22.214.171.124 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.
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.
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.
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).