Figure 1 gives a general overview of the global emission pathways in the NDC scenarios, compared with the no-climate policy reference and frozen emission factor cases (model specific results are provided in Supplement S4; sectoral and regional emissions are further discussed in the next section).
By design, CO2 emissions are the same in the no-SLCF and SLCF policy cases. Any feedbacks from SLCF policy on CO2 emissions have been neglected. For all models, CO2 emissions follow one of three distinct pathways, increasing emissions in the reference case, stable to declining emissions in the NDC scenarios, and strongly declining emissions after 2030 in the NDC + 2degC case. CO2 reduction measures can indirectly reduce CH4 and to a much lesser extent BC emissions, via the reduction of fossil fuel production and use and the reduction of deforestation (Rogelj et al. 2014). This CO2 indirect effect reduces the SLCF reduction potential in the mitigation scenarios.
All models project that without climate policy (reference in Fig. 1), CH4 emissions in 2030 are higher than in 2010. In the NDC scenario, 2030 emissions are comparable to those in 2010, although model results differ. Only limited (non-climate policy related) reductions in CH4 emission factors are assumed in the reference case, indicated by the relatively small difference with the frozen emission factor scenario. The NDCs are projected to lead to a decrease in emissions compared with the reference case, even in the absence of additional SLCF policy (see next section for a detailed description).
All models consistently project steadily decreasing BC emissions for the next decades in all scenarios. Even in a no-climate policy reference scenarios as SSP2, a considerable decrease in emission factors is expected (by 23% in 2030, model mean) resulting from stricter air pollutants emission legislation and increasing access to clean energy for cooking in growing economies (Rao et al. 2017). This also reduces the mitigation potential in 2030 in the NDC scenarios. The projected difference in emissions between the reference and NDC scenario is small (< 5%), since BC is not included as an offset under the Kyoto Protocol and mitigation efforts concentrate on Kyoto GHGs in the NDC case. In the NDC scenario, the very limited CO2 reduction measures also do not have much impact on BC.
The projected HFC emissions differ from CH4 and BC in the sense that they are not indirectly mitigated by CO2 reduction measures, but only directly, depending on the climate policy stringency (i.e. the carbon price level). In the next section, the model specific assumptions and results are discussed.
Figure 2 shows the sectoral and regional CH4 emissions for the NDC and reference scenarios. In the bar charts, the emissions and emission reductions are subdivided into 9 sectors and 5 aggregated world regions given for 2030. Here, no distinction is made between the two degree and NDC extrapolated scenarios, as these follow the same trajectory until 2030. The bars show the model means per sector and region and model range in total emissions (individual model results are provided in Supplement S4).
In the NDC scenario, CH4 emission reductions are found in all sectors (44 to 155-Mt CH4 or an average 20% reduction, compared with the reference case) and are the largest in fossil fuel production both in absolute and relative terms (29% of the energy supply emissions on average). With SLCF policy, additional reductions are realized in all sectors (125 to 235-Mt CH4 or an average 41% in total), leading to lower emissions in 2030 compared with 2010 for all models. Also, in the NDC + SLCF case, emission reductions are mainly realized in the fossil fuel sectors (59% of the energy supply emissions on average, compared with reference). Partly, this is achieved by direct CH4 measures, and partly, it is an indirect impact of CO2 policy. Generally, the models estimate slightly larger reductions from the former activity; however, there is a large model spread in the indirect effects of CO2 mitigation on CH4 emissions.Footnote 1 Roughly half the emissions come from land-use sources in the historical and reference case, whereas the relative share of land-use emissions increases as climate policy intensifies, due to the larger challenges surrounding land-use mitigation (predominantly enteric fermentation in ruminants, which is approximately two thirds of the land-use CH4). Note that agricultural emission reductions in the 2degC and SLCF scenarios largely come from end-of-pipe measures (lowering emission factors, rather than activities), and a large global decrease in ruminant meat and dairy demand could further bring down emissions (see Supplement S5 for further discussion).
About 40% of the emissions originate from Asia (Fig. 2). In the NDC scenario, however, emission reductions in Asia are expected to be relatively modest. Due to the assumption of global methane actions in NDC + SLCF, this increases significantly, and reductions are more evenly distributed over the world regions. In total, the models project an average reduction of 95 Mt CH4/year (or 26%) from additional SLCF policy in 2030.
