3.3. Sea-Level Rise Scenarios
Sea-level rise scenarios are required to evaluate a diverse range of threats
to human settlements, natural ecosystems, and landscape in the coastal zone.
Relative sea-level scenarios (i.e., sea-level rise with reference to movements
of the local land surface) are of most interest for impact and adaptation assessments.
Tide gauge and wave-height records of 50 years or more are required, along with
information on severe weather and coastal processes, to establish baseline levels
or trends. Recent techniques of satellite altimetry and geodetic leveling have
enhanced and standardized baseline determinations of relative sea level over
large areas of the globe. [3.6.2]
Although some components of future sea-level rise can be modeled regionally
by using coupled ocean-atmosphere models, the most common method of obtaining
scenarios is to apply global mean estimates from simple models. Changes in the
occurrence of extreme events such as storm surges and wave setup, which can
lead to major coastal impacts, sometimes are investigated by superimposing historically
observed events onto a rising mean sea level. More recently, some studies have
begun to express future sea-level rise in probabilistic terms, enabling rising
levels to be evaluated in terms of the risk of exceeding a critical threshold
of impact. [3.6.3, 3.6.4,
3.6.5, 3.6.6]
3.4. Climate Scenarios
Three main types of climate scenarios have been employed in impact assessments:
incremental scenarios, analog scenarios, and climate model-based scenarios.
Incremental scenarios are simple adjustments of the baseline climate according
to anticipated future changes that can offer a valuable aid for testing system
sensitivity to climate. However, because they involve arbitrary adjustments,
they may not be realistic meteorologically. Analogs of a changed climate from
the past record or from other regions may be difficult to identify and are seldom
applied, although they sometimes can provide useful insights into impacts of
climate conditions outside the present-day range. [3.5.2]
The most common scenarios use outputs from general circulation models (GCMs)
and usually are constructed by adjusting a baseline climate (typically based
on regional observations of climate over a reference period such as 1961-1990)
by the absolute or proportional change between the simulated present and future
climates. Most recent impact studies have constructed scenarios on the basis
of transient GCM outputs, although some still apply earlier equilibrium results.
The great majority of scenarios represent changes in mean climate; some recent
scenarios, however, also have incorporated changes in variability and extreme
weather events, which can lead to important impacts for some systems. Regional
detail is obtained from the coarse-scale outputs of GCMs by using three main
methods: simple interpolation, statistical downscaling, and high-resolution
dynamical modeling. The simple method, which reproduces the GCM pattern of change,
is the most widely applied in scenario development. In contrast, the statistical
and modeling approaches can produce local climate changes that are different
from large-scale GCM estimates. More research is needed to evaluate the value
added to impact studies of such regionalization exercises. One reason for this
caution is the large uncertainty of GCM projections, which requires further
quantification through model intercomparisons, new model simulations, and pattern
scaling methods. [3.5.2,
3.5.4, 3.5.5]
|
