The construction of scenarios to investigate alternative future developments under a set of assumed conditions dates far back.

Scenarios are one of the main tools used to address the complexity and uncertainty of future challenges. The first scenarios were probably designed to help plan military operations, often called "war games." Today, scenarios are used regularly by military organizations around the world for training and planning purposes. Military strategists and teachers often use very sophisticated computer models to develop scenarios for a multitude of different purposes.

Scenarios are also increasingly used by enterprises around the world for many commercial purposes. Perhaps the most famous example is that of the Shell Group in the wake of the so-called oil crisis, which used scenarios to plan the corporate response strategies (Jefferson, 1983; Schwartz, 1991). Today, the use of scenarios is quite widespread.. Many scenarios, particularly those developed for enterprises in the energy sector, are quantitative and include GHG emissions. Recently, the World Business Council for Sustainable Development (WBCSD) presented a set of scenarios developed in collaboration with 35 major corporations (WBCSD, 1998). The SRES scenario database documents a number of such scenarios that are in the public domain and have been published.

During the past three decades many global studies have used scenarios as a tool to assess future CO₂ (and in a few cases also other GHG) emissions. One of the first such global studies was Energy in a Finite World, conducted by the International Institute of Applied Systems Analysis (IIASA) during the late 1970s (Häfele et al., 1981). Another influential series of scenarios that included the assessment of CO₂ emissions was developed by the World Energy Council (WEC, 1993). Recently, IIASA and WEC jointly presented a set of global and regional scenarios that were developed with a set of integrated assessment models and then reviewed and revised through 11 regional expert groups (Nakicenovic et al., 1998b). Another recent set of three scenarios, based on elaborate narrative stories that described alternative futures, was developed by the Global Scenario Group (Raskin et al., 1998) and received considerable attention.

Scenarios of future emissions played an important role from the beginning of the IPCC work. In 1990, the IPCC initiated the development of its first set of GHG emissions scenarios designed to serve as inputs to general circulation models (GCMs) and facilitate the assessments of climate-change impacts (Houghton et al., 1990). Two years later, in 1992, the IPCC approved six new emissions scenarios (IS92) that provided alternative emissions trajectories for the years 1990 through 2100 for such radiatively active gases as CO₂ , carbon monoxide (CO), methane (CH₄), nitrous oxide (N₂O), nitrogen oxides (NO_x), and sulfur dioxide (SO₂) (Leggett et al., 1992). They were widely used by atmospheric and climate scientists in the preparation of scenarios of atmospheric composition and climate change (Alcamo et al., 1995). In many ways, the IS92 scenarios were pathbreaking. They were the first global scenarios to provide estimates of the full suite of GHGs and at the time were the only scenarios to provide emissions trajectories for SO₂ . The IS92 scenarios are marked for reference in many of the illustrations herein that show the variation of emissions and their driving forces across the scenarios in the SRES database.

An important group of emissions scenarios included in this literature review was compiled from two international scenario and model comparison activities. This first group is from the IEW and involves structured comparisons of energy and emissions scenarios since 1981 (Manne and Schrattenholzer, 1996, 1997). The participating groups provide information for a standardized scenario poll from which the ranges and other sample statistics are reported for the main driving forces and emissions. The other group is the EMF (Weyant, 1993) and also involves regular scenario comparisons, in addition to standardized input assumptions, such as the international oil price or carbon emissions taxes. Both of these international scenario comparison activities provide a large share of the data for this scenario review and comparison. They include most of the global and regional emissions scenarios developed by formal modeling approaches. A large part of these activities is based on the use of scenarios for the purpose of climate-change research. A third scientific effort that involves scenario comparisons is the Energy Technology Systems Analysis Programme (ETSAP; Kram, 1993) supported by the International Energy Agency (IEA). The ETSAP work involves scenario analysis by more than 40 scientific groups from about 20 countries using the same modeling approach.

In addition to the many scientific, governmental, and private organizations throughout the world engaged in scenario-building, some international governmental organizations regularly develop global and regional scenarios that include GHG emissions. For example, the IEA regularly publishes global energy scenarios that include CO₂ emissions (IEA, 1998). Most of these scenarios are of shorter term and so are not suitable for the requirements of IPCC (see Chapter 1 for further details). Nevertheless, they are included in this assessment to facilitate a more comprehensive evaluation of emissions and their driving forces during the next few decades.

Some studies consider scenarios that involve explicit policies and measures to reduce emissions of GHGs or adapt to climate change. Such climate change intervention, control, or mitigation scenarios are an important tool for the assessment of policies and measures that would be required to reduce future GHG emissions. In this report, we use the terminology from the most recent IPCC evaluation of emissions scenarios (Alcamo et al., 1995). Those scenarios that include some form of policy intervention are referred to as intervention scenarios, while those that do not assume any climate policy measures, such as the 40 SRES scenarios, are referred to as non-intervention scenarios. In some cases, intervention scenarios go even further and investigate more radical emissions reductions required to stabilize atmospheric concentrations of these gases (in accordance with Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC, 1992)). In contrast, the SRES scenarios do not include any explicit additional climate policy initiatives in accordance with the Terms of Reference (see Appendix I).

The SRES writing team used a general approach to identify intervention scenarios. According to this approach, a scenario is identified as an intervention scenario if it meets one of the following two conditions:

• it incorporates specific climate change targets, which may include absolute or relative GHG limits, GHG concentration levels (e.g., CO₂ stabilization scenarios), or maximum allowable changes in temperature or sea level; and
• it includes explicit or implicit policies and/or measures of which the primary goal is to reduce GHG emissions (e.g., a carbon tax or a policy encouraging the use of renewable energy).

Note that this classification system is only a first step, and further work is needed to refine this taxonomy.

Some scenarios in the literature are difficult to classify as intervention or non-intervention, such as those developed to assess sustainable development. These studies consider futures that require radical policy and behavioral changes to achieve a transition to a sustainable development path; Greenpeace formulated one of the first (Lazarus et al., 1993). This class of scenarios describes low emissions futures that sometimes, but not always, result from specific climate policy measures. Such sustainable development scenarios are also included in this assessment of the scenario literature. Where they do not include the explicit policies of the SRES criteria, they can be classified as non-intervention scenarios. However, there is a great deal of ambiguity as to what constitutes policies directed at climate change, as opposed to those directed at achieving sustainable development in general. Thus, some of these sustainable development scenarios are "non-classified" (i.e., the information available is insufficient to determine whether or not the scenarios included any additional climate policy initiatives).

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