Figure 1a shows 2040 and 2050 changes in global radiative forcing for each scenario relative to each model’s reference scenario using central climate model parameters (see SM for 2030 and 2050). As found in previous work (Smith and Mizrahi 2013; Rogelj et al. 2014b), the major SLCF contributor to radiative forcing changes is methane reductions (e.g., the CH4-Only scenario). The BCOC-EndU scenario, focused on BC reductions, results in a net reduction in most models; however, they are significantly smaller than for CH4-Only. There is little sectoral or species overlap in the CH4- and BC-reduction scenarios, with the sum of CH4-Only and BCOC-EndU forcing reductions close to the reductions in combined SLCF scenario.
The reduction in total forcing from the SLCF scenario is similar to that from the ClimPolicy scenario in 2030 (see SM), but from 2040 onward, the ClimPolicy scenario results in much larger forcing reductions. There is considerable overlap in terms of sectoral and emission species reduction between the ClimPolicy and SLCF scenarios, which will be examined in the temperature change section below.
We split our analysis of forcing changes into consideration of the direct forcing changes, which are the forcing changes from the targeted emission species, and auxiliary reductions, which are defined as the sum of all other forcing changes that occur in other species (see SM). We now examine results for the CH4 and BC emission reduction scenarios below.
In the CH4-Only scenario, a methane price is phased in from 2015 to 2030 and has been set to a magnitude sufficient to induce near-maximal reductions in methane emissions (SM §A). The methane emission reductions in the CH4-Only scenario result in an average 2040 CH4 forcing a reduction of 0.23 W/m2, with relatively little scatter other than one model (Fig. 2). Further analyses of methane emission reductions are provided in Harmsen et al. (2019a), although see SM for a note on differences in the CH4 scenario definition between these two works.
There are also significant auxiliary forcing reductions that are in addition to the change in CH4 forcing itself. The largest are reductions in CO2 and tropospheric ozone forcing (Fig. 2). Neglecting one outlier model whose results in this case are due to a specific model configuration, total auxiliary forcing reductions amplify the direct CH4 emission reductions by 30–50% in 2040 and 35–70% in 2050. The MESSAGE model is an outlier here due to the inclusion of land-use-related CO2 emission reductions in the CH4 reduction case. (see discussion in SM.)
The auxiliary CO2 reductions vary across the models with one group of models showing relatively small reductions (< 0.02 W/m2 in 2040) with the second group showing much larger reductions, which range from 15 to 20% of the direct reduction in CH4 forcing in 2040, and 20–50% in 2050. The relative impact of these auxiliary CO2 reductions increases with time as CO2 concentration changes accumulate in the atmosphere.
There are two sets of mechanisms for these auxiliary effects. The first set of mechanisms are economic auxiliary effects, which are reductions due to economic feedbacks and technology shifts induced by the CH4 emission reduction policy. In some models, the methane emission price has an economic impact on the cost of activities that produce methane. As the methane price increases, those activities become more expensive and their consumption is reduced, which also can reduce other emissions such as CO2. For example, reductions in (1) fossil fuel and (2) beef production, two sectors with substantial methane emissions, would tend to reduce net CO2 emissions through reducing (1) fossil fuel consumption and (2) land-use change. The strength of these reductions will depend on model structure and the extent to which the CH4 price is passed through to consumers. Note that in some cases, these economic feedbacks can be a modeling choice and in other cases, it is inherent to the structure of the model (see model descriptions in the SM).
In four of the models, there are limited or no economic feedbacks from a methane reduction policy. This can be thought of as a CH4 policy formulation where CH4 reductions are incentivized and this is assumed to occur in a revenue neutral manner with respect to the CH4-producing activity. This might represent, for example, an offset or best available technology (BAT) regulation regime where the direct costs of methane mitigation activities are compensated but any non-mitigatable emissions are not priced. While there may still be economic feedbacks in such a regime in reality, these can be of a different magnitude (e.g., general equilibrium effects instead of direct effects at the technology level) or simply not be included in the model formulation.
A second set of auxiliary reduction mechanisms are physical effects related to changing methane concentrations. Decreasing methane emissions decreases temperatures, the oxidation of CH4 to CO2, the production of tropospheric ozone, and the production of stratospheric water vapor (all included in MAGICC). All of these forcing mechanisms contribute to the auxiliary forcing reductions and their magnitude is quantified in the lower set of model auxiliary results noted above (e.g., ~ 0.03 W/m2 in 2050). The largest contributor to the CO2 decrease is smaller carbon-cycle feedbacks due to decreased global temperatures. Note that there is additional forcing uncertainty for some of these mechanisms as compared with methane forcing itself. We also note that in the configuration used here, all anthropogenic methane is assumed to be oxidized to CO2, where in reality only fossil CH4 emissions should be considered to add to atmospheric CO2 concentration (Boucher et al. 2009), which will lead to an overestimate of the effect of CH4 oxidation.
Consideration of these auxiliary effects is clearly a potentially important element of a policy focused on CH4 emission reductions. The economic auxiliary effects are the dominant contributor, where these are included, which continue to increase throughout the century as CO2 accumulates in the atmosphere. We note that this scenario included a relatively high CH4 emission price. A different scenario formulation, with a more modest CH4 price, will result in a lower level of economic auxiliary emission reductions.
BCOC-EndU—black carbon–focused reductions
The BCOC-EndU scenario includes two policies targeting emission sectors with high BC emissions: a complete phase-out of end-use coal and traditional biomass in the buildings sector and the imposition of stringent particulate emission controls on vehicles, both by 2030. These BC-focused reductions show a wide range of results across the models, with a total forcing reduction relative to reference that peaks in 2030 and ranges from 0.06–0.29 W/m2. The reasons for the range in BC and OC emission reductions are discussed further by Smith et al. (in prep, this issue), so we concentrate here on examining the various radiative forcing components.
BC and OC are emitted together, although in different proportions depending on the source. Because the SLCF scenario focuses on BC-rich sources, the impact of OC emissions is relatively low in these scenarios, with the magnitude of the OC forcing differences (BCOC-EndU—reference) ranging from 10 to 20% of the BC forcing differences.
For the BCOC-EndU scenario, we define all forcing changes other than direct BC and OC to be auxiliary reductions. The largest auxiliary changes are cloud indirect effects, which act to reduce the forcing impact of the BC emission reductions while tropospheric ozone changes add to the forcing reductions. Overall, up until about 2050, auxiliary forcing changes are net positive, reducing the impact of the BC and OC forcing reductions by up to 50% (Fig. 3, Fig. S9, S10). Auxiliary forcing changes in the BCOC-EndU scenario generally peak in 2030 and decrease in absolute magnitude thereafter. The two models with the largest auxiliary forcing (DNE21+ and ENV-Linkages) also have the largest reductions in BC forcing, so their total forcing impact is still among the highest among these models.
Note that one factor that was not included in these models was that the sulfur content of road diesel fuel would need to be reduced to very low levels in order to facilitate the operation of the particulate control devices necessary to achieve low BC emission levels from the transportation sector. This would further reduce the temperature impact of BC-focused reductions (Smith and Mizrahi 2013). Overall, given the role of auxiliary reductions, we conclude that it is important to include a comprehensive representation of air pollutant and GHG emission changes when examining the impact of potential BC-focused policies.