Working Group II: Impacts, Adaptation and Vulnerability

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3.6.3. Global Average Sea-Level Rise

The major components of average global sea-level rise scenarios are thermal expansion, glaciers and small ice caps, the Greenland and Antarctic ice sheets, and surface and groundwater storage (Warrick et al., 1996; TAR WGI Chapter 11). These phenomena usually are modeled separately. Using GCM output, the thermal component of sea-level rise has been estimated by Bryan (1996), Sokolov et al. (1998), and Jackett et al. (2000). Contributions from glaciers and ice sheets usually are estimated via mass-balance methods that use coupled atmosphere-ocean and atmosphere-ice relationships. Such studies include: for glaciers and the Greenland ice sheet, Gregory and Oerlemans (1998); for Greenland only, Van de Wal and Oerlemans (1997) and Smith (1998); for the Antarctic ice sheet, Smith et al. (1998); and for Greenland and Antarctica, Ohmura et al. (1996) and Thompson and Pollard (1997).

Simple models that integrate these separate components through their relationship with climate, such as the upwelling diffusion-energy balance model of Wigley and Raper (1992, 1993, 1995) used in Warrick et al. (1996), can be used to project a range of total sea-level rise. De Wolde et al. (1997) used a two-dimensional model to project a smaller range than in Warrick et al. (1996); the major differences were related to different model assumptions. Sokolov and Stone (1998) used a two-dimensional model to achieve a larger range. Some new estimates are presented in Section 3.8.2.

3.6.4. Regional Sea-Level Rise

Regional sea-level rise scenarios require estimates of regional sea-level rise integrated with estimates of local land movements. Currently there are too few model simulations to provide a range of regional changes in sea level, restricting most scenarios to using global mean values (de Wolde, 1999). An exception is Walsh et al. (1998), who produced scaled scenarios of regional sea-level rise for the Gold Coast of eastern Australia on the basis of a suite of runs from a single GCM. Because relative sea-level rise scenarios are needed for coastal impact studies, local land movements also must be estimated. This requires long-term tide gauge records with associated ground- or satellite-based geodetic leveling. Geophysical models of isostatic effects, incorporating the continuing response of the Earth to ice-loading during the last glaciation, also provide estimates of long-term regional land movements (Peltier, 1998; Zwartz et al., 1999).

3.6.5. Scenarios Incorporating Variability

Most impacts on the coast and near coastal marine environments will result from extreme events affecting sea level, such as storm surges and wave set-up. The magnitude of extreme events at any particular time is influenced by tidal movements, storm severity, decadal-scale variability, and regional mean sea level. These phenomena are additive. Because it is impossible to provide projections of all of these phenomena with any confidence, many assessments of coastal impacts simply add projections of global average sea level to baseline records of short-term variability (e.g., Ali, 1996; McDonald and O'Connor, 1996; McInnes and Hubbert, 1996; Lorenzo and Teixiera, 1997). Moreover, several coastal processes also are stochastic, and locally specific scenarios may have to be constructed for these (e.g., Bray and Hooke, 1997).

Table 3-7: Illustration of importance of some different feedback processes. Values are for the year 2100, obtained from a baseline scenario implemented in the IMAGE-2 integrated assessment model (adapted from Alcamo et al., 1998a). The no-feedbacks case excludes CO2 fertilization and accelerated ice melt and includes an intermediate adaptation level of vegetation.
Simulation [CO2]

Net Ecosystem
(Pg a-1)a

All feedbacks 737 6.5 2.8 43 41
No CO2 fertilization 928 0.1 3.6 52 39
Vegetation adapts immediately 724 7.0 3.1 45 40
No adaptation of vegetation 762 5.3 3.2 46 41
No land-use change 690 6.9 2.9 41 39
No feedbacks 937 0.0 3.5 29 45
No land-use change/no feedbacks 889 0.2 3.4 28 45
Range 690-937 0.0-7.0 2.8-3.6 28-52 39-45
a 1 Pg a-1 = 1015 grams per year.
b Percentage of vegetated area for which climate change induces a change of vegetation class.

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