Emissions Scenarios - IPCC

300 Summary Discussions and Recommendations
18
16
14
2.5
OECD90&REF
IS92f
A2 (OASA)
-
2.0 /
- 1.5
B2 (Ш)
/'
Bl. Al (11 AS A) /' /
IS92a,b,e / /
IS92f
A2 (Í1ASA)
в
о
и
12
10
1.0
^ 1 1 1 1
IS92c,d / /
0.5
1900 1950 200П 2050 2100 / ' / '
lS92a,b,e
B2 (UN)
о
lô
Bl, Al (HASA)
Starting Point of à
lS92c,d
time horizon (1990)гГ
World
1 1 1
1900 1950 2000 2050 2100
Figure 6-2rPopulation 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.
6.2.2. Economie Development
The SRES scénarios span a wide range of future levels of
economic activity (expressed in gross world product). The Al
scenario family with a ("hannonized") gross world product of
US$529 trillion (all values in 1990 US dollars unless otherwise
indicated) in 2100 delineates the SRES upper bound, whereas
B2 with ("harmonized") US$235 trillion in 2100 represents its
lower bound. The range of gross world product across all
scenarios is even higher, from US$197 to US$550 by 2100.
Although the SRES scenarios span a wide range, still lower
and higher gross world product levels can be found in the
literature (see Chapters 2, 3, and 4). Uncertainties in future
gross world product levels are governed by the pace of future
productivity growth and population growth, especially in
developing regions. Different assumptions on conditions and
possibilities for development "catch-up" and for narrowing per
capita income gaps in particular explain the wide range in
projected future gross world product levels. Given a
qualitatively negative relationship between population growth
and per capita income growth discussed in Chapters 2 and 3,
uncertainties in future population growth rates tend to narrow
the range of associated gross world product projections. High
population growth would, ceteris paribus, lower per capita
income growth, whereas low population growth would tend to
increase it. This relationship is evident in empiric data - high
per capita income countries are generally also those that have
completed then demographic transition. The affluent live long
and generally have few children. (Exceptions are some
countries with small populations, high birth rates, and
significant income from commodity exports.) This relationship
between affluence and longevity again identities development
as one of the most important indicators of human well being.
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).
Two of the SRES scenario families, Al and Bl, explicitly
explore alternative pathways to gradually close existing
income gaps. As a reflection of uncertainty, development
"catch-up" diverges in terms of geographically distinct
economic growth patterns across the four SRES scenario
families. Table 6-1 summarizes per capita income for SRES
and IS92 scenarios for the four SRES world regions. SRES
scenarios indicate a smaller difference between the now
industrialized and developing countries compared with the
IS92 scenarios. This tendency toward a substantially narrower
income "gap" compared with the IS92 scenarios overcomes
one of the major shortcomings of the previous IPCC scenarios
cited in the literature (Parikli, 1992).
6.2.3. Structural and Technological Change
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.