Climate change impacts generally are agreed to be the difference between conditions with and without climate change. However, there is controversy among researchers about how to set the baseline for estimating impacts (or evaluating adaptation).
Most studies apply scenarios of future climate change but estimate impacts on the basis of current environmental and socioeconomic baselines. Although this approach is expedient and provides information about the sensitivity of current systems, it skirts the issue of evolving sensitivity to climatic variations (Parry and Carter, 1998). Even without climate change, the environment and societal baselines will change because of ongoing socioeconomic development and, with climate change, because of system responses and autonomous adaptation (e.g., as described for BangladeshWarrick and Ahmad, 1996). Strictly speaking, the effects of climate change should be evaluated by taking the moving baseline into account (further discussion on socioeconomic, climate, and sea-level rise scenarios appears in Chapter 3).
Given the uncertainty of the future and the complexity of the various driving forces affecting any given exposure unit, a wide range of different assumptions about future baselines is plausible. The emission scenarios in the Special Report on Emissions Scenarios (SRES) reflect this perspective and are based on multiple projections of "alternative futures" (see Chapter 3). Framing local concerns for adaptation to changing risks may require exploratory scenarios, extending the coarse driving forces inherent in the SRES suite. For example, coping with water shortages in Bangladesh is sensitive to scenarios of regional collaboration with India and Nepal (e.g., Huq et al., 1999). For vulnerability and adaptation assessment, there is little apparent consistency regarding elements or procedures for development of these future baselines, including who is exposed, how to select sensitive sectors, and the drivers of social and institutional change at the scale of stakeholders exposed to climate impacts.
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