Sea-level rise scenarios are constructed to assess climate change impacts and adaptations in the coastal zone. Variations in sea level are measured in two ways. Eustatic sea level represents the level of the ocean independent of land movements. Relative sea level is measured relative to the local land surface (Klein and Nicholls, 1998), so it consists of two components: eustatic sea-level change and local land movements. Climate modelers largely concentrate on estimating eustatic sea-level change, whereas impact researchers focus on relative sea-level change.

Based on historical tide gauge records and allowing for land movements, eustatic sea level has risen at an estimated rate of 1.0-2.0 mm yr^-1 during the past century (TAR WGI Chapter 11). This rate of sea-level rise is consistent with recent satellite altimeter data (Nerem et al., 1997), which directly measures eustatic variations in sea level. Tide gauge records are the main source of information on relative sea level; records are archived by the Permanent Service for Mean Sea Level (PSMSL) (Spencer and Woodworth, 1993). These records exhibit variations in interannual and multi-decadal variability (e.g., Delcroix, 1998; Bell et al., 1999; Nerem, 1999). The land surface forming the coastline at any point may be subsiding, static, or rising. Subsidence can be caused by tectonic movements, isostatic subsidence, compaction of sediments, or extraction of groundwater, oil, and/or gas. Uplift, as a result of postglacial isostatic rebound or tectonic processes, reduces or reverses relative sea-level rise. To allow for these influences, Douglas (1997) recommends that tide gauge records be at least 50 years in length before they are used to establish long-term trends or a nonstationary baseline.

Most studies of vulnerability to sea-level rise use the mean sea level at a reference date. For instance, studies employing the IPCC Common Methodology (WCC 1993, 1994) use the level in 1990 (Nicholls, 1995; Bijlsma, 1996). For more comprehensive assessments of coastal vulnerability, however, baseline time series of sea-level variability are required. These reflect tidal variations and the influences of water temperature, wind, air pressure, surface waves, and Rossby and Kelvin waves in combination with the effects of extreme weather events. Baseline information for coastal processes also may be necessary where the coastline is accreting, eroding, or changing in form as a result of previous environmental changes. Where an earlier climate or sea-level shift can be related directly to a response in coastal or adjacent marine processes, this may serve as a historical or palaeo-analog for assessment of future changes.

Table 3-6: Approximate chronology of IPCC process in relation to GCM simulations, their adoption in impact studies, and the development of IPCC emissions scenarios. Abbreviations follow: AGCM = atmospheric GCM with simple ocean; AOGCM = coupled atmosphere-ocean GCM; GHG = greenhouse gas; IS92 = IPCC emissions scenarios published in 1992 (Leggett et al., 1992); SRES = Special Report on Emissions Scenarios (Nakicenovic et al., 2000).
Date	IPCC Process	Working Group I GCM Simulations	Working Group II GCM-Based Scenarios used in Impact Studies	Working Group III Emissions Scenarios
1988-1990	First Assessment Report (FAR), 1990	Equilibrium high-resolution AGCM	Equilibrium low-resolution 2 x CO2	Scenarios A-D (A = Business-as-Usual)
1991-1992	FAR Supplement, 1992	Transient AOGCM cold start GHG-only (Scenario A emissions)	Equilibrium low-resolution 2 x CO2	IS92a-f
1993-1996	Second Assessment Report (SAR), 1996	Transient AOGCM warm-start GHG + aerosol (0.5 or 1% per year emissions)	Equilibrium low/high-resolution; transient cold-start	IS92a-f (modified)
1997-1998	Regional Impacts Special Report, 1998	Transient AOGCM ensemble/ multi-century control	Equilibrium low/high-resolution; transient cold-start/ warm-start	IS92a-f (modified)
1999-2001	Third Assessment Report (TAR), 2001	Transient AOGCM CO2-stabilization; SRES-forced	Transient warm-start; multi-century control and ensembles	SRES; stabilization

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