IPCC Special Report on Emissions Scenarios

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1.7. SRES Emissions Scenarios

Figure 1-3: Global energy-related and industrial CO2 emissions - historical development and future scenarios, shown as an index (1990 = 1). The median (50th), the 5th, 25th,75th and 95th 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). 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 "error" 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 CO2 emissions were about 6 GtC. The coverage of CO2 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.

1.7.1. Literature Review and Analysis

The first step in formulation of the SRES emissions scenarios was to review both the published scenario literature and the development of the scenario database accessible through the web site (www-cger.nies.go.jp/cger-e/db/ipcc.html). Chapters 2 and 3 give a more detailed description of the literature review and analysis. Figure 1-3 shows the global energy-related CO2 emission paths from the database as "spaghetti" curves for the period to 2100 against the background of the historical emissions from 1900 to 1990. These curves are plotted against an index on the vertical axis rather than as absolute values because of the large differences and discrepancies for the values assumed for the base year 1990. These sometimes arise from genuine differences among the scenarios (e.g., different data sources, definitions) and sometimes from different base years assumed in the analysis or in alternative calibrations 7. The differences among the scenarios in the specification of the base year illustrate the large genuine scientific and data uncertainty that surrounds emissions and their main driving forces captured in the scenarios. The literature includes scenarios with additional climate polices, which are sometimes referred to as mitigation or intervention scenarios. There are many ambiguities associated with the classification of emissions scenarios into those that include additional climate initiatives and those that do not. Many cannot be classified in this way on basis of the information available from the database. Figure 1-3 indicates the ranges of emissions in 2100 from scenarios that apparently include additional climate initiatives (designated as intervention 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. The range of the whole sample of scenarios has significant overlap with the range of those that cannot be classified and they share virtually the same median (15.7 and 15.2 GtC in 2100, respectively) but the non-classified scenarios do not cover the high part of the range. Also, the range of the scenarios that apparently do not include climate polices (non-intervention) has considerable overlap with the other two ranges (lower bound is higher) but with a significantly higher median (of 21.3 GtC in 2100).

Historically, gross CO2 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 95th 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 CO2 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 IS92a as a reference for scenario studies.



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