3.2.2.3 Non-CO₂ greenhouse gas emissions

The emissions scenario chapter in the TAR (Morita et al., 2001) recommended that future research should include GHGs other than CO₂ in new scenarios work. The reason was that, at that time, certainly regarding mitigation, most of the scenarios literature was still primarily focused on CO₂ emissions from energy. Nevertheless, some multi-gas scenario work existed, including the SRES baseline scenarios, but also some other modelling efforts (Manne and Richels, 2001; Babiker et al., 2001; Tol, 1999). The most important non-CO₂ gases include: methane (CH₄), nitrous oxide (N₂O), and a group of fluorinated compounds (F-gases, i.e., HFCs, PFCs, and SF₆). Since the TAR, the number of modelling groups producing long-term emission scenarios for non-CO₂ gases has dramatically increased. As a result, the quantity and quality of non-CO₂ emissions scenarios has improved appreciably.

Unlike CO₂ where the main emissions-related sectors are few (i.e. energy, industry, and land use), non-CO₂ emissions originate from a larger and more diverse set of economic sectors. Table 3.2 provides a list of the major GHG emitting sectors and their corresponding emissions, estimated for 2000. Note that there is significant uncertainty concerning emissions from some sources of the non-CO₂ gases, and the table summarizes the central values from Weyant et al. (2006) which has been used in long-term multi-gas scenario studies of the EMF-21. To make the non-CO₂ emissions comparable to those of CO₂, the common practice is to compare and aggregate emissions by using global warming potentials (GWPs).

Table 3.2: Global Anthropogenic GHG Emissions for 2000 at sector level, as used in EMF-21 studies (MtCO₂-eq/yr).

Notes: ^a LUCF is Land-use change and forestry.

^b HFCs are used as substitutes for ODSs in a range of applications

Sources: Weyant et al, 2006.

The most important work on non-CO₂ GHG emissions scenarios has been done in the context of EMF-21 (De la Chesnaye and Weyant, 2006). The EMF-21 study updated the capability of long-term integrated assessment models for modelling non-CO₂ GHG emissions. The results of the study are illustrated in Figure 3.11.

Figure 3.11: Development of baseline emissions in the EMF-21 scenarios (left) and comparison between EMF21 and SRES scenarios (right).

Source: De la Chesnaye and Weyant , 2006; see also Van Vuuren et al., 2006b

Evaluating the long-term projections of anthropogenic methane emissions from the EMF-21 data shows a significant range in the estimates, but this range is consistent with that found in the SRES. The methane emission differences in the SRES are due to the different storylines. The differences in the EMF-21 reference cases are mainly due to changes in the economic activity level projected in key sectors by each of the models^[6]. This could include, for example, increased agriculture production or increased supply of natural gas and below-ground coal in the energy sector. In addition, different modelling groups employed various methods of representing methane emissions in their models and also made different assumptions about how specific methane emission factors for each economic sector change over time. Finally, the degree to which agricultural activities are represented in the models differs substantially. For example, some models represent all agricultural output as one large commodity, ‘agriculture’, while others have considerable disaggregation. Interestingly, the latter group of models tend to find slower emissions growth rates (see Van Vuuren et al., 2006b).

The range of long-term projections of anthropogenic nitrous oxide emissions is wider than for methane in the EMF-21 data. Note that for N₂O, base year emissions of the different models differ substantially. Two factors may contribute to this. First, different definitions exist as to what should be regarded as human-induced and natural emissions in the case of N₂O emissions from soils. Second, some models do not include all emission sources.

The last group of non-CO₂ gases are fluorinated compounds, which include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur hexafluoride (SF₆). The total global emissions of these gases are almost 450 MtCO₂-eq, or slightly over 1% of all GHG for 2000. While the emissions of some fluorinated compounds are projected to decrease, many are expected to grow substantially because of the rapid growth rate of some emitting industries (e.g. semiconductor manufacture and magnesium production and processing), and the replacement of ozone-depleting substances (ODSs) with HFCs. Long-term projections of these fluorinated GHGs are generated by a fewer number of models, but still show a wide range in the results over the century. Total emissions of non-CO₂ GHGs are projected to increase, but somewhat less rapidly than CO₂ emissions, due to agricultural activities growing less than energy use.