2.3.3. What should be the Baseline for Comparison?
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.