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The main characteristics of the four SRES storylines and scenario families j Al A2 ,,� B1 B2 Storyline ^ "" Storyline Storyline Storyline Al F1 A A1B p B1 B2 Scenario Groups Illustrative I I Illustrative i Illustrative Illustrative Illustrative Illustrative Scenario i i Scenario Marker Marker Marker Marker Scenario Scenario Scenario Scenario OS HS OS HS OS HS OS HS OS HS OS HS 1 5 1 2 2 6 4 2 2 7 4 4 Number of Scenarios This figure shows a schematic illustration of SRES scenarios. Four qualitative storylines yield four sets of scenarios called "families ": Al, A2, Bl, and B2. Altogether 40 SRES scenarios have been developed by six modeling teams. All are equally valid with no assigned probabilities of occurrence. The set of scenarios consists of six scenario groups drawn from the four families: one group each in A2, B 1, B2, and three groups within the Al family, characterizing alternative developments of energy technologies: A1FI (fossil fuel intensive), A1B (Balanced), and A1T (predominantly non -fossil fuel). Within each family and group of scenarios, some share "harmonized" assumptions on global population, gross world product, and final energy. These are marked as "HS" for harmonized scenarios. "OS" denotes scenarios that explore uncertainties in driving forces beyond those of the harmonized scenarios. The number of scenarios developed within each category is shown. For each of the six scenario groups an Illustrative scenarios (which is always harmonized) is provided. Four illustrative marker scenarios, one for each scenario family, were used in draft form in the 1998 SRES open process and are included in revised form in this Report. Two additional illustrative scenarios for the groups A1FI and A1T are also provided and complete a sex of six that illustrates all scenario groups. All are equally sound. By 2100 the world will have changed in ways that are difficult to imagine — as difficult 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 under- lying uncertainties in the main driving forces. They cover a wide range of key future characteristics such as demographic change, 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 Al Storyline and scenario family describes a future world of very rapid economic growth, global population that peaks in mid - century and declines thereafter, 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 Al scenario family develops into three groups that describe alternative directions of technological changes in the energy system. The three Al groups are distinguished by their technological emphasis: fossil intensive (A1FI), non - fossil energy sources (A1T ), or a balance across all sources (A1B)'. • 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 continuously increasing global population. Economic development is primarily regionally oriented and per capita economic growth and technological change are more fragmented and slower than in other storylines. • The B 1 Storyline and scenario family describes a convergent world with the same global population that peaks in mid - century and declines thereafter, as in the Al Storyline, but with rapid changes in economic structures toward a service and information economy, with reduction 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 continuously increasing global population at a rate lower than A2, intermediate levels of economic develop- ment, and less rapid and more diverse technological change than in the BI and Al storylines. While the scenario is also oriented toward environmental protection and social equity, it focuses on local and regional levels. 'Balanced is defined as not relying too heavily on one particular energy source, on the assumption that similar improvement rates apply to all energy and end use technologies. Figure 1 -1. Schematic illustration of emissions scenarios and storylines from the Special Report on Emissions Scenarios (Source: Nakicenovic et al. 2000). These scenarios are used to make predictions about future green- house gas emissions, which are then incorporated into projections of future global warming.