2.3.8. How can Vulnerability Assessments be Related to Policies for Reducing
One approach to mitigation policy is to evaluate targets for reducing GHG emissions
on the basis of reductions in vulnerability, rather than GHG concentrations
or similar indirect measures of dangerous climate change. By applying existing
methods for impact analysis, it is possible to invert the assessment procedure
and start with defined sets or windows of impacts that are judged to be tolerable
for humankind. This procedure results in emission corridors that embrace all
future GHG emissions that are compatible with changes defined to be tolerablethe
"safe landing" approach (WBGU, 1995; Alcamo and Kreileman, 1996; Petschel-Held
et al., 1999). This approach can be extended to include economic, social,
or equity aspectsthat is, to define tolerable windows for climate-related
facets of these sectors and obtain emission corridors that simultaneously satisfy
all possible windows (Toth et al., 1997).
Such approaches require that climate impacts should be differentiated between
smooth changes and thresholds that mark abrupt shifts in the system's functioning.
In the latter case, the definition of tolerable windows appears to be quite
obvious: Damaging, abrupt shifts should be avoided. In the case of smooth changes,
specification of tolerable windows is more difficult, confounded in part by
uncertainty about adaptation.
Normative decisions on tolerable windows must be a consultative process involving
scientists in close cooperation with stakeholders, decisionmakers, nongovernmental
organizations (NGOs), and others. There are various designs for this participatory
process, such as policy exercises (Toth, 1986, 1988a,b) or what is known as
the Delft Process (van Daalen et al., 1998). Nevertheless, specification
of windows remains somewhat arbitrary and preferably is used as an assumption
in an "if-then" analysis rather than as an ultimate specification.
Methodological challenges include development and validation of reduced-form
models, devising robust damage functions, identifying thresholds in adaptive
systems, and concerns for equity in relating the distribution of impacts to