3.2.3.4. Time Horizons
The appropriate time horizon for socioeconomic scenarios depends on the use
to which they are put. Climate modelers often use scenarios that look forward
100 years or more. Socioeconomic scenarios with similar time horizons may be
needed to drive models of climate change, climate impacts, and land-use change.
However, policymakers also may wish to use socioeconomic scenarios as decision
tools in framing current policies for climate change adaptation. In this context,
time horizons on the order of 20 years may be more appropriate, reflecting the
immediate needs of decisionmakers.
Short-term socioeconomic scenarios can still be very uncertain. "Surprises"
such as economic slumps or booms, wars, or famines frequently occur in social
and economic systems. Over the course of 50-100 years, even the most basic
scenario drivers, such as population and aggregate economic activity, are highly
uncertain, and their future development can be projected with any credibility
only by using alternative scenarios. Moreover, technologies will have been replaced
at least once, and those in use 100 years hence could have unimagined effects
on climate sensitivity and vulnerability. Politically led developments in local,
regional, and international systems of governance also will unfold along unpredictable
paths.
3.2.3.5. Spatial Resolution
Global emissions scenarios form the framework for predicting climate change
and variability impacts at the national level. To assess vulnerability and adaptation
potential, national scenarios must account for biophysical and socioeconomic
impacts. The potential for autonomous adaptations must be understood, reflecting
the ability of nature and society to cope with climate change and climate variability.
Many of the impacts of climate change on the coping ability of human systems
are likely to be location-specific. Impact assessors therefore should make use
of local/regional scenarios, where appropriate, and be wary of generalizing
experiences from one location to another. Matching of regional scenarios may
be difficult, however—for example, if data on population and land use are
available at different levels of resolution.
3.2.3.6. Sectoral Scenarios
As illustrated in Table 3-1, scenario exercises
often make specific assumptions about individual sectors. These sectors usually
are chosen because they are considered particularly sensitive to climate change
(e.g., water, agriculture/food) or because they are important sources or sinks
for GHGs (e.g., energy, forestry). Detailed quantitative assumptions often are
made about levels of future economic activity or the price of key commodities,
which will influence adaptation strategies.
Formal modeling work generally is used to improve the detail, coherence, and
internal consistency of socioeconomic variables that are susceptible to quantification.
Expert judgment or stakeholder consultations may be used to build consensus
around the characterization of more subjective and less quantifiable variables
that relate to values and institutions. Stakeholder engagement also can provide
a wealth of local expertise about specific impacts and vulnerabilities.
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