IPCC Special Report on Emissions Scenarios

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6.3.1.2. Four Categories of Cumulative Emissions

This comparison of some of the SRES scenario characteristics implies that similar future emissions can result from very different socio-economic developments, and similar developments of driving forces can result in different future emissions. Uncertainties in the future development of key emission driving forces create large uncertainties in future emissions, even within the same socio-economic development paths. Therefore, emissions from each scenario family overlap substantially with emissions from other scenario families. Figure 6-6 shows this for CO₂ emissions. For example, comparison of the A1B and B2 marker scenarios indicates that they have similar emissions of about 13.5 and 13.7 GtC by 2100, respectively. The dynamics of the paths, however, are different so that they have different cumulative CO₂ emissions and different emissions of other GHG gases and SO₂ .

To facilitate comparisons of emissions and their driving forces across the scenarios, the writing team grouped them into four categories of cumulative emissions between 1990 and 2100. However, any categorization of scenarios based on emissions of multiple gases is quite difficult. Figure 6-7 shows total CO₂ emissions from all sources (from Figures 6-6a and b). Most of the scenarios are shown aggregated into seven groups, the four A1 groups and the other three families. The scenarios that remain outside the seven groups adopted alternative interpretations of the four scenario storylines. The emission trajectories ("bands") of the seven groups display different dynamics, from monotonic increases to non-linear trajectories in which there is a subsequent decline from a maximum. The dynamics of the individual scenarios are also different across gasses, sectors, or world regions. This particularly diminishes the significance of focusing scenario categorization on any given year, such as 2100. In addition, all gases that contribute to radiative forcing should be considered, but methods of combining gases such as the use of global warming potentials (GWP) are appropriate only for near-term GHG inventories⁴. In light of these difficulties, the classification approach presented here uses cumulative CO₂ emissions between 1990 and 2100. CO₂ is the dominant GHG and cumulative CO₂ emissions are expected to be roughly proportional to CO₂ radiative forcing over the time scale of a century. According to the IPCC SAR, "any eventual stabilised concentration is governed more by the accumulated anthropogenic CO₂ emissions from now until the time of stabilisation than by the way emissions change over the period" (Houghton et al., 1996). Therefore, the writing team also grouped the scenarios according to their cumulative emissions (see Figure 6.8).

This categorization can guide comparisons using either scenarios with different driving forces yet similar emissions, or scenarios with similar driving forces but different emissions. This characteristic of SRES scenarios also has very important implications for the assessment of climate-change impacts, mitigation, and adaptation strategies. Two future worlds with fundamentally different characteristic features, such as the A1 and B2 marker scenarios, also have different cumulative CO₂ emissions, but very similar CO₂ emissions in 2100. In contrast, scenarios that are in the same category of cumulative emissions can have fundamentally different driving forces and different CO₂ emissions in 2100, but very similar cumulative emissions. Presumably, adverse impacts and effective adaptation measures would vary among the scenarios from different families that share similar cumulative emissions, but have different demographic, socio-economic, and technological driving forces. This is another reason for considering the entire range of emissions in future assessments of climate change.

Figure 6-9 shows the histogram of cumulative CO₂ emissions from 1990 to 2100 for the SRES scenarios subdivided into the four emissions categories. Relative positions of the four marker scenarios, and the ranges of the four families and the six IS92 scenarios, are marked. The SRES scenarios have a bimodal structure similar to the distribution of cumulative emissions from the scenarios in the literature. The groups of SRES scenarios span the whole range of cumulative emissions from the scenarios in the literature. The range of the four markers is from 1000 to 1900 GtC. In comparison, the IS92 cumulative emissions range from 700 GtC for IS92c to 2140 GtC for IS92e.

The SRES emissions scenarios encompass emissions of other GHGs and chemically active species such as carbon monoxide, nitrogen oxides, and non-methane volatile organic compounds. The emissions of other gases follow dynamic patterns much like those shown in Figures 6-5 and 6-6 for CO₂ emissions. Further details of GHG emissions are given in Chapter 5.