3.3.5.4 The role of non-CO₂ GHGs

As also illustrated by the scenario assessment in the previous sections, more and more attention has been paid since the TAR to incorporating non-CO₂ gases into climate mitigation and stabilization analyses. As a result, there is now a body of literature (e.g. Van Vuuren et al., 2006b; De la Chesnaye and Weyant, 2006; De la Chesnaye et al., 2007) showing that mitigation costs for these sectors can be lower than for energy-related CO₂ sectors. As a result, when all these options are employed in a multi-gas mitigation policy, there is a significant potential for reduced costs, for a given climate policy objective, versus the same policy when CO₂ is the only GHG directly mitigated. These cost savings can be especially important where carbon dioxide is not the dominant gas, on a percentage basis, for a particular economic sector and even for a particular region. While the previous sections have focused on the joint assessment of CO₂ and multi-gas mitigation scenarios, this section explores the specific role of non-CO₂ emitting sectors.^[15]

A number of parallel numerical experiments have been carried out by the Energy Modelling Forum (EMF-21; De la Chesnaye and Weyant, 2006). The overall conclusion is that economic benefits of multi-gas strategies are robust across all models. This is even true, despite the fact that different methods were used in the study to compare the relative contribution of these gases in climate forcing (see Section 3.3.3). The EMF-21 study specifically focused on comparing stabilization scenarios aiming for 4.5 W/m² compared to pre-industrial levels. There were two cases employed to achieve the mitigation target:

1. Directly mitigate CO₂ emissions from the energy sector (with some indirect reduction in non-CO₂ gases).

2. Mitigate all available GHG in costs-effective approaches using full ‘what’ flexibility.

In the CO₂-only mitigation scenario, all models significantly reduced CO₂ emissions, on average by about 75% in 2100 compared to baseline scenarios. Models still indicated some emission reductions for CH₄ and N₂O as a result of systemic changes in the energy system. Emissions of CH₄ were reduced by about 20% and N₂O by about 10% (Figure 3.26).

Figure 3.26: Reduction of emissions in the CO₂-only versus multi-gas strategies.

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

In the multi-gas mitigation scenario, all models found that an appreciable percentage of the emission reductions occur through reductions of non-CO₂ gases, which then results in smaller required reductions of CO₂. The emission reduction for CO₂ in 2100 therefore drops (on average) from 75% to 67%. This percentage is still rather high, caused by the large share of CO₂ in total emissions (on average, 60% in 2100) and partly due to the exhaustion of reduction options for non-CO₂ gases. The reductions of CH₄ across the different models averages around 50%, with remaining emissions coming from sources for which no reduction options were identified, such as CH₄ emissions from enteric fermentation. For N₂O, the increased reduction in the multi-gas strategy is not as large as for CH₄ (almost 40%). The main reason is that the identified potential for emission reductions for the main sources of N₂O emissions, fertilizer use and animal manure, is still limited. Finally, for the fluorinated gases, high reduction rates (about 75%) are found across the different models.

Although the contributions of different gases change sharply over time, there is a considerable spread among the different models. Many models project relatively early reductions of both CH₄ and the fluorinated gases under the multi-gas case. However, the subset of models that does not use GWPs as the substitution metric for the relative contributions of the different gases to the overall target, but does assume inter-temporal optimization in minimizing abatement costs, do not start to reduce CH₄ emissions substantially until the end of the period. The increased flexibility of a multi-gas mitigation strategy is seen to have significant implications for the costs of stabilization across all models participating in the EMF-21. These scenarios concur that multi-gas mitigation is significantly cheaper than CO₂-only. The potential reductions of the GHG price ranges in the majority of the studies between 30% and 85% (See Figure 3.27).

Figure 3.27: Reduction in GHG abatement price (%) in multi-gas stabilization scenarios compared to CO₂-only cases. Ranges correspond to alternative scenarios for a stabilization target of 4.5 W/m².

Data source: De la Chesnaye and Weyant, 2006

Finally, the EMF-21 research also showed that, for some sources of non-CO₂ gases, the identified reduction potential is still very limited (e.g. most agricultural sources for N₂O emissions). For long-term scenarios (and more stringent targets) in particular, identifying how this potential may develop in time is a crucial research question. Attempts to estimate the maximum feasible reductions (and the development of potential over time) have been made in Van Vuuren et al. (2007).