13.1 Introduction
13.1.1 Definition and Nature of Scenarios
For the purposes of this report, a climate scenario refers to a plausible future
climate that has been constructed for explicit use in investigating the potential
consequences of anthropogenic climate change. Such climate scenarios should
represent future conditions that account for both human-induced climate change
and natural climate variability. We distinguish a climate scenario from a climate
projection (discussed in Chapters 9 and 10),
which refers to a description of the response of the climate system to a scenario
of greenhouse gas and aerosol emissions, as simulated by a climate model. Climate
projections alone rarely provide sufficient information to estimate future impacts
of climate change; model outputs commonly have to be manipulated and combined
with observed climate data to be usable, for example, as inputs to impact models.
To further illustrate this point, Box 13.1 presents a
simple example of climate scenario construction based on climate projections.
The example also illustrates some other common considerations in performing
an impact assessment that touch on issues discussed later in this chapter.
|
Box 13.1: Example of scenario construction.
Example of basic scenario construction for an impact study: the case
of climate change and world food supply (Rosenzweig and Parry, 1994).
Aim of the study
The objective of this study was to estimate how global food supply might
be affected by greenhouse gas induced climate change up to the year 2060.
The method adopted involved estimating the change in yield of major crop
staples under various scenarios using crop models at 112 representative
sites distributed across the major agricultural regions of the world.
Yield change estimates were assumed to be applicable to large regions
to produce estimates of changes in total production which were then input
to a global trade model. Using assumptions about future population, economic
growth, trading conditions and technological progress, the trade model
estimated plausible prices of food commodities on the international market
given supply as defined by the production estimates. This information
was then used to define the number of people at risk from hunger in developing
countries.
Scenario information
Each of the stages of analysis required scenario information to be provided,
including:
- scenarios of carbon dioxide (CO2) concentration, affecting crop growth
and water use, as an input to the crop models;
- climate observations and scenarios of future climate, for the crop
model simulations;
- adaptation scenarios (e.g., new crop varieties, adjusted farm management)
as inputs to the crop models;
- scenarios of regional population and global trading policy as an input
to the trade model.
To the extent possible, the scenarios were mutually consistent, such
that scenarios of population (United Nations medium range estimate) and
Gross Domestic Product (GDP) (moderate growth) were broadly in line with
the transient scenario of greenhouse gas emissions (based on the Goddard
Institute for Space Studies (GISS) scenario A, see Hansen et al., 1988),
and hence CO2 concentrations. Similarly, the climate scenarios were based
on 2xCO2 equilibrium GCM projections from three models, where the
radiative forcing of climate was interpreted as the combined concentrations
of CO2 (555 ppm) and other greenhouse gases (contributing about 15% of
the change in forcing) equivalent to a doubling of CO2, assumed to occur
in about 2060.
Construction of the climate scenario
Since projections of current (and hence future) regional climate from
the GCM simulations were not accurate enough to be used directly as an
input to the crop model, modelled changes in climate were applied as adjustments
to the observed climate at a location. Climate change by 2060 was computed
as the difference (air temperature) or ratio (precipitation and solar
radiation) of monthly mean climate between the GCM (unforced) control
and 2xCO2 simulations at GCM grid boxes coinciding with the crop
modelling sites (Figure 13.1b). These estimates
were used to adjust observed time-series of daily climate for the baseline
period (usually 1961 to 1990) at each site (Figure 13.1b,c).
Crop model simulations were conducted for the baseline climate and for
each of the three climate scenarios, with and without CO2 enrichment (to
estimate the relative contributions of CO2 and climate to crop yield changes),
and assuming different levels of adaptation capacity.
|
Figure 13.1: Example of the stages in the formation of a simple climate
scenario for temperature using Poza Rica (20.3° N, 97.3° W) as a
typical site used in the Mexican part of the Rosenzweig and Parry (1994)
study.
(a) Mean monthly differences (D) (2xCO2 minus control) of average temperature
(°C) as calculated from the control and 2xCO2 runs of the Geophysical
Fluid Dynamics Laboratory (GFDL) GCM (Manabe and Wetherald, 1987) for the
model grid box that includes the geographic location of Poza Rica. The climate
model spatial resolution is 4.4° latitude by 7.5° longitude.
