What are Scenarios and What is Their Role?
A scenario is a coherent, internally consistent, and plausible description
of a possible future state of the world. Scenarios commonly are required in
climate change impact, adaptation, and vulnerability assessments to provide
alternative views of future conditions considered likely to influence a given
system or activity. A distinction is made between climate scenarioswhich
describe the forcing factor of focal interest to the Intergovernmental Panel
on Climate Change (IPCC)and nonclimatic scenarios, which provide socioeconomic
and environmental "context" within which climate forcing operates.
Most assessments of the impacts of future climate change are based on results
from impact models that rely on quantitative climate and nonclimatic scenarios
Types of Scenarios
Socioeconomic scenarios can serve multiple roles within the assessment
of climate impacts, adaptation, and vulnerability. Until recently, they have
been used much more extensively to project greenhouse gas (GHG) emissions than
to assess climate vulnerability and adaptive capacity. Most socioeconomic scenarios
identify several different topics or domains, such as population or economic
activity, as well as background factors such as the structure of governance,
social values, and patterns of technological change. Scenarios make it possible
to establish baseline socioeconomic vulnerability, pre-climate change; determine
climate change impacts; and assess post-adaptation vulnerability.
Land-use and land-cover scenarios should be a major component of scenarios
for climate change impact and adaptation assessments. A great diversity of land-use
and land-cover change scenarios have been constructed. However, most of these
scenarios do not address climate change issues explicitly; they focus instead
on other issuesfor example, food security and carbon cycling. Large improvements
have been made since the Second Assessment Report (SAR) in defining current
and historic land-use and land-cover patterns, as well as in estimating future
scenarios. Integrated assessment models currently are the most appropriate tools
for developing land-use and land-cover change scenarios.
Environmental scenarios embrace changes in environmental factors other
than climate that will occur in the future regardless of climate change. Because
these changes could have an important role in modifying the impacts of future
climate change, scenarios are required to portray possible future environmental
conditions, such as atmospheric composition [e.g., carbon dioxide (CO2),
tropospheric ozone (O3), acidifying compounds, and ultraviolet (UV)-B
radiation]; water availability, use, and quality; and marine pollution. Apart
from the direct effects of CO2 enrichment, changes in other environmental
factors rarely have been considered alongside climate changes in past impact
assessments, although their use is increasing with the emergence of integrated
Climate scenarios of three main types have been employed in impact assessments:
incremental scenarios, analog scenarios, and climate model-based scenarios.
Of these, the most common 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.
Regional detail is obtained from the coarse-scale outputs of GCMs by using three
main methods: simple interpolation, statistical downscaling, and high-resolution
dynamic 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 the 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. Such research could facilitate future evaluation
of impacts in a risk assessment framework.
Sea-level rise scenarios are required to evaluate a diverse range of
threats to human settlements, natural ecosystems, and landscape in coastal zones.
Relative sea-level scenarios (i.e., sea-level rise with reference to movements
of the local land surface) are of the 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. Although some components of future sea-level rise
can be modeled regionally, 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 set-up, which can lead to major coastal impacts, sometimes are investigated
by superimposing historically observed events onto 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 that they
will exceed a critical threshold of impact.