Emissions Scenarios - IPCC

An Overview of the Scenario Literature 95
2050, which indicates that the scenarios agree less about the
central estimated gross world product (Toi, 1995; Yohe, 1995).
For 2050 and 2100 the gross world products for the IS92a and
b scenarios are the same as the median for all scenarios
reviewed (Pepper et al., 1992). 148 different scenarios were
used to derive the histogram for the year 2100.
2.4.7. Population and Gross World Product Relationships
The scenarios in the database portray a weak relationship
between population and economic growth; the correlation is
slightly negative. Scenarios that lead to a very high gross world
product are generally associated with central to low population
projections, while high population projections do not lead to
the highest gross world product scenarios. At extremely high
levels of average global income the correlation is strongly
negative. The highest per capita incomes in 2100 - in the range
between US$30,000 and US$45,000 - are achieved with a lowto-medium
population growth.
Figure 2-7 illustrates some of the relationships between
population and gross world product in the scenarios. It
compares only 39 scenarios as information about population
and gross world product assumptions is available for only a few
scenarios. In most of these, global population transition is
achieved during the 2F' century and stabilization occurs at a
population between 10 to 12 billion people in the year 2100.
Generally, this is associated with relatively high levels of
economic development, in the range US$200-500 trillion in the
year 2100. Scenarios at the lower end of this scale are labeled
collectively as the "mid-range cluster," which includes all
IIASA-WEC scenarios (IIASA-WEC, 1995; Nakicenovic et ai,
1998b), IS92a and b (Pepper et ai, 1992), and AIM96 (Matsuoka
et ai, 1994). The two highest scenarios are labeled as the "extra
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Global Population (billions)
Figure 2-7: Gross world product and population growth,
historical development, 1950 to 1990, and scenarios in the
database to 2100, in trillion US dollars and billion people. All
endpoints of the curves correspond to 2100.
high growth" cases, namely IS92e (Pepper et al., 1992) and
IMAGE 2.1, Baseline-C (Alcamo and Kreileman, 1996).
One scenario, IS92f, shows high population growth (over 18
billion people by 2100) with comparatively low economic
growth (about the same level as the mid-range cluster of
scenarios, approximately US$300 trillion). At the other side of
the scale are the two IS92 variants (c and d (Pepper et ai, 1992»
with low population projections (about 6 billion people by 2100).
2.4.8. Primary Energy Consumption Ranges
Primary energy consumption is another fundamental
determinant of GHG emissions. Clearly, high energy
consumption leads to high emissions. However, what is more
important for emissions is the structure of future energy
systems. High carbon intensities of energy - namely high
shares of fossil energy sources, especially coal, in total energy
consumption - lead to scenarios with the highest COj
emissions. The primary energy paths of different scenarios are
compared here, and the issue of energy carbon intensity is
considered in the next section.
Figure 2-8 shows the primary energy consumption paths in the
scenarios and its historical development since 1900. It gives the
whole distribution of the 153 scenarios in the SRES database
that report primary energy consumption, the median, and the
95*, 75"', 25*, and 5* percentiles. As a result of the relatively
large differences in the base-year values, the primary energy
consumption paths are plotted as an index and spliced to the
historical data in 1990. In 1990, primary energy was about 370
EJ, including non-commercial energy (Nakicenovic et al.,
1996).
On average the global primary energy consumption has
increased at more than 2% per year (fossil energy alone has
risen at almost 3% per year) since 1900. Also, the short-term
trend from 1975 to 1995 shows a similar increase. In the
scenarios the average growth rates to 2100 range from 2.4%
per year to -0.1% per year, with a median value of 1.3% per
year.
For the full range of scenarios, the factor increase above the
1990 level is 0.9 to 10 by 2100.'° However, Figure 2-8
indicates that this full range includes a few noticeable outliers,
especially toward the high end of energy consumption levels.
The rest of the scenarios are grouped more closely together,
which compresses the range to a factor increase of about 1.5 to
7.5 times the 1990 level. The degree of clustering is discussed
in greater detail below.
Note that the highest scenario in the database reports 3400 EJ for
primary energy consumption by 2100. Relative to the base year of
this scenario (340 EJ), this level corresponds to a 10-fold increase.
However, relative to the base-year value including non-commercial
biomass (370 EJ), this level corresponds to a nine-fold increase only.