3.9. State of the Science and Future Needs for Scenario Development
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.
3.9.1. Well-Developed Features
Some features of scenario development and application are well established
- 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).
3.9.2. Deficiencies in Knowledge and Future Needs
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
- 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