IPCC Special Report on Emissions Scenarios

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6. Narrative Scenarios and Storylines

Given these large ranges of future emissions and their driving forces, there is an infinite number of possible alternative futures to explore. The SRES scenarios cover a finite, albeit a very wide range, of future emissions. The approach involved the development of a set of four alternative scenario "families" comprising 40 SRES scenarios subdivided into seven scenario groups. During the approval process of the SPM in March 2000 at the 5th Session of WGIII in Katmandu, it was decided to combine two of these groups into one, resulting in six groups. To facilitate the process of identifying and describing alternative future developments, each scenario family includes a coherent narrative part called a "storyline," and a number of alternative interpretations and quantifications of each storyline developed by six different modeling approaches. All the interpretations and quantifications of one storyline together are called a scenario family (see also Box 1-1 in Chapter 1 on terminology). Each storyline describes a demographic, social, economic, technological, environmental, and policy future for one of these scenario families. Storylines were formulated by the writing team in a process which identified driving forces, key uncertainties, possible scenario families, and their logic. Within each family different scenarios explore variations of global and regional developments and their implications for GHG and sulfur emissions. Each of these scenarios is consistent with the broad framework of that scenario family as specified by the storyline. Consequently, each scenario family and scenario group is equally sound. Chapters 4 and 5 give a more detailed description of the storylines, their quantifications, and the resultant 40 emissions scenarios.

The main reasons for formulating storylines are:

The SRES writing team reached broad agreement that there could be no "best guess" scenarios; that the future is inherently unpredictable and that views will differ on which of the storylines could be more likely. The writing team decided on four storylines: an even number helps to avoid the impression that there is a "central" or "most likely" case. The team wanted more than two in order to help illustrate that the future depends on many different underlying dynamics; the team did not want more than four, as they wanted to avoid complicating the process by too many alternatives. There is no "business-as-usual" scenario. Nor should the scenarios be taken as policy recommendations. The storylines represent the playing out of certain social, economic, technological, and environmental paradigms, which will be viewed positively by some people and negatively by others. The scenarios cover a wide range, but not all possible futures. In particular, it was decided that possible "surprises" would not be considered and that there would be no "disaster" scenarios that are difficult to quantify with the aid of formal models.

The titles of the storylines have been kept simple: A1, A2, B1, and B2. There is no particular order among the storylines; Box TS-1 lists them in alphabetic order. Figure TS-2 schematically illustrates the four storylines and scenario families. Each is based on a common specification of the main driving forces. They are shown, very simplistically, as branches of a two-dimensional tree. The two dimensions shown indicate the global-regional and the development-environmental orientation, respectively. In reality, the four scenario families 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. 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 within a given scenario family. For example, four scenario "groups" within the A1 scenario family were explored. As mentioned, two of these four groups that explore fossil-intensive developments in the energy system were merged in the SPM. Together with the other three scenario families this results in seven groups of scenarios effectively six equally sound groups after the merging of the two fossil-intensive groups in the SPM - that share common assumptions of some of the key driving forces and are thus not independent of each other.

Figure TS-2: Schematic illustration of SRES scenarios. The four scenario "families" are shown, 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. The A1 storyline branches out into four groups of scenarios to illustrate that alternative development paths are possible within one scenario family. Two of these groups were merged in the SPM.

All four storylines and scenario families describe future worlds that are generally more affluent compared to the current situation. They range from very rapid economic growth and technological change to high levels of environmental protection, from low to high global populations, and from high to low GHG emissions. What is perhaps even more important is that all the storylines describe dynamic changes and transitions in generally different directions. Although they do not include additional climate initiatives, none of them are policy free. As time progresses, the storylines diverge from each other in many of their characteristic features. In this way they allow us to span the relevant range of GHG emissions and different combinations of their main sources.

Box TS-1: The Main Characteristics of the Four SRES Storylines and Scenario Families
By 2100 the world will have changed in ways that are hard to imagine - as hard as it would have been at the end of the 19th century to imagine the changes of the 100 years since. Each storyline assumes a distinctly different direction for future developments, such that the four storylines differ in increasingly irreversible ways. Together they describe divergent futures that encompass a significant portion of the underlying uncertainties in the main driving forces. They cover a wide range of key "future" characteristics such as population growth, economic development, and technological change. For this reason, their plausibility or feasibility should not be considered solely on the basis of an extrapolation of current economic, technological, and social trends.

  • 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. Two of the fossil-intensive groups were merged in the SPM.
  • 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.

After determining the basic features and driving forces for each of the four storylines, the team began modeling and quantifying the storylines. This resulted in 40 scenarios, each of which constitutes an alternative interpretation and quantification of a storyline. All the interpretations and quantifications associated with a single storyline are called a scenario family (see Chapter 1 for terminology and Chapter 4 for further details).

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