The scenarios cover a wide range of driving forces, from demographic to social and economic developments. This section summarizes the assumptions on important scenario drivers. For simplicity, only three important driving forces are presented separately following the exposition in Chapters 2, 3, and 4. Nonetheless, it is important to keep in mind that the future evolution of these and other main driving forces is interrelated in the SRES scenarios (see Table 6.2a for a summary of the ranges of the main driving forces across the scenario groups in 2020, 2050, and 2100).

The SRES scenarios span a wide range of assumptions for the most salient scenario drivers, and thus reflect the uncertainty of the future. Evidently, views of the future are a time-specific phenomenon, and this report and its scenarios are no exception. However, it is important to emphasize that this is an explicit part of the Terms of Reference for the SRES writing team - to reflect a range of views, based on current knowledge and the most recently available literature (see Appendix I). The scenario quantification results reflect well the literature range, except for extreme scenarios.

6.2.1. Population Projections

Three different population trajectories were chosen for SRES scenarios to reflect future demographic uncertainties based on published population projections (Lutz, 1996; UN, 1998; see Chapter 3). The population projections are exogenous input to all the models used to develop the SRES scenarios. The models used do not develop population from other assumptions within the model. Figure 6-2 shows the three population projections in comparison with the three population projections used in the IS92 scenarios. Global population ranges between 7 and 15 billion people by 2100 across the scenarios, depending on the rate and extent of the demographic transition. The insert in Figure 6.2 shows population development in the industrialized (i.e., developed) regions. The range of future populations is smaller than in the IS92 scenarios, particularly in the industrialized regions, for which the lowest scenario indicates a very modest population decline compared to IS92 scenarios. The greatest uncertainty about future growth lies in the developing regions across all scenarios in the literature. An equally pervasive trend across all scenarios is urbanization (see Chapter 3). Altogether three different population projections were used in the 26 harmonized scenarios. Other scenarios explored alternative population projections consistent with the storylines.

The lowest population trajectory is assumed for the A1 and B1 scenario families and is based on the low population projection in Lutz (1996), which combines low fertility with low mortality and central migration rate assumptions. After peaking at 8.7 billion in the middle of the 21^st century, world population declines to 7.1 billion by the year 2100. As discussed in Chapters 3 and 4, this population development is somewhat higher than the previous low population used in the IS92 scenarios. The B2 scenario family is based on the UN median 1998 population projection (UN, 1998). The global population increases to about 9.4 billion people by 2050 and to about 10.4 billion by 2100. This population scenario is characteristic of recent median global population projections, which describe a continuation of historical trends towards a completion of the demographic transition that would lead to a level global population, and is consistent with recent faster fertility declines in the world together with declining mortality rates. Hence, the population is somewhat lower than previous UN median projections, as used in the IS92 scenarios. This median scenario projects very low population growth in today's industrialized countries, with stabilization of growth in Asia in the second half of the 21^st century and in the rest of the world towards the end of the 21^st century. The A2 scenario family is based on the high population growth of 15 billion by 2100 reported in Lutz (1996), which assumes a significant decline in fertility for most regions and a stabilization at above replacement levels. It falls below the long-term 1998 UN high projection of 18 billion. It is also lower than in the highest IS92 scenario (17.6 billion by 2100). Nevertheless, this scenario represents very high population growth compared with that in current demographic literature. Demographers attach a probability of more than 90% that actual population will be lower than the trajectory adopted in the A2 scenario family (Lutz et al., 1997). A more detailed discussion of the population projections used to quantify the four scenario families is given in Chapters 3 and 4.

Figure 6-2: Population projections - historical data from 1900 to 1990 (based on Durand, 1967; Demeny; 1990; UN, 1998), SRES scenarios (based on Lutz, 1996, for high and low, and UN, 1998, for medium), and IPCC IS92 scenarios (Leggett et al., 1992; Pepper et al., 1992) from 1990 to 2100.

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