Emissions Scenarios - IPCC

An Overview of Scenarios 197
growth rate translates into a 25-fold expansion of global GDP
that would reach US$529 trillion by 2100.
As a byproduct of rapid economic development and fast
demographic transition, income inequities between industrial
and developing countries are virtually eradicated. Per capita
income ratios are 1:1.6 in 2100, compared to a ratio of 1:16 in
1990 in terms of the GDP/capita difference between cunent
developed (IND) and developing (DEV) regions across the four
SRES regions. However, even if relative income differences are
reduced drastically, absolute differences remain large, not least
because of the high incomes characteristic of the Al scenario
family (per capita income differences are also larger when
considered at a more disaggregated level). When measured
across the four SRES regions in 1990, income per capita
differences are nearly 1:40 (between ASIA and OECD90). Per
capita income differences are yet higher for differences across
countries or between different social strata. The poorest 20% of
Bangladesh's population, for instance, earn per capita incomes
that are a factor of 700 lower than that of the 20% richest Swiss
population (UNDP, 1993). A distinguishing feature of the Al
scenarios is to explore pathways of reductions in present
disparities. In AI, per capita income in industrial countries
(IND) increases to about US$107,300 and in now developing
countries (DEV) to US$66,500. Non-OECD GDP growth rates
rise to a peak of about 8% between 2010 to 2030 in scenario
Al, and decline once the industrial and infrastructural bases of
their economies are established. By and lai'ge, the Al scenario
implies a rephcation across all developing regions of the post-
World War II experience of Japan and Korea or the recent
economic development of China.
4.4.4.2. Harmonized and Other Al Scenarios
The high economic growth characteristics of the AIB marker
scenario reproduce well in the scenarios calculated with
different models. The AIB-MESSAGE scenario tracks the AIB
marker scenario closely at the global and regional levels ("fully
harmonized" input assumptions). At the global GDP level, most
growth trajectories agree within a range up to 15%, except the
Alv2-MiniCAM scenario (see Box 4-5).^^ Differences at the
regional level are larger. All models with comparable regional
aggregation levels agree well for ASIA (except Alv2-
MiniCAM). Differences in economic growth prospects also
agree well for OECD90 (except Alv2-MimCAM). For REF
the AIB scenarios group into two clusters - one group
reproduces the GDP growth scenario of the AIB marker, while
the other group suggests a GDP level by 2100 about one-third
lower than that of the AIB marker. Mostly this reflects
different assumptions used in the models on future labor
productivity growth that have not been harmonized with the
values adopted for the AIB marker scenario. For ALM, again
one group of scenarios tracks closely the AIB marker, whereas
other groups indicate either higher (AlB-ASF, AlB-
" The Alv2-MinCAM scenario reaches only about 64% of the AIB
marker global GDP in 2100.
MiniCAM) or lower (Alv2-MiniCAM, AIT-MARIA) GDP
growth. The reasons are similar to those discussed above. For
the SRES region REF, the different regional aggregations
across the models required complicated "inverse" calculations
on regional growth rates for harmonization with the respective
marker scenario at the level of the aggregated SRES region. For
ALM, such calculations were neither possible for all models
nor considered desirable by various modeling teams, which
prefeired to emphasize the inherent uncertainties in regional
economic growth perspectives even for an otherwise shared
vision of rapid global economic growth and development
catch-up.
4.4.4.3. A2 Scenarios
As compared to the other SRES scenario families, the A2
world is characterized by relatively slower productivity growth
rates and resultant lower per capita incomes (see Table 4-6).
Yet, the global average (1990-2100) growth rate in per capita
income of 1.3% is still somewhat higher than that observed
from 1970 to 1995 (1.2%; World Bank, 1998). The
comparatively conservative assumptions on per-capita-income
growth reflect both the more fragmented economic outlook of
the A2 storyline (see Section 4.3) and the slow pace of the
demographic transition that underlies A2's high population
growth trajectory. The fastest growth in per capita incomes (on
average over 2.3% per year) occurs in the ASIA region, while
the slowest growth is observed in the OECD90 region (on
average 1.0% per year). In a reversal of current short-term
trends, the REF and ALM regions experience a stable increase
of their per capita income levels over the 2P' century at a rate
that is almost twice as high as in the OECD90 region (see Table
4-7). The A2 world is also characterized by a slow convergence
of incomes among regions. Nonetheless, present income
disparities become nairower, from a factor of 40 difference in
1990 per capita income levels between the richest and the
poorest of the four SRES regions, to a factor of seven or eight
by 2100. The increase of global population from 6 to 15 billion
by 2100 translates into an increase of global GDP by a factor
of 12 over a century. The average (1990 to 2100) annual growth
rate of total GDP is 2.2%, which is lower than the 2.9%
average annual growth rate observed between 1970 and 1995
(World Bank, 1998) and the 4% rate observed from 1950 to
1990 (see Table 4-5).
4.4.4.4. Harmonized and Other A2 Scenarios
Four non-marker A2 scenarios (A2-AIM, A2-IMAGE with the
exception of 2050, A2-MiniCAM, and A2-MESSAGE) have
global GDPs within 5% of the A2 marker. The A2-A1-
MiniCAM scenario has much lower global GDP than the A2
marker, reflecting a different viewpoint on future labor
productivity growth (see Box 4-5) and a different interpretation
of the A2 scenario storyline altogether (see Box 4-6). While
this particular scenario illustrates important uncertainties with
respect to economic growth and development catch-up for
developing countries, it remains controversial within the
writing team, especially as to whether it reflects the overall