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Although each of the four SRES storylines can unfold only if certain values are emphasized more than others, no explicit judgments have been made by the SRES team as to their desirability or probability. Storylines in the literature, however, are often explicitly intended by their authors to have a positive or negative connotation, and sometimes explicitly include assumed dominant or preferred types of governance. Thus, the four SRES storylines are fundamentally different in this respect compared to many narrative scenarios in the underlying literature.
Two arguments are pertinent to the linkage between the scenario storylines and the underlying literature. First, a future regarded as negative by some people may be perceived as positive by others. Second, the storylines represent families of scenarios that can include both success and failure, depending on the perspective of the beholder. To quote Wilkerson (1995): "Like the real life from which they are drawn, the scenarios are mixed bags, at once wonderfully dreadful and dreadfully wonderful." Importantly, the "neutral" or "agnostic" character of SRES scenarios is an explicit departure from most of the underlying literature about storylines and narrative scenarios.
Another important departure and an innovation unique to the SRES approach is the use of the storylines in conjunction with multiple (six) modeling approaches to develop and formulate a set of quantifications or scenarios that are overall consistent with the underlying storylines. This approach provides a rigorous modeling test of the underlying logic and structure of the storyline, while the narrative aspect of the storyline provides a broader, descriptive context for better understanding and interpretation of the scenarios.
The A1 storyline is a case of rapid and successful economic development, in which regional average income per capita converge - current distinctions between "poor" and "rich" countries eventually dissolve. The primary dynamics are:
The transition to economic convergence results from advances in transport and communication technology, shifts in national policies on immigration and education, and international cooperation in the development of national and international institutions that enhance productivity growth and technology diffusion.
This may be the type of scenario best represented in recent literature (e.g., Shinn, 1985; UN, 1990; Schwartz, 1991; Peterson, 1994; Gallopin et al., 1997; Glenn and Gordon, 1997, 1999; Lawrence et al., 1997; Hammond, 1998; Raskin et al., 1998). Such scenarios are dominated by an American or European entrepreneurial, progress-oriented perspective in which technology, especially communication technology, plays a central role. Wilkerson (1995) designed various scenarios that share features with A1. They emphasize market-oriented solutions, high consumption of both tangible and intangible commodities, advanced technology, and intensive mobility and communication. In some examples of this type of scenario, high economic growth leads to shifts of economic power from traditional core countries to the current economic "periphery," as in the "Global Shift" scenario by CPB (1992) and de Jong and Zalm (1991). The Shell scenario "New Frontiers" (Shell, 1993) is also representative of this family. The IPCC Scenarios IS92a and IS92e are well-known examples of futures with high levels of economic growth (Leggett et al., 1992). IIASA and World Energy Council (WEC) jointly developed three High Growth Scenarios that share assumptions on rapid technological progress, liberalized trade markets, and rising income levels (Nakic�enovic� et al., 1998).
In the A1 scenario family, demographic and economic trends are closely linked, as affluence is correlated with long life and small families (low mortality and low fertility). Global population grows to some nine billion by 2050 and declines to about seven billion by 2100. Average age increases, with the needs of retired people met mainly through their accumulated savings in private pension systems.
The global economy expands at an average annual rate of about 3% to 2100, reaching around US$550 trillion (all dollar amounts herein are expressed in 1990 dollars, unless stated otherwise). This is approximately the same as average global growth since 1850, although the conditions that lead to this global growth in productivity and per capita incomes in the scenario are unparalleled in history. Global average income per capita reaches about US$21,000 by 2050. While the high average level of income per capita contributes to a great improvement in the overall health and social conditions of the majority of people, this world is not necessarily devoid of problems. In particular, many communities could face some of the problems of social exclusion encountered in the wealthiest countries during the 20 th century, and in many places income growth could produce increased pressure on the global commons.
Energy and mineral resources are abundant in this scenario family because of rapid technical progress, which both reduces the resources needed to produce a given level of output and increases the economically recoverable reserves. Final energy intensity (energy use per unit of GDP) decreases at an average annual rate of 1.3%. Environmental amenities are valued and rapid technological progress "frees" natural resources currently devoted to provision of human needs for other purposes. The concept of environmental quality changes in this storyline from the current emphasis on "conservation" of nature to active "management" of natural and environmental services, which increases ecologic resilience.
With the rapid increase in income, dietary patterns shift initially toward increased consumption of meat and dairy products, but may decrease subsequently with increasing emphasis on the health of an aging society. High incomes also translate into high car ownership, sprawling suburbia, and dense transport networks, nationally and internationally.
Several scenario groups considered in the A1 scenario family reflect uncertainty in the development of energy sources and conversion technologies in this rapidly changing world. Some scenario groups evolve along the carbon-intensive energy path consistent with the current development strategy of countries with abundant domestic coal resources. Other scenario groups intensify the dependence on (unconventional) oil and (in the longer-run) natural-gas resources5 . A third group envisages a stronger shift toward renewable energy sources and conceivably also toward nuclear energy. A fourth group (which includes the A1B marker scenario) assumes a balanced mix of technologies and supply sources, with technology improvements and resource assumptions such that no single source of energy is overly dominant. The implications of these alternative development paths for future GHG emissions are challenging: the emissions vary from the carbon-intensive to decarbonization paths by at least as much as the variation of all the other driving forces across the other SRES scenarios.
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