5.1 Introduction

In this chapter emission estimates for radiatively important gases generated in 40 Special Report on Emission Scenarios (SRES) scenarios are present. These gases are carbon dioxide (CO₂), methane (CH₄ ), nitrous oxide (N₂O), nitrogen oxides (NO^x), carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs), sulfur dioxide (SO₂ ), chlorofluoro-carbons (CFCs) and hydrochlorofluorocarbons (HCFCs)¹, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆ ) (see Table 5-1). Emission estimates presented here span the interval from 1990 to 2100 at the global level and at the level of four SRES macro-regions (OECD90, REF, ASIA, and ALM; see Appendix IV). In addition, sulfur emission estimates are presented in the regional gridded format to assist in quantifying the effects at the local level. Links between emissions and the underlying driving forces presented in Chapter 4 are illustrated also.

As a result of differences in modeling and estimation approaches, base-year (1990) emission values in SRES scenarios developed using different models show substantial variation. To facilitate both use of the scenarios and comparisons across scenarios and families, base-year emissions in all 40 SRES scenarios were standardized using one common set of values (see Box 5-1). With a few clearly indicated exceptions, only standardized emission values are discussed in this chapter. A complete set of standardized emissions, along with other quantitative scenario information, is provided in Appendix VII.

The first section in this chapter presents a "roadmap" that serves as an orientation to the 40 SRES scenarios. The roadmap gives a simple taxonomy that compares input parameters, as represented by the four scenario families, and emission outputs, as represented by the 1990 to 2100 cumulative CO₂ emissions. The subsequent sections discuss in detail emissions of each gas over the next 100 years to 2100.

5.2. Roadmap to the Scenarios

A classification scheme is presented here to assist the reader in understanding the links between driving forces and scenario outputs. This scheme can also be used to help select appropriate scenarios for further analysis (see Chapter 6).

The SRES scenarios were developed as quantitative interpretations of the four alternative storylines that represent possible futures with different combinations of driving forces. These broad scenario families are broken down further into seven scenario groups², used here to classify the input driving forces (see also Table 4-20 in Chapter 4).

The scenario outputs of most interest are emissions of GHGs, SO₂ , and other radiatively important gases. However, the categorization of scenarios based on emissions of multiple gases is quite difficult. 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 inventories³. In addition, emission trajectories may display different dynamics, from monotonic increases to non-linear trajectories in which a subsequent decline from a maximum occurs. This particularly diminishes the significance of a focus on any given year, such as 2100. 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 considered (Houghton et al. 1996).

Total cumulative CO₂ emissions from the 40 SRES scenarios fall into the range from 773 to 2538 gigatonnes of carbon (GtC) with a median of 1509 GtC. To represent this range, the scenario classification uses four intervals:

• Figure 5-1: Global cumulative CO2 emissions in the 40 SRES scenarios, classified into four scenario families (each denoted by a different color code: A1, red; A2, brown; B1, green; B2, blue). Marker scenarios are shown with thick lines without ticks, globally harmonized scenarios with thin lines, and non-harmonized scenarios with thin, dotted lines. Black lines show percentiles, means, and medians for the 40 SRES scenarios. For numbers on the two additional illustrative scenarios A1FI and A1T see Appendix VII.

  Less than 1100 GtC (low).
• Between 1100 and 1450 GtC (medium-low).
• Between 1450 and 1800 GtC (medium-high).
• 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 5-1 shows how cumulative CO₂ emissions from the 40 SRES scenarios fit within the selected emission intervals.

Table 5-2 provides an overview of this scenario classification and links the scenario outcomes with factors that drive them, organized by family and scenario group (see also Table 4-20). The rows in Table 5-2 represent the emission categories, while the columns represent the scenario families. The analysis of Table 5-2 reveals two key results:

• Similar future cumulative CO₂ emissions may emerge from different sets of driving forces;
• Conversely, similar future states of the world with respect to socio-economic development may yield different outcomes in terms of cumulative CO₂ emissions.

Similar cumulative CO₂ emissions can be attained in very different social, economic, and technological circumstances. High emission levels of the A2 marker scenario (A2-ASF) are also attained in all the A1 family scenarios with high fossil-fuel use (A1C and A1G groups), for example in A1G-MiniCAM. Medium-high emissions are attained in most of the A1 group scenarios (including the A1B marker, A1B-AIM), but also in scenarios from the B2 scenario family with high fossil-fuel use (e.g., B2-ASF). Medium-low emissions, which are characteristic of the B2 family, including the B2 marker (B2- MESSAGE), are also attained in the A2-A1-MiniCAM scenario, which illustrates the transition between the A2 and A1 families. Finally, low emission levels result from almost all the B1 family scenarios (including the B1 marker, B1-IMAGE) as well as from scenarios that belong to the A1T high-technology scenario group (e.g., A1T-MARIA). In Table 5-2, italics are used for examples of scenarios within each emission category that illustrate alternative ways to achieve cumulative CO₂ emissions similar to those of the marker scenarios.

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