3.6. Sea-Level Rise Scenarios
3.6.1. Purpose
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.
3.6.2. Baseline Conditions
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 |
| |
