated by the change in vertical land motion at about 40° latitude from uplift in the north to subsidence in the south. The sea-level fingerprint effect reduces the projected sea levels along the entire coast and is most pronounced in Washington. The fingerprint effect has not been included in previous studies and projections of sea level for the west coast (e.g., Mote et al., 2008; Cayan et al., 2009; Tebaldi et al., 2012). The ocean components have little effect on the north-south gradient in projected sea-level change.

The committee’s projections for the west coast of the United States are significantly different from global projections (Figure 5.10). The difference is largest off the Washington coast, where sea-level fingerprint effects lower the height of the ocean surface and regional tectonics raises the height of the land surface, resulting in rates of relative sea-level rise that are substantially lower than the global mean. Off the California coast, where subsidence is lowering the land surface, the projected relative sea-level rise is slightly higher than the global mean. The committee’s projected values for California are somewhat lower than the Vermeer and Rahmstorf (2009) projections, which are being used by California state agencies on an interim basis for coastal planning (CO-CAT, 2010). For California and Washington, the committee’s projections fall within the range presented in Cayan et al. (2009) and Mote et al. (2008), respectively. The committee’s projected values for 2030 and 2050 also are comparable to those of Tebaldi et al., (2012), although the committee found a larger north-south difference in the magnitude of sea-level rise.

UNCERTAINTY

Projections of future sea-level rise carry numerous sources of uncertainty. This uncertainty arises from an incomplete understanding of the global climate system, the inability of global climate models to accurately represent all important components of the climate system at global or regional scales, a shortage of data at the temporal and spatial scales necessary to constrain the models, and the need to make assumptions about future conditions (e.g., population growth, technological developments, large volcanic eruptions) that drive the climate system. Although a systematic analysis of these uncertainties was beyond the ability of the committee, this report attempts to describe and combine the most important uncertainties. For the committee’s global sea-level rise projections, important uncertainties are associated with assumptions about the growth of concentrations of greenhouse gases and sulfate aerosol, which affect the steric contribution, and future ice loss rates and the effect of rapid dynamic response, which affect the land ice contribution. Additional, unquantified uncertainties arise from neglecting the terrestrial water component in the projections and from combining model-projected steric contributions with extrapolation-projected land ice contributions (e.g., model projections account for future emissions whereas extrapolations do not).

Regional projections carry additional uncertainties because more components are included and some components are estimated from global scale analyses. The uncertainties are larger for the committee’s projections for California, Oregon, and Washington than they are for the global projections, primarily because uncertainties in the steric component are larger at smaller spatial scales and because some of the additional components (e.g., vertical land motion) have relatively large uncertainties.

For both global and regional projections of sea-level rise, uncertainties grow as the projection period increases because the chances of the observations and models deviating from actual climate changes increases. Currently, all projection methods—including process-based numerical models, extrapolations, and semi-empirical methods—have large uncertainties at 2100. Although the actual value of sea-level rise will almost surely fall somewhere within these wide uncertainty bounds, confidence in specifying the exact value is relatively low. At short timescales, the models more closely represent the future climate system, so uncertainties are smaller and confidence is higher. Confidence in the committee’s projections is likely to be highest in 2030 and perhaps 2050, which are likely of greatest interest to coastal planners, engineers, and other decision makers tasked with planning for sea-level rise along the west coast of the United States.



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