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 scenarios—which 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 as inputs.

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 issues—for 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 assessment methods.

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.

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