The writing team decided to describe their scenarios coherently by narrative storylines, based on the futures and scenario literature, in order to help the user groups in interpreting the scenarios (see why storylines?). After the team had made some progress in reviewing historical demographic, economic and technological developments, they met to begin their development of scenario storylines. Each storyline is basically a short "history" of a possible future development, of (a combination of) key scenario characteristics. These descriptions are stylized and have been designed for the specific purposes of the web site (see writing team brief and terms of reference ). The storylines identify particular dynamics, visible in the world today, that might have important influences on future GHG emissions; they deliberately explore what might happen if political, economic, technical and social developments took a particular alternative direction at the global level; they also pay attention to regional differences and interactions, especially between developing and industrialized countries.

The storylines evolved over several writing team meetings as the historical review was developed and the first model runs were carried out. During these meetings many options were discussed. For example, participants asked: Should there be a "best guess", "business-as-usual" or "central" case? Should the storylines represent idealized worlds or more plausible futures? Should there be two, three, four, or more storylines? How should the storylines be characterized? Should the scenarios explore the full range of imaginable rates of economic growth, population growth, and technological change?

The group decided that there could be no "best guess"; that the future is inherently unpredictable and that views will differ on which of the storylines could be more likely. Nor should the scenarios be taken as policy recommendations. The storylines represent the playing out of certain economic, social and environmental paradigms, which will be viewed positively by some people and negatively by others. The writing team decided on four storylines: an even number helps to avoid the impression that there is a "central" or "most likely" case. They wanted more than two to help to illustrate that the future depends on many different underlying dynamics; they did not want more than four, as they wanted to avoid complicating the process by too many alternatives. The scenarios would cover a wide range - but not all - possible futures. In particular, there would be no "disaster" scenarios. Nevertheless, the team decided to carry out sensitivity tests within some of the storylines by considering alternative scenarios with differing fossil fuel reserves, rates of economic growth, or rates of technical change.

Many attempts have been made to capture the spirit of each storyline with a short and snappy title. However, no one title has been found to reflect adequately the complex mix of characteristics of any storyline. The storylines describe developments in many different economic, technical, environmental and social dimensions. The titles of the storylines have therefore been kept simple: A1, A2, B1 and B2.

A1 storyline and scenario family describes a future world of very rapid economic growth, low population growth and rapid introduction of new and more efficient technology. Major underlying themes are economic and cultural convergence and capacity building, with a substantial reduction in regional differences in per capita income. In this world, people pursue personal wealth rather than environmental quality.

A2 storyline and scenario family is a very heterogeneous world. The underlying theme is that of strengthening regional cultural identities, with an emphasis on family values and local traditions, high population growth, and less concern for rapid economic development.

B1 storyline and scenario family describes a convergent world with rapid change in economic structures, "dematerialization" and introduction of clean technologies. The emphasis is on global solutions to environmental and social sustainability, including concerted efforts for rapid technology development, dematerialization of the economy, and improving equity.

B2 storyline and scenario family is a world in which the emphasis is on local solutions to economic, social, and environmental sustainability. It is again a heterogeneous world with less rapid, and more diverse technological change but a strong emphasis on community initiative and social innovation to find local, rather than global solutions.

After determining the basic features and driving forces for each of the four storylines, the team began modeling and quantifying the storylines. In all, six models were used to generate many variant quantifications of the four storylines. Each model quantification of a storyline constitutes a scenario and all scenario variants of one storyline constitute a "scenario family". The models used were AIM (National Institute of Environmental Studies, Japan); ASF (ICF Kaiser, USA); IMAGE (RIVM, The Netherlands); MESSAGE (IIASA, Austria); MARIA (Science University of Tokyo, Japan), and MINICAM (PNNL, USA). These six models are representative of emissions scenario modeling approaches in the literature and include top-down and bottom-up models.

For purposes of presentation on the web site, however, the writing team has chosen one model run that best represents each scenario family. This model run is designated as a "marker scenario." There are thus four such marker scenarios on this web site.

The six models have different regional aggregation. 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 are consistent with the allocation of countries into different regions in the United Nations Framework Convention on Climate Change (UNFCCC):

1. OECD region groups together all countries belonging to the Organisation for Economic Cooperation and Development and corresponds to Annex II countries under UNFCCC;
2. REFs region stands for countries undergoing economic reform and groups together the East European countries and the Newly Independent States of the former Soviet Union, it corresponds to Annex I outside the Annex II countries.
3. Asia region stands for all non-Annex I countries in Asia; and
4. ROW region stands for rest of the world and includes all non-Annex I countries in Africa, Latin America and Middle East.

In other words, the OECD and REFs regions together correspond to the Annex I or developed countries, while the Asia and ROW regions together correspond to the non-Annex I, or developing countries.