Emissions Scenarios - IPCC

Background and Overview 69
1900 1950 2000 2050 2100
Figure 1-3: Global energy-related and industrial COj emissions - historical development and future scenarios, shown as an
index (1990 = 1). The median (50*), the 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 occuiTcnce (e.g., the frequency distribution
of the scenarios may be influenced by the use of IS92a as a reference for many subsequent studies). The emissions paths
indicate a wide range of future emissions. The range is also large in the base year 1990 and is indicated by an "eiTor" bar. To
separate the variation due to base-year specification from different future paths, emissions are indexed for the year 1990, when
actual global energy-related and industrial COj emissions were about 6 GtC. The coverage of COj emissions sources may vary
across the 256 different scenarios from the database included in the figure. The scenario samples used vary across the time
steps (for 1990 256 scenarios, for 2020 and 2030 247, for 2050 220, and for 2100 190 scenarios). Also shown, as vertical bars
on the right of the figure, are the ranges of emissions in 2100 for scenarios from the literature that apparently include
additional climate initiatives (designated as "intervention" scenarios emissions range), those that do not ("non-intervention"),
and those that cannot be assigned to either of these two categories ("non-classified"). This classification is based on the
subjective evaluation of the scenarios in the database by the members of the writing team and is explained in Chapter 2. Data
sources: Morita and Lee, 1998a, 1998b; Nakicenovic et al., 1998.
Historically, gross COj emissions have increased at an average
rate of about 1.7% per year since 1900 (Nakicenovic et al.,
1996); if that historical trend continues global emissions would
double during the next three to four decades and increase more
than sixfold by 2100. Many scenarios in the database describe
such a development. However, the range is very large around
this historical trend so that the highest scenarios envisage more
than a sevenfold increase of global emissions by 2100 as
compared with 1990, while the lowest have emissions lower
than those of today. The median and the average of the
scenarios lead to about a threefold emissions increase over the
same time period or to about 16 GtC. This is lower than the
median of the IS92 set and is lower than the IS92a scenario,
often considered as the "central" scenario with respect to some
of its tendencies. However, the distribution of emissions is
asymmetric. The thin emissions "tail" that extends above the
95* percentile (i.e., between the six and tenfold increase of
emissions by 2100 compared to 1990) includes only a few
scenarios. The range of other emissions and the main scenario
driving forces (such as population growth, economic
development and energy production, conversion and end use)
for the scenarios documented in the database is also large and
comparable to the variation of CO^ emissions. Statistics
associated with scenarios from the literature do not imply
probability of occurrence or likelihood of the scenarios. The
frequency distribution of the database may be influenced by the
use of lS92a as a reference for scenario studies.
1.7.2. Narrative Storylines and Scenario Quantifications
Given these large ranges of future emissions and their driving
forces, there are an infinite number of possible alternative
futures to explore. The SRES scenarios cover a finite, albeit a
very wide, range of future emissions. To facilitate the process
of identifying alternative future developments, the writing team
decided to describe their scenarios coherently by narrative
storylines. The storylines describe developments in many
different economic, technical, environmental and social
dimensions. The main reasons for formulating storylines are to:
• help the writing team to think more coherently about
the complex interplay between scenario driving forces
within each and across alternative scenarios;