Emissions Scenarios - IPCC

194 An Overview of Scenarios
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 m 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 2P' 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 Secfion 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 2P' century a pronounced shift
occurs in the population distribution, from ASIA to the ALM
region (specificaUy Africa).
4.4.3.3. Harmonized and Other A2 Scenarios
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 milUon
versus 706 million in the marker. An altemative demographic
interpretation at the global level was attempted in the
"transitional" A2-Al-MiniCAM scenario, in which the
implications of delayed development pattems are explored. In
this scenario global population is assumed to reach 10 billion
by 2050, and 12 billion by 2100 (see Box 4-6).
4.4.3.4. B2 Scenarios
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 2100^^. 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 2P'
century. The projection is consistent with recent demographic
data and scenarios; it reflects faster declines in world fertiUty
together with declining mortality rates. Hence, the scenario is
somewhat lower than previous UN median projections, as used
^' The SRES writing team gratefully acknowledges the assistance of
Thomas Büttner of the UN Population Division, New York, in
developing more detailed regional population projections based on
the UN 1998 medium projection, and in making these data available
to the SRES writing team in electronic form.
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 industiialized countries. In Asia,
population size stabilizes in the second half of the 2U' 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
scenaiio 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.
4.4.4. Economic Development
The SRES scenarios span a wide range of future economic
growth rates (Table 4-5) and resultant levels of economic
output. The Al scenario family, with a global GDP of US$520
to 550 trillion in 2100, defineates the SRES upper bound,
whereas the A2 and B2 scenarios, with a range of US$230 to
250 trillion in 2100, represent its lower bound. The ВI scenario
family is intermediary. Although the SRES scenarios span a
wide range, both lower and higher global GDP levels can be
found in the literature (see Chapter 2).
Uncertainties in future GDP levels are governed by the rates of
future productivity growth and population growth, especially
those in developing countries. Different assumptions on
conditions and possibilities for development "catch-up" and
for naiTower per capita income gaps in particular explain the
wide range in projected future economic growth rates. Given
the weak inverse relationship between population growth and
per capita income growth discussed in Chapter 2, uncertainties
in future population growth rates tend to restrict the range of
associated GDP projections. High population growth, all else
being equal, lowers per capita income growth, whereas low
population growth tends to increase this growth. This
relationship is evident in empiric data - high per capita income
countries are generally also those that have completed their
demographic transition. The affluent live long and generally
have few children. Notable exceptions are countries with small
populations and significant income from commodity exports.
Yet even assuming this relationship holds for an extended time
into the future, its quantification is subject to considerable
theoretic and empiric uncertainties (Alcamo et al., 1995).
As outUned above, two of the SRES scenarios explicitly
explore alternative pathways of the gradually closure of
existing income gaps. As a reflection of uncertainty, the
development "catch-up" diverges in terms of geographically
distinct economic growth pattems across the four SRES
scenario families, as summarized in Tables 4-5, 4-6, and 4-7.
The scenarios of rapid development and "catch-up" remain in