Estimates of thermosteric sea-level rise for the upper 700 m of the ocean have lower uncertainties than estimates for the full ocean depth due to the paucity of deep-ocean measurements. Studies suggest that sampling problems cause a low bias in upper-ocean thermosteric sea-level rise estimates, and also make it difficult to assess the uncertainty in the deep-ocean thermosteric sea-level rise. Data assimilation and model results are not yet robust enough to be used to fill in missing data.
FIGURE 3.4 Glacier and ice sheet mass balance components. Ice accumulates at high elevations and is lost at lower elevations through melting, sublimation, or iceberg calving. The boundary between areas of net gain and loss is called the equilibrium line.
Loss of land-based ice is a major contributor to global sea-level rise, equal to or exceeding the contribution of thermal expansion. The equivalent of at least 65 m of sea level is stored in glaciers, ice caps, and ice sheets. The Greenland and Antarctic ice sheets store the equivalent of about 7 m and 57 m of sea level, respectively (Bamber et al., 2001; Zhang et al., 2003),1 and glaciers and ice caps store the equivalent of 0.6 ± 0.07 m, about one-third of which is around the periphery of Greenland and Antarctica (Radic and Hock, 2010).
The response of glaciers and ice sheets to climate change depends on processes acting at the upper surface; at the base, where glacial meltwater and the properties of the bedrock affect the rate of ice flow; and, in some locations, at the marine margin, where iceberg calving and melting occur (Figure 3.4). Glaciers, ice caps, and ice sheets typically gain mass through snow accumulation and lose mass through melting and runoff (ablation), iceberg calving, and, to a lesser extent, sublimation and wind erosion and transport. Calving can be the dominant mechanism of mass loss, accounting for 50–100 percent of the loss on the Antarctic Ice Sheet, about 50 percent of the loss on the Greenland Ice Sheet (Rignot and Jacobs, 2002; van den Broeke et al., 2010), and, where it has been measured, about 50 percent of loss from ocean-terminating ice cap complexes (Blaszczyk et al., 2009). In general, mass is gained at higher elevations and on the upper surface of a glacier or ice sheet, and mass is lost at lower elevations and at the base. The difference between accumulation and ablation is called the mass balance, and it is determined through a combination of in situ and satellite measurements (Box 3.2), often combined with models.
To determine the contributions of land ice to sea-level rise, mass balance estimates are converted to sea-level equivalent (SLE), the change in global average sea level that would occur if a given amount of water or ice were added to or removed from the oceans. SLE is computed by dividing the observed mass change of the ice by the surface area of the world’s oceans (362 × 106 km2). When working with changing ice volume (e.g., rates of iceberg flux), the volume is converted to mass using the density of ice (900 kg m3). Using these values, 1.11 km3 ice = 1 km3 of water = 109 kg water = 1 GT water, and 362 GT water = 1 mm SLE. For glacier ice resting on bedrock below sea level, a
1 See also data compiled for the Sea-level Response to Ice Sheet Evolution (SeaRISE) assessment project, http://websrv.cs.umt.edu/isis/index.php/SeaRISE_Assessment.