ice sheet models with far more sophisticated dynamic capabilities have long been in use, they are difficult to drive in a realistic fashion with only climatic forcing variables, and are still not a feature of integrated atmosphere-ice-ocean models. Consequently, the IPCC (2007) treated ice sheets as fxed geographic features that could gain and lose mass through accumulation and ablation, but would not otherwise change size or undergo variations in flow. The IPCC (2007) attempted to account for rapid transfers of ice from land to ocean by scaling up certain components of the modeled results, shown in Table 5.1 under “Scaled-up ice sheet discharge.” However, the estimates were not based on physical models of ice sheet processes, and they were not included in the projections of global sea-level rise. The IPCC (2007) projections of the cryospheric contribution to sea-level rise are widely regarded as too low (e.g., Kerr, 2007; Pfeffer et al., 2008; van der Veen and IMASS, 2010; AMAP, 2011; Price et al., 2011).
FIGURE 5.1 Thermal expansion contribution to global sea-level rise calculated by a range of models for three emission scenarios: A1B, A2, and B1. SOURCE: Figure 10.31 from Meehl et al. (2007).
Extrapolation of Land Ice Contributions
As noted above, some aspects of the cryospheric system are not yet understood well enough to be confidently represented in physical models, and many of the observations needed to characterize boundary and initial conditions or other model parameters are not available. Consequently, some investigators use extrapolation methods to project the cryospheric contribution to sea-level rise. Extrapolations carry past and present-day observed rates of change forward in time at rates that remain constant or vary according to assumed rules.
A number of recent studies have projected the future contributions of land ice to sea-level change by extrapolating observed trends in ice loss rates. Meier et al. (2007) extrapolated loss rates for the Greenland and Antarctic ice sheets and for aggregate glaciers and ice caps, and estimated that land ice would contribute ca. 8–16 cm to sea-level rise by 2050 and 17–56 cm by 2100 under plausible future conditions. The lower estimate assumed that present-day loss rates continued unchanged in the future. The higher estimate assumed that the present-day loss rate continued to increase in the future. Future sea-level rise could be less than the lower estimate only if global loss rates actually decreased in the future, an unlikely outcome of most climate and mass balance and ice dynamics modeling. Whether the higher estimate, which was not proposed as a firm upper limit, bounds the true upper range of outcomes is uncertain.
Pfeffer et al. (2008) made extrapolations that were intended to constrain the upper limits of glacier