For the SRES scenario quantification three different population trajectories were chosen to reflect future demographic uncertainties based on published population projections (Lutz, 1996; UN, 1998; see Chapter 3). Global population ranges between 7 and 15 billion people by 2100 (Figure 4-5) depending on the speed and extent of the demographic transition. Fertility rates were assumed to converge to replacement levels (UN, 1998 medium scenario) in the B2 scenario or to below replacement levels in the A1 and B1 scenario families that adopt a variant of the low population scenario of Lutz (1996). The A2 scenario family is based on the high population projection described in Lutz (1996), which is characterized by heterogeneous fertility patterns that remain above replacement levels in many regions but nonetheless decline compared to current levels.
Across all scenarios, the concentration of future population growth and its uncertainty lies primarily in the developing countries. An equally pervasive trend across all scenarios is urbanization (see Chapter 3). Since the population trajectories are exogenous input to all the models used for this report, and are subject to a stringent harmonization criterion across different models, no further demographic variants to these are reported here, with exception of the A2 scenario family16.
The population trajectory assumed for the A1 and B1 scenario families is based on a variant of the low population projection reported in Lutz (1996), which combines low fertility with low mortality and central migration rate assumptions. As is the case for other population scenarios used in this report, it is well within the uncertainty ranges as discussed in the demographic literature and the UN (1998) long-range population projections. After peaking at 8.7 billion in the middle of the 21 st century, world population declines to 7.1 billion in the year 2100 (for comparison, the lowest UN Long Range projection indicates 5.6 billion by 2100; UN, 1998). As discussed in Chapter 3, the scenario population is somewhat higher than the previous low population scenario used in the IS92 scenario series, as only a combination of low fertility with high mortality rates could result in a global population as low as six billion people by 2100. Such a development is judged to be inconsistent with demographic theory (see Chapter 3) and to be inconsistent with the scenario storylines (see Section 4.2).
The pace of demographic transition in developing countries is fastest among all the SRES scenarios, reflecting the emphasis on social and educational development (scenario B1) and economic development (A1). The use of the same population projection for two SRES scenario families thus reflects different views of the driving forces of the demographic transition (with causality links in both directions), but it does not imply that such a demographic scenario is considered more likely compared to that of other projections. Better education and unproved social development, in particular concerning the role of women in society, lead the demographic transition (and consequently also economic development) in scenario B1. Accelerated rates of economic development and its required favorable social environment (education, reduction of income disparities, etc.) in turn lead the demographic transition in scenario A1. In both scenarios, low (infant) mortality rates are a necessary precondition to lower fertility rates (consistent with the Cairo targets of the UN Conference on Population and Development, discussed in Chapter 2). A distinguishing feature of the IIASA low population projection is the assumption of below replacement fertility levels, on the basis of the actual experience of industrial countries17. The implications of this trend are visible in both the absolute decline of global population (from its peak at 8.7 billion people in the middle of the 21 st century) and the significant population aging. In the long-term this trend affects not only the industrialized countries but also the currently developing countries (see the discussion in Chapter 3).
For A1 and B1 scenarios, regional population trajectories are (almost for all years) within the proposed 5% interval of their respective marker scenarios, except for two of the scenarios18.
For the A2 scenario family, future population levels are based on the high scenario (15 billion) reported in Lutz (1996)19, which is somewhat lower than the high population projection used in the previous IS92 scenarios (17.6 billion by 2100) for the reasons outlined in Chapter 3. The A2 population trajectory is the highest among the SRES scenarios, but well within the range of the UN long-range population projections20 and corresponding uncertainties estimated by demographers (see Chapter 3). For instance, Lutz et al. (1997) attach a probability of about 90% that actual world population will be lower (and 10% that it will be higher) than the value adopted for the A2 scenario family. Thus, the scenario represents well an upper bound of population growth scenarios found in the current scenario literature, although higher population scenarios exist in the demographic literature (see Chapter 3). As mentioned above, the SRES writing team is not in a position to attach any judgment concerning probability or likelihood to this or to any other demographic scenario. Population growth in the A2 world remains uninterrupted across all the SRES regions (Figure 4-5). The average global population growth rate over the 21st century is 0.96% per year, half that observed during the period between 1950 and 1990 (1.86%; UN, 1998). In the A2 marker, fertility rates vary considerably from one region to another; this reflects the regional orientation toward specific values, lifestyles, etc. described in the A2 scenario storyline (see Section 4.3). In the A2 world, in the year 2100 less than one-tenth of the world population lives in OECD90 countries, and toward the end of the 21st century a pronounced shift occurs in the population distribution, from ASIA to the ALM region (specifically Africa).
The A2 scenarios share, with one exception, the same global population trajectory, but some of them show variation in population across the four SRES regions because of differences in the regional breakdown of the underlying models. For example, the A2-IMAGE scenario has a smaller population in the REF region as compared with the marker, 405 million versus 706 million in the marker. An alternative demographic interpretation at the global level was attempted in the "transitional" A2-A1-MiniCAM scenario, in which the implications of delayed development patterns are explored. In this scenario global population is assumed to reach 10 billion by 2050, and 12 billion by 2100 (see Box 4-6).
The B2 marker scenario adopted the UN median 1998 population projection (UN, 1998), wherein global population increases to about 9.4 billion people by 2050 and to about 10.4 billion by 210021. The scenario is characteristic of recent median global population projections (see discussion in Chapter 3), and describes a continuation of historical trends toward a completion of the demographic transition in the 21st century. The projection is consistent with recent demographic data and scenarios; it reflects faster declines in world fertility together with declining mortality rates. Hence, the scenario is somewhat lower than previous UN median projections, as used in the previous IS92 scenario series. A distinguishing feature of the UN population projections is the assumption that, in the long-term, fertility levels converge toward replacement levels globally (see Chapter 3). Future population growth is assumed to be slow in today's industrialized countries. In Asia, population size stabilizes in the second half of the 21st century, and in the rest of the world population growth slows down toward the end of that century.
The UN median population projection is shared across all B2 scenario quantifications, although differences remain at the regional level. The different regional aggregations used across various models did not coincide with the regional aggregation of the original UN projection, which suggests that a more detailed regional breakdown of demographic projections is highly desirable for long-term global scenario studies.
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