The cumulative CO2 emissions over the periods of 2011–2050 and 2051–2100 among the models are shown on Fig. 1. With regard to the global models, although the median across the models is almost consistent with the national budgets, cumulative emissions between 2011 and 2050 vary to some extent across the models. In addition, the range of cumulative emissions by 2050 also differs by the different near-term policies. For example, cumulative emissions in the NDC1000 scenario were found to exceed the carbon budget in some models; therefore, this scenario is characterized by a deeper emission reduction in the second half of this century, and even the net removal of CO2 between 2051 and 2100. A discussion of the effort-sharing schemes in the global models and an assessment of the longer-term pathways are beyond the scope of this study; however, it should be noted that uncertainties remain in the number of national budgets.
Although the trajectories after 2050 are not the main focus of this study, the global 1000 Gt-CO2 budgets scenario suggests that cumulative emissions in Japan in the second half of this century need to be nearly zero. In particular, if near-term emissions follow the level of the NDC, the net removal of CO2 would be required in the second-half of the century in the global 1000 Gt-CO2 scenario, which is equivalent to about 4.1 Gt-CO2 (median), because cumulative emissions in the first half of the century exceed the national budget by 2100.
The CO2 emissions pathways in Japan over the period of 2010–2050, and their ranges across the models in 2030 and 2050, are depicted in Fig. 2. The figures for total GHG emissions and detailed results can be found in ESM Fig. S2, Table S5 and S6. Because the trend in the emission reduction from 2010 is broadly consistent between CO2 and GHGs, we mainly focus on CO2 emissions in this section. CO2 emissions in the NoPOL and NPi scenarios remain almost stable, and their reductions in 2050 from the 2010 level are 8% (median; full range − 17 to 33%) and 14% (full range − 4 to 34%), respectively. In the NDC scenario, CO2 emissions continue to fall after 2030 in most models, decreasing by 29% (full range 10–45%) in 2050 with respect to the 2010 level.
Additional mitigation efforts are required to meet the carbon budget that corresponds to the global climate objective of keeping the temperature rise below 2 °C. To meet the high budget, the median CO2 emission reduction reaches 61% below the 2010 level (full range 35–74%) by 2050. In these scenarios the emissions of total GHGs also fall by 63% (median 33–76%) (see ESM Fig. S2). The low budget scenarios (both NDC100 and NPi1000), as well as the high budget scenarios, require a substantial emission reduction, which is equivalent to a 75% (full range 47–89%) reduction by 2050 below the 2010 level. As shown in ESM Fig. S2, a similar trend is observed in the reduction of total GHG emissions, where the median reduction in 2050 is 73% (range 45–91%) below the 2010 level. This reduction is broadly similar to the national long-term goal in Japan to reduce GHG emissions by 80%.Footnote 2
For the sectoral CO2 emissions, while emissions from the energy demand sectors vary across models, all models show rapid and significant emissions reduction in the energy supply sector in 2050 (Fig. 2, bottom, ESM Fig. S3). In particular, energy supply is nearly decarbonized by 2050 in the low budget scenarios (median 93–97% reduction below 2010). In the energy demand sectors, the median CO2 emissions in the low budget scenarios are reduced by 59–64% by 2050 relative to 2010, while those in the high budget scenarios are halved in this period.
With regard to the near-term policy dimension, in the high budget scenarios, the median CO2 emissions in 2030 in the NPi1600 scenario (− 24% below 2010) are almost identical to those in the NDC1600 scenario (− 23% below 2010), suggesting that Japan’s NDC is broadly on track to meet the emission pathway corresponding to the global 1600 Gt-CO2 budget. In contrast, given the stringency of the carbon budget in the low budget scenarios, median CO2 emissions in the NPi1000 scenario fall by 34% (full range 14–39%) by 2030, whereas the NDC1000 scenarios show a 23% reduction. Although all models explored the pathways to meet the low budget with no additional effort beyond the NDC by 2030, these pathways involve a rapid emission reduction after 2030. Figure 3 shows that the average annual changes in CO2 emissions in the NDC1000 scenario represent − 5.2 and − 6.1% (median) over the periods of 2030–2040 and 2040–2050, respectively, whereas in the NPi1000 scenario, the corresponding figures are − 4.4 and − 4.7%, respectively. Consequently, the median CO2 emissions in the NDC1000 scenario reach the same extent as those in the NPi1000 scenario by 2035, and are lower after 2040 (Fig. 2). For the last half-century, Japan’s economy has experienced such drastic changes only in the oil crisis in the 1970s and 1980s and in the global economic recession at the end of 2000.
In both the high and low budget scenarios, upscaling of low-carbon energies, including nuclear, renewable, and carbon capture and storage (CCS), is a key mitigation option for the most models (Fig. 4, top, ESM Fig. S4). While the share of low-carbon energy accounts for about 10 and 6% of the total primary energy supply in 2010 and 2015, respectively, the median share in 2050 increases to 44% (full range 25–61%) and 54% (full range 42–75%) in the high and low budgets scenarios, respectively. In addition, the share of low-carbon energy has to be approximately tripled over the period of 2030–2050 both in the high and low budget scenarios. A more rapid increase is required if the emissions by 2030 follow the NDC. To achieve this drastic emission reductions in the energy supply sector, about 86% (full range 72–100%) and 97% (full range 77–100%) of electricity comes from low-carbon energy sources by 2050 in the high and low budget scenarios, respectively. Especially in the low budget scenarios, electricity is nearly decarbonized by 2050 and, in some scenarios, net CO2 emissions from electricity become negative due to the deployment of bioenergy with CCS (BECCS). With regard to the near-term, more than 60% of electricity comes from low-carbon sources in 2030 in the NPi1000 scenario, without depending on CCS (the share of each low-carbon energy can be found in ESM Figs. S5, S6).
The development of the shares of various energy sources in the primary energy supply and electricity generation in the high and low budget scenarios over the period of 2010–2050 is shown in Fig. 4 (bottom). In both the high and low budget scenarios, fossil fuel without CCS is substituted by low-carbon energies over time; however, the share of the low-carbon energy source varies among all models, both national and global. The share of nuclear and renewables in primary energy in 2050 ranges from 5 to 23% and from 14 to 39%, respectively, in the NDC1000 scenario (see ESM Table S7 for more detail). Some models are characterized by the conservative assumption of nuclear and CCS accounting for ≤ 20% of the primary energy supply, together with a dependency on renewables. In contrast, other models depend largely on nuclear and/or CCS, while the share of renewables in the primary energy supply remains at < 20% in 2050. The detailed results on the low-carbon energy share can