In this brief summary of the SRES scenarios, structural and technological changes are illustrated by using energy and land use as examples. These examples are characteristic for the driving forces of emissions because the energy system and land use are the major sources of GHG and sulfur emission.

Chapter 4 gives a more detailed treatment of the full range of emissions driving forces across the SRES scenarios.

Figure TS-4 illustrates that the change of world primary energy structure diverges over time. It shows the contributions of individual primary energy sources - the percentage supplied by coal, that by oil and gas, and that by all non-fossil sources taken together (for simplicity of presentation and because not all models distinguish between renewables and nuclear energy). Each corner of the triangle corresponds to a hypothetical situation in which all primary energy is supplied by a single source - oil and gas at the top, coal to the left, and non-fossil sources (renewables and nuclear) to the right. Historically, the primary energy structure has evolved clockwise according to the two "grand transitions" (discussed in Chapter 3) that are shown by the two segments of the "thick black" curve. From 1850 to 1920 the first transition can be characterized as the substitution of traditional (non-fossil) energy sources by coal. The share of coal increased from 20% to about 70%, while the share of non-fossils declined from 80% to about 20%. The second transition, from 1920 to 1990, can be characterized as the replacement of coal by oil and gas (while the share of non-fossils remained essentially constant). The share of oil and gas increased to about 50% and the share of coal declined to about 30%.

Figure TS-4 gives an overview of the divergent evolution of global primary energy structures between 1990 and 2100, regrouped into their respective scenario families and four A1 scenarios groups (three in the SPM) that explore different technological developments in the energy systems. The SRES scenarios cover a wider range of energy structures than the previous IS92 scenario series, which reflects advances in knowledge on the uncertainty ranges of future fossil resource availability and technological change.

In a clockwise direction, A1B, A1T, and B1 scenario groups map the structural transitions toward higher shares of non-fossil energy in the future, which almost closes the historical "loop" that started in 1850. The B2 scenarios indicate a more "moderate" direction of change with about half of the energy coming from non-fossil sources and the other half shared by coal on one side and oil and gas on the other. Finally, the A2 scenario group marks a stark transition back to coal. Shares of oil and gas decline while non-fossils increase moderately. What is perhaps more significant than the diverging developments in these three marker scenarios is that the whole set of 40 scenarios covers virtually all possible directions of change, from high shares of oil and gas to high shares of coal and non-fossils. In particular, the A1 scenario family covers basically the same range of structural change as all the other scenarios together. In contrast, the IS92 scenarios cluster into two groups, one of which contains IS92c and IS92d and the other the four others. In all of these the share of oil and gas declines, and the main structural change occurs between coal on the one hand and non-fossils on the other. This divergent nature in the structural change of the energy system and in the underlying technological base of the SRES results in a wide span of future GHG and sulfur emissions.

Figure TS-5 illustrates that land-use patterns are also diverging over time. It shows the main land-use categories - the percentages of total land area use that constitute the forests, the joint shares of cropland and energy biomass, and all the other categories including grasslands. As for the energy triangle in Figure TS-4, in Figure TS-5 each corner corresponds to a hypothetical situation in which land use is dedicated to a much greater extent than today to two of the three land-use categories - 40% to cropland and energy biomass and 20% to forests at the top, 60% to forests and 40% to other categories (including grasslands) to the left, and 80% to other categories (including grasslands) to the right.

In most scenarios, the current trend of shrinking forests is eventually reversed because of slower population growth and increased agricultural productivity. Reversals of deforestation trends are strongest in the B1 and A1 families. In the B1 family pasture lands decrease significantly because of increased productivity in livestock management and dietary shifts away from meat, thus illustrating the importance of both technological and social developments.

The main driving forces for land-use changes are related to increasing demands for food because of a growing population and changing diets. In addition, numerous other social, economic, and institutional factors govern land-use changes such as deforestation, expansion of cropland areas, or their reconversion back to forest cover (see Chapter 3). Global food production can be increased, either through intensification (by multi-cropping, raising cropping intensity, applying fertilizers, new seeds, improved farming technology) or through land expansion (cultivating land, converting forests). Especially in developing countries, there are many examples of the potential to intensify food production in a more or less ecological way (e.g. multi-cropping; agroforestry) that may not lead to higher GHG emissions.

Different assumptions on these processes translate into alternative scenarios of future land-use changes and GHG emissions, most notably CO₂ , methane (CH₄), and nitrous oxide (N₂O). A distinguishing characteristic of several models (e.g., AIM, IMAGE, MARIA, and MiniCAM) used in SRES is the explicit modeling of land-use changes caused by expanding biomass uses and hence exploration of possible land-use conflicts between energy and agricultural sectors. The corresponding scenarios of land-use changes are illustrated in Figure TS-5 for all SRES scenarios. In some contrast to the structural changes in energy systems shown in Figure TS-4, different land-use scenarios in Figure TS-5 appear to be rather model specific, following the general trends as indicated by the respective marker scenario developed with a particular model.

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