Emissions Scenarios

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1.7.2. Narrative Storylines and Scenario Quantifications

Given these large ranges of future emissions and their driving forces, there are an infinite number of possible alternative futures to explore. The SRES scenarios cover a finite, albeit a very wide, range of future emissions. To facilitate the process of identifying alternative future developments, the writing team decided to describe their scenarios coherently by narrative storylines. The storylines describe developments in many different economic, technical, environmental and social dimensions. The main reasons for formulating storylines are to:

  • help the writing team to think more coherently about the complex interplay between scenario driving forces within each and across alternative scenarios;
  • make it easier to explain the scenarios to the various user communities by providing a narrative description of alternative futures that goes beyond quantitative scenario features;
  • make the scenarios more useful, in particular to analysts who contribute to IPCC WGII and WGIII; the social, political and technological context described in the scenario storylines is all-important in analyzing the effects of policies either to adapt to climate change or to reduce GHG emissions; and
  • provide a guide for additional assumptions to be made in detailed climate impact and mitigation analyses, because at present no single model or scenario can possibly respond to the wide variety of informational and data needs of the different user communities of long-term emissions scenarios.

The writing team consciously applied the principle of Occam's Razor (i.e., the economy of thought, Eatwell et al., 1998). They sought the minimum number of scenarios that could still serve as an adequate basis to assess climate change and that would still challenge policy makers to test possible response strategies against a significant range of plausible futures. The team decided on four storylines, as an even number helps to avoid the impression that there is a "central" or "most likely" case. The writing team wanted more than two storylines to help to illustrate that the future depends on many different underlying dynamics; the team did not want more than four, as it wanted to avoid complicating the process by too many alternatives. The scenarios would cover a wide range of - but not all possible - futures. In particular, there would be no "disaster" scenarios. None of the scenarios include new explicit climate policies. The team decided to carry out sensitivity tests within some of the storylines by considering alternative scenarios with different fossil-fuel reserves, rates of economic growth, or rates of technical change.

The storylines describe developments in many different social, economic, technological, environmental and policy dimensions. The titles of the storylines have been kept simple: A1, A2, B1 and B2. There is no particular order among the storylines; they are listed in the alphabetic and numeric order:

  • The A1 storyline and scenario family describes a future world of very rapid economic growth, low population growth, and the rapid introduction of new and more efficient technologies. Major underlying themes are convergence among regions, capacity building and increased cultural and social interactions, with a substantial reduction in regional differences in per capita income. The A1 scenario family develops into four groups that describe alternative directions of technological change in the energy system 8
  • The A2 storyline and scenario family describes a very heterogeneous world. The underlying theme is self-reliance and preservation of local identities. Fertility patterns across regions converge very slowly, which results in high population growth. Economic development is primarily regionally oriented and per capita economic growth and technological change are more fragmented and slower than in other storylines.
  • The B1 storyline and scenario family describes a convergent world with the same low population growth as in the A1 storyline, but with rapid changes in economic structures toward a service and information economy, with reductions in material intensity, and the introduction of clean and resource-efficient technologies. The emphasis is on global solutions to economic, social, and environmental sustainability, including improved equity, but without additional climate initiatives.
  • The B2 storyline and scenario family describes a world in which the emphasis is on local solutions to economic, social, and environmental sustainability. It is a world with moderate population growth, intermediate levels of economic development, and less rapid and more diverse technological change than in the B1 and A1 storylines. While the scenario is also oriented toward environmental protection and social equity, it focuses on local and regional levels.

Figure 1-4: Schematic illustration of SRES scenarios. The four scenario "families" are illustrated, very simplistically, as branches of a two-dimensional tree. In reality, the four scenario families share a space of a much higher dimensionality given the numerous assumptions needed to define any given scenario in a particular modeling approach. The schematic diagram illustrates that the scenarios build on the main driving forces of GHG emissions. Each scenario family is based on a common specification of some of the main driving forces.

Figure 1-4 schematically illustrates the SRES scenarios. It shows that the scenarios build on the main driving forces of GHG emissions. Each scenario family is based on a common specification of the main driving forces. The four scenario families are illustrated, very simplistically, as branches of a two-dimensional tree. The two dimensions indicate global and regional scenario orientation and development and environmental orientation, respectively. In reality, the four scenarios share a space of a much higher dimensionality given the numerous driving forces and other assumptions needed to define any given scenario in a particular modeling approach..

After determining the basic features and driving forces for each of the four storylines, the teams began to model and quantify them. This resulted in 40 scenarios, each constituting an alternative interpretation and quantification of a storyline. All the interpretations and quantifications associated with a single storyline are called a scenario "family" (see also Box 1-2 on terminology and Chapter 4 for further details).

In all, six models were used to generate the 40 scenarios:

  • Asian Pacific Integrated Model (AIM) from the National Institute of Environmental Studies in Japan (Morita et al., 1994);
  • Atmospheric Stabilization Framework Model (ASF) from ICF Consulting in the USA (Lashof and Tirpak, 1990; Pepper et al., 1992, 1998; Sankovski et al., 2000);
  • Integrated Model to Assess the Greenhouse Effect (IMAGE) from the National Institute for Public Health and Environmental Hygiene (RIVM) (Alcamo et al., 1998; de Vries et al., 1994, 1999, 2000), used in connection with the Dutch Bureau for Economic Policy Analysis (CPB) WorldScan model (de Jong and Zalm, 1991), the Netherlands;
  • Multiregional Approach for Resource and Industry Allocation (MARIA) from the Science University of Tokyo in Japan (Mori and Takahashi, 1999; Mori, 2000);
  • Model for Energy Supply Strategy Alternatives and their General Environmental Impact (MESSAGE) from the International Institute of Applied Systems Analysis (IIASA) in Austria (Messner and Strubegger, 1995; Riahi and Roehrl, 2000); and the
  • Mini Climate Assessment Model (MiniCAM) from the Pacific Northwest National Laboratory (PNNL) in the USA (Edmonds et al., 1994, 1996a, 1996b).

These six models are representative of emissions scenario modeling approaches and different IA frameworks in the literature and include so-called top-down and bottom-up models.

The six models have different regional aggregations. The writing team decided to group the various global regions into four "macro-regions" common to all different regional aggregations across the six models. The four macro-regions (see Appendix III) are broadly consistent with the allocation of the countries in the United Nations Framework Convention on Climate Change (UNFCCC, 1997), although the correspondence is not exact because of changes in the countries listed in Annex I of the UNFCCC:

  • The OECD90 region groups together all countries that belong to the OECD as of 1990, the base year of the participating models, and corresponds to Annex II countries under UNFCCC (1992).
  • The REF region comprises those countries undergoing economic reform and groups together the East European countries and the Newly Independent States of the former Soviet Union. It includes Annex I countries outside Annex II as defined in UNFCCC (1992).
  • The ASIA region stands for all developing (non-Annex I) countries in Asia.
  • The ALM region stands for rest of the world and includes all developing (non-Annex I) countries in Africa, Latin America and the Middle East.

In other words, the OECD90 and REF regions together correspond to the developed (i.e., industrialized) countries while the ASIA and ALM regions together correspond to the developing countries. The OECD90 and REF regions are consistent with the Annex I countries in the Framework Convention on Climate Change, while the ASIA and ALM regions correspond to the non-Annex I countries.

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