Climate Change 2001:
Working Group I: The Scientific Basis
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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.



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