Emissions Scenarios - IPCC

An Overview of the Scenario Literature 87
about a third (2 GtC) compared to the 1990 level. This range of
2-37 GtC in 2100 indicates the high degree of uncertainty with
respect to the level of future GHG emissions. The emissions
range is somewhat smaller, from 6.5 to 22 GtC, for the 25* and
75* percentiles, respectively. However, the statistics associated
with scenarios from the literature do not imply probability of
their occunence.
Some additional information about the range of future
emissions can be obtained by examining the results from
detailed analyses (see, e.g., Alcamo et al., 1996, 1998)
conducted by Nordhaus and Yohe (1983), Edmonds et al.
(1986), de Vries et al. (1994), and Mamie and Richels (1994).
These analyses confirm that the range is very lai'ge. The
analysis of Edmonds et al. (1986) shows a range of 87 to 2 GtC
by the year 2070 for the 95* and 5* percentiles, respectively;
the range is from 27 to 4 GtC for the 75* and 25* percentiles.
Nordhaus and Yohe (1983) estimated the range to be from 55 to
7 GtC for the 95* and 5"' percentiles, and from 27 to 12 GtC for
the 75* and 25* percentiles, respectively. In contrast, de Vries
et al. (1994) estimated a fundamentally smaller range of 24 to
11 GtC by 2050 for the 95* and 5* percentiles, respectively, but
they standardized some of the scenario assumptions, such as
population and economic growth, relative to a base case.
The SRES team also applied the criteria presented in Section
2.2 to identify intervention scenarios from among the CO,
emissions scenarios in the database. Of the 190 scenarios that
reported COj emissions through the year 2100, 62 were
classified as intervention scenarios and 88 as non-intervention
scenarios. For 40 scenarios the infonnation available was
insufficient to determine whether or not they included any
climate policies. These are referred to as non-classified
scenarios. The statistics for these three scenario samples are
given in Table 2-2. The medians for these samples are also
shown in Figures 2-1 с to 2-If The analysis indicates that many
of the 88 non-intervention scenarios are emulations of IS92
scenaiios, and many of these are emulations of IS92a. For
example, 42 of the 88 non-intervention scenarios were
produced by EMF-14, of which 25 are based on IS92a.
Figure 2-lc shows spaghetti cuives for the non-intervention
scenarios found in Table 2-2. In the year 2100 these scenarios
cover almost the same range (1.2 GtC to 59.4 GtC) as does the
entire sample of 190 scenarios (-2.1 GtC to 59.4 GtC). The
sample of non-intervention scenarios includes some that have
very low emissions, which suggests that emissions can be low
even in the absence of explicit climate intervention policies.
However, the non-intervention sample also has a higher median
60
1900 1950 2000 2050 2100
Figure 2-lc: Global energy-related and industrial CO2 emissions for 138 non-intervention scenarios identified from the SRES
database. Historical development and future scenarios are shown as absolute values in GtC. Median (50*), 5*, 25*, 75*, and
95* percentiles of the frequency distribution are shown. The statistics associated with scenarios from the literature do not
imply probability of occurrence (e.g., the frequency distribution of the scenarios may be influenced by the use of IS92a as a
reference for many subsequent studies). Again, the emissions paths indicate a wide range of future emissions. The actual
coverage of CO, emissions sources may vary across the 138 scenarios from the database included in the figure. The scenario
samples used vary across the time steps (for 1990 138 scenarios; for 2100 88 scenarios were analyzed). Data sources; Morita
and Lee, 1998; Nakicenovic et al, 1998a.