Black carbon emissions
Figure 3 gives an overview of the sectoral and regional BC emissions, with a similar setup as Fig. 2. In the NDC scenario, BC emissions are only slightly reduced, due to CO2 reduction measures (3% on average, compared with reference). In the NDC + SLCF case, BC emissions are reduced considerably (1.1- to 4.3-Mt BC or an average 34% in total). The largest mitigation options are the reduction of emissions from coal and traditional biomass use in the residential sector (on average, 70% reduction). Further emission reductions in transport sector are more limited, since even in several developing countries relatively stringent emission standards are assumed in the reference case, especially for road vehicles.
BC emissions in the NDC case are highest in Asia and Africa on average 49 and 32% of the total emissions in 2010 and 48 and 39% of the total emissions in the reference case in 2030. However, 95% of the reduction potential in the NDC + SLCF case in 2030 is also found in these regions (65% in Asia, 30% in Africa). In total, the models project an average reduction of 2.1-Mt BC (or 32%) from additional SLCF policy in 2030. In Asia, the relative reduction in NDC + SLCF is found to be higher than the global average: 42%.
The selected BC reduction measures also reduce OC emissions, however to a lesser extent in relative terms: 22% or 4.7 Mt of total OC emissions in the NDC + SLCF (not shown).
HFC emission reduction potentials and measures vary considerably across the different models. Partly, this is because of the historically small contribution of HFCs to global warming; partly, this has resulted from uncertainty about the inclusion of HFCs in the Montreal protocol. Supplement S6 shows the HFC emission pathways for the models that ran the additional HFC reduction scenario (NDC + 2 DC + SLCF + HFC). All models indicate that without strong climate policy, HFC emissions would either remain constant or steadily increase. Note that the model projections exclude the recently pledged reductions under the Kigali Amendment to the Montreal protocol (UNEP 2016b), and mitigation of HFC is dependent on the carbon price development. The emissions in the NDC case are projected to be at or slightly below the no policy reference level (by AIM and REMIND) or stabilize around 800- to 1000-kt HFC134a equivalent emissions per year (by IMAGE and MESSAGE). For the 2 degree scenarios, four models project a large short-term potential for HFC reduction compared with the baseline emissions in 2030 around 88–89% for AIM/CGE, IMAGE, REMIND, and POLES in line with Höglund-Isaksson et al. (2017), who estimated this at 80%. Early, ambitious HFC reduction could lead to substantially lower emissions (in 2030: 80–88% lower than NDC + 2degC for the four models). This reduction estimate is significantly higher (in 2030: 30–35%) than what can be expected from the Kigali Amendment (in 2030: 54–58% reduction (Höglund-Isaksson et al. 2017; PBL 2015). As such, considering very deep HFC reductions as a SLCF measure additional to the Kigali Amendment and the NDCs seems legitimate.
DNE21+ projects the least HFC mitigation in the HFC reduction scenario. This follows from the use of US-EPA marginal abatement cost curves (US-EPA 2013) not only with more conservative estimates of the reduction potential but also with most reductions occurring at lower carbon prices, so most reductions occur already in the NDC scenario without additional HFC policy.
The HFC emission reductions projected by MESSAGE also differ less across scenarios. MESSAGE includes mitigation options, such as refrigerant recovery, but does not represent substitution by non-GHG gases. With a maximum reduction in 2030 of just below 600-kt HFC134a eq., the strong HFC mitigation case in MESSAGE results in higher residual HFC emissions than in all other models, but can be considered comparable with the ambition in the Kigali Amendment.
Figure 4 shows the GMT and GMT rate of change for the reference and NDC scenarios. In both sets, a comparison is made between the scenarios with and without additional SLCF policy. Table 2 provides detailed outcomes for all models.
In the short-term (the year 2040), when the GMT reducing effect of SLCF policy is the largest, models project a potential GMT reduction of 0.03–0.15 °C in NDC + SLCF and NDC + 2degC + SLCF (or 2 to 9% of total GMT change, which is 6 to 30% of the GMT increase between now and 2040, when considering the NDC scenario as a benchmark). This effect is projected to be slightly larger in the NDC than in the NDC + 2degC scenario, due to a lower SLCF reduction potential in the latter 2 degree case, where the SLCF and no-SLCF case converge stronger after 2030.
