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:
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:
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:
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:
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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