This chapter outlines the current practice of scenario development for climate impact, vulnerability, and adaptation assessment. Methods of scenario construction and application are evolving rapidly, so it is useful to identify which aspects are well developed and which aspects still are deficient.

Some features of scenario development and application are well established and tested:

• Extensive monitoring efforts and continued development of global and regional databases has improved the quality and consistency of baseline observational data required for some scenario exercises, even in some data-sparse regions.
• Many impact studies apply incremental scenarios to explore the sensitivity of an exposure unit to a range of climate futures; studies seldom rely exclusively on a single, model-based scenario.
• Estimates of long-term mean global changes are available and widely applied for a limited number of variables (e.g., population, economic development, CO₂ concentration, global mean temperature), based on projections produced by specialized international organizations or the use of simple models.
• A growing volume of information now is available to enable scientists to construct regional scenarios of many features of global change, even though uncertainties in most projections remain high. A notable example is the IPCC-DDC, which was established in 1998 to facilitate the timely distribution of a consistent set of up-to-date projections of changes in climate and related environmental and socioeconomic factors for use in climate impact and adaptation assessment. Some of the studies reported in this volume use scenarios derived from information held in the DDC (see, e.g., Table 3-5).

There are many shortcomings of current scenario development, but there also are promising new methods that may address these problems and require further attention. These include:

• Future socioeconomic, environmental, and land-use changes have not been represented satisfactorily in many recent impact studies and need to be integrated into the process of scenario development.
• Many impact studies fail to consider adequately uncertainties embedded in the scenarios they adopt. New techniques are emerging to explore the role of scenarios, conditional probabilities, and conditional forecasts in providing policy-relevant advice in impact assessments in an environment of high uncertainty.
• There is a mismatch between the time and space scales at which scenario information commonly is provided and the resolution at which it is required for impact assessments. Methods of obtaining higher resolution scenarios of global change from broad-scale projections are being actively developed and refined. However, in some regions of the world the coverage and availability of baseline global change data are still poor, which has hampered efforts at scenario development.
• Most global change scenarios consider long-term and broad-scale changes in mean conditions. Scenarios of changes in variability and the frequency of extreme events (climatic or nonclimatic) seldom are constructed because it is difficult to simulate such events and because they are complicated to formulate and explain. More research is required into methods of representing variability change in scenarios.
• Scenarios for impact studies lag new developments in climate modeling. There is a need to reduce this time lag to deliver up-to-date scenarios for impact assessment (e.g., constructing regional climate and sea-level scenarios by using outputs from AOGCM simulations that are based on SRES emissions scenarios).
• Few comprehensive scenarios have been developed to date for examining the consequences of stabilizing GHG concentrations at different concentrations, in line with Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC).
• Climate change mitigation conventionally has been treated separately from impacts and adaptation, except in some studies that use IAMs. However, these two methods of responding to climate change are inextricably linked, and this linkage should be reflected in scenarios. Efforts to develop the SRES scenarios with well-elaborated narratives and improved appreciation of important interactions in the climate system seem likely to generate greater consistency among scenarios.
• Few scenarios directly address adaptation, but existing scenario methods could be refined to do so (e.g., by combining scenarios of climate change with decision support and similar systems being used to foster adaptation under current climate variability).
• Improved guidance material and training is required in the construction of integrated global change scenarios (see, e.g., IPCC-TGCIA, 1999; Hulme et al., 2000), especially concerning the development of nonclimatic scenarios.
  

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