In an NDC + 2degC + SLCF scenario, SLCF policy is found to have a relatively small effect on reducing the maximum temperature before 2100 (1 to 3% of total GMT change or 0.02 to 0.08 °C), confirming earlier work (Bowerman et al. 2013). The main reason also here is that later in the century, SLCF reductions are almost the same as in the NDC + 2degC case, because models exhaust their assumed mitigation options (under a sufficiently high carbon equivalent price), with maximized direct CH4 reduction measures and indirect CH4 reductions due to CO2 mitigation. This is also shown in Fig. 4, where the two 2 degree scenarios converge near 2060. Table 2 shows that most models project that additional SLCF policy will cause the GMT peak year to occur a few years later in time (3 on average, ranging from − 5 to 8 years).
While the effect on lowering peak GMT is limited, the selected SLCF measures can contribute to lowering the maximum GMT change rate in both the continued NDC scenario (2 to − 15% change in the seven IAMs) as in the NDC + 2degC scenario (− 3 to −15% change)(see lower panel Table 2). In the NDC + 2degC case for all models, this peak in GMT change rate occurs before 2040 and in most cases between 2020 and 2030. Most models do not project a change in the timing of the maximum GMT rate of change due to additional SLCF reductions. If the peak value without SLCF policy is expected in the next decade, this is generally not expected to change when SLCF policy is employed. Notable exceptions are projections from DNE21+ and REMIND of the NDC case where SLCF policy shifts the maximum rate of change to mid-century. In the NDC + 2degC case, where the highest GMT rate of change generally is projected to occur in this decade, no major shifts in peak rate of change have been projected.
An assessment of the RF levels of individual SLCF groups, CH4, BC/OC, HFCs, and tropospheric ozone (O3), shows that all three SLCF groups can contribute to lowering GMT (O3 is indirectly decreased by lowering CH4 and nitrogen oxide (NOx), which is co-emitted with BC and OC). In Fig. 5, the decrease in RF resulting from SLCF measures in the NDC + 2degC scenarios is shown (model specific results are provided in Supplement S7). The figure shows the average RF difference between “NDC + 2degC” and “NDC + 2degC + SLCF + HFCs” and is thus only based on results from the seven models that submitted the latter scenario. The relative contribution of forcers is similar in the NDC extrapolated case (not shown). The largest RF reducing effect occurs in 2030. In that year, the relative impact of additional methane reduction is 41%, of BC/OC reduction 26% and of HFC reduction 6%. A large share of the decrease in RF can be accounted to a reduction in O3 (27%), which is mainly caused by the reduction of the O3 precursors NOx and CH4. Depending on the abundance of other species, NOx reduction is generally dominant in the reduction of O3. For instance, reducing projected NOx levels in 2100 to the 2000 value is projected to more than halve the ozone abundance (Ehhalt et al. 2001).
All models project a substantial forcing reduction that can be attributed to CH4. Considering that still a substantial part of the O3 forcing reduction is indirectly caused by CH4 reduction, more than half of the total forcing reduction comes from CH4 mitigation.
With the exception of DNE21+, the models agree on the smaller effect of BC measures. While the direct BC forcing decrease can be considerable (roughly twice as large as BC/OC forcing decrease in Fig. 3), several effects reduce the net impact. Partly, it results from the reduction of OC. Secondly, aerosol emission reductions (i.e. BC, OC, nitrate, and to al smaller extent sulphate, all lowered by BC measures) are projected to decrease the cloud indirect (negative) forcing effect, thus increase forcing. For example, in MESSAGE, the effect of BC/OC is relatively small. While BC reduction in the SLCF case is considerable, OC reduction is very large as well. Due to large aerosol emission reductions, the cloud indirect aerosol forcing is considerably increased, adding up to a small net forcing decrease. In DNE21+, the opposite is happening. Here, there is also a very large and sustained difference in BC emissions between NDC + 2DC and NDC + 2 DC + SLCF. However, OC and other aerosols are mitigated much less, leading to a much smaller change in OC direct and aerosol indirect effect.
The absolute impact of HFC reductions appears to be limited; however, the model differences in terms of emission reductions are relatively large. Models that assume a reduction potential of more than 80% of the HFCs (AIM, IMAGE, POLES, WITCH-GLOBIOM) project a RF decrease of roughly 0.03 W/m2, which is roughly 15% of the total decrease. Considering that this percentage would be achieved when maximizing HFC reductions (in 2030, far beyond the Kigali amendment of the Montreal protocol), HFC abatement measures can potentially provide only a much smaller additional contribution to reducing GMT than more important CH4 and BC measures.