cause sea level to rise and semi-empirical models on the observed relationship between temperature and sea level—so it is not surprising that they do not agree. Moreover, the IPCC (2007) projections are likely underestimates because they do not account fully for cryospheric processes. The highest projections made by semi-empirical models (more than 2 m of sea-level rise) are likely overestimates because they would require unrealistically rapid acceleration of glaciological processes (Pfeffer et al., 2008).

An advantage of semi-empirical models is that, by parameter fitting, they reproduce the observed past sea-level rise. However, the simple empirical connection found for the past may not hold in the future. In particular, the ice sheets appear to have been negligible sea-level contributors during the observational periods used by Gornitz and Lebedeff (1987), Rahmstorf (2007), and Vermeer and Rahmstorf (2009), but ice sheet dynamic response is widely regarded as the most uncertain aspect of sea-level change. Indeed, some events, such as ice shelf melting triggering an instability of the West Antarctic Ice Sheet, would not be factored into semi-empirical models.

COMMITTEE PROJECTIONS OF GLOBAL SEA-LEVEL RISE

The committee was charged with projecting both the individual contributions to global sea-level rise (e.g., thermal expansion, melting of land ice) and the total global sea-level rise for the years 2030, 2050, and 2100 (Task 1, see Box 1.1). Given the state of knowledge and the limited time and computational capability available for a National Research Council study, the committee chose a combination of approaches for its projections. The output of GCMs was used to project the steric contribution (primarily thermal expansion) to global sea-level rise over the three time frames. For the land ice projections, the committee extrapolated mass balance estimates. Like the IPCC (2007), the committee did not project land hydrology contributions because uncertainties are too large, and a recent comprehensive assessment (Milly et al., 2010) found that the primary sources (groundwater depletion) and sinks (reservoir storage) appear to effectively cancel out. The individual components were then summed and compared with results from semi-empirical methods. The projections are for individual years (2030, 2050, and 2100, relative to 2000), and were derived using single-year values from low-order curves, except for the steric values, as explained below. The projections are given in Table 5.2 and discussed below.

Steric Contribution

The most recent GCM results for the steric contribution that were available to the committee were from the Coupled Model Intercomparison Project Phase 3 (CMIP3), which were used in the IPCC Fourth Assessment Report. Although outputs from a new generation of GCMs are beginning to be available, performing computations of derived quantities like global sea-level changes from these new outputs is beyond the charge and capability of the committee. Consequently, the committee drew on the work of Pardaens et al. (2010), who analyzed an ensemble of IPCC (2007) model projections using the A1B emission scenario (Figure 5.3). Drs. Pardaens and Gregory1 provided the gridded annual mean sea-level data used in their paper, and the committee analyzed the combine steric and ocean dynamic height data for the globe.

The models in Pardaens et al. (2010) yielded time series of annual mean sea level spanning roughly the 21st century: the first year in the various model simulations ranged from 2000 to 2004, and the final year was 2099. The committee performed a quadratic fit on each model’s time series at each grid point and, using the values on the quadratic curves, obtained steric sea-level changes for 2030, 2050, and 2100 relative to year 2000 for each model. The results are presented in the first row of Table 5.2.

Uncertainties

The committee endeavored to incorporate and describe as accurately as possible the known sources of uncertainty in the steric projections. These uncertainties are related to future greenhouse gas and aerosol emissions and concentrations (human forcing), the response of global temperatures to human forcing, and the response of the ocean to those global temperature distributions. The IPCC (2007) treated uncertainty in

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1 See <http://www.met.rdg.ac.uk/~jonathan/data/ar4_sealevel/>.



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