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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).
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