(b) The average 17-year (1973 to 1989) observed mean monthly maximum temperature
for Poza Rica (solid line) and the 2xCO2 mean monthly maximum temperature
produced by adding the differences portrayed in (a) to this baseline (dashed
line). The crop models, however, require daily climate data for input.
(c) A sample of one year’s (1975) observed daily maximum temperature
data (solid line) and the 2xCO2 daily values created by adding the monthly
differences in a) to the daily data (dashed line). Thus, the dashed line
is the actual daily maximum temperature time-series describing future climate
that was used as one of the weather inputs to the crop models for this study
and for this location (see Liverman et al., 1994 for further details). |
We also distinguish between a climate scenario and a climate
change scenario. The latter term is sometimes used in the scientific literature
to denote a plausible future climate. However, this term should strictly refer
to a representation of the difference between some plausible future climate
and the current or control climate (usually as represented in a climate model)
(see Box 13.1, Figure 13.1a). A
climate change scenario can be viewed as an interim step toward constructing
a climate scenario. Usually a climate scenario requires combining the climate
change scenario with a description of the current climate as represented by
climate observations (Figure 13.1b). In a climate impacts
context, it is the contrasting effects of these two climates – one current
(the observed “baseline” climate), one future (the climate scenario) on the exposure unit1 that determines the impact of the climate change
(Figure 13.1c).
A treatment of climate scenario development, in this specific sense, has been
largely absent in the earlier IPCC Assessment Reports. The subject has been
presented in independent IPCC Technical Guidelines documents (IPCC, 1992, 1994),
which were briefly summarised in the Second Assessment Report of Working Group
II (Carter et al., 1996b). These documents, while serving a useful purpose in
providing guidelines for scenario use, did not fully address the science of
climate scenario development. This may be, in part, because the field has been
slow to develop and because only recently has a critical mass of important research
issues coalesced and matured such that a full chapter is now warranted.
The chapter also serves as a bridge between this Report of Working Group I
and the IPCC Third Assessment Report of Working Group II
(IPCC, 2001) (hereafter TAR WG II) of climate change impacts,
adaptation and vulnerability. As such it also embodies the maturation in the
IPCC assessment process – that is, a recognition of the interconnections
among the different segments of the assessment process and a desire to further
integrate these segments. Chapter 3 performs a similar
role in the TAR WG II (Carter and La Rovere, 2001) also
discussing climate scenarios, but treating, in addition, all other scenarios
(socio-economic, land use, environmental, etc.) needed for undertaking policy-relevant
impact assessment. Chapter 3 serves in part as the other
half of the bridge between the two Working Group Reports.
Scenarios are neither predictions nor forecasts of future conditions. Rather
they describe alternative plausible futures that conform to sets of circumstances
or constraints within which they occur (Hammond, 1996). The true purpose of
scenarios is to illuminate uncertainty, as they help in determining the possible
ramifications of an issue (in this case, climate change) along one or more plausible
(but indeterminate) paths (Fisher, 1996).
Not all possibly imaginable futures can be considered viable scenarios of future
climate. For example, most climate scenarios include the characteristic of increased
lower tropospheric temperature (except in some isolated regions and physical
circumstances), since most climatologists have very high confidence in that
characteristic (Schneider et al., 1990; Mahlman, 1997). Given our present state
of knowledge, a scenario that portrayed global tropospheric cooling for the
21st century would not be viable. We shall see in this chapter that what constitutes
a viable scenario of future climate has evolved along with our understanding
of the climate system and how this understanding might develop in the future.
It is worth noting that the development of climate scenarios predates the issue
of global warming. In the mid-1970s, for example, when a concern emerged regarding
global cooling due to the possible effect of aircraft on the stratosphere, simple
incremental scenarios of climate change were formulated to evaluate what the
possible effects might be worldwide (CIAP, 1975).
The purpose of this chapter is to assess the current state of climate scenario
development. It discusses research issues that are addressed by researchers
who develop climate scenarios and that must be considered by impacts researchers
when they select scenarios for use in impact assessments. This chapter is not
concerned, however, with presenting a comprehensive set of climate scenarios
for the IPCC Third Assessment Report.
