IPCC Special Report on Emissions Scenarios

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9.1.3. Scenario Groups and Four Categories of Cumulative Emissions

This comparison of 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.

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 TS-8 shows total CO₂ emissions from all sources (from Figures TS-7a and b). The 40 scenarios are shown aggregated into six groups, three for the A1 family (that result by merging the two fossil-intensive groups A1C and A1G into one A1FI group as in the SPM). The scenarios that remain outside the six groups adopted alternative interpretations of the four scenario storylines. The emission trajectories ("bands") of the scenario 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 (GWPs) are appropriate only for near-term GHG inventories ⁶ . 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 stabilized concentration is governed more by the accumulated anthropogenic CO₂ emissions from now until the time of stabilization 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.

Cumulative SRES carbon emissions through to 2100 range from less than 800 GtC to more than 2500 GtC with a median of about 1500 GtC. To represent this range, the scenario classification uses four intervals as follows: less than 1100 GtC (low), between 1100 and 1450 GtC (medium-low), between 1450 and 1800 GtC (medium-high), and greater than 1800 GtC (high). Each CO₂ interval contains multiple scenarios and scenarios from more than one family. Each category also includes one of the four marker scenarios. Figure TS-9 shows how cumulative carbon emissions from the 40 SRES scenarios fit within the selected emission intervals (see Chapter 4 for further details). The 40 SRES scenarios extend the IS92 range toward higher emissions (SRES maximum of 2570 GtC compared to 2140 GtC for IS92), but not toward lower emissions. The lower bound for both scenario sets is just below 800 GtC.

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 A1B and B2 marker scenarios, also have different cumulative CO₂ emissions and radiative forcing, 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 and radiative forcing. 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.

9.2. Other Greenhouse Gases

Of the GHGs, CO₂ is the main contributor to anthropogenic radiative forcing because of changes in concentrations from pre-industrial times. According to Houghton et al. (1996) well-mixed GHGs (CO₂ ,CH₄ ,N₂O, and the halocarbons) induced additional radiative forcing of around 2.5 W/m ² on a global and annually averaged basis. CO₂ accounted for 60% of the total, which indicates that the other GHGs are significant as well. Whereas CO₂ emissions are by-and-large attributable to two major sources, energy consumption and land-use change, other emissions arise from many different sources and a large number of sectors and applications (e.g. see Table 5-3 in Chapter 5).

The SRES emissions scenarios also have different emissions for other GHGs and chemically active species such as CO, NO_x (nitrogen oxides), and NMVOCs. The uncertainties that surround the emissions sources of the other GHGs, and the more complex set of driving forces behind them are considerable and unresolved. Therefore, the models and approaches employed for the SRES analyses cannot produce unambiguous and generally approved estimates for different sources and world regions over a century. Keeping these caveats above in mind, Table TS-4 (see later) shows the emissions of all relevant direct and indirect GHGs for the four marker scenarios and, in brackets, the range of the other scenarios in the same family (or scenario groups for the A1 family). Chapter 5 gives further detail about the full range of GHG emissions across the SRES scenarios. The emissions of other gases follow dynamic patterns much like those shown in Figures TS-7 and TS-8 for carbon dioxide emissions. A summary of GHG emissions is given in Chapter 6 and further details in Chapter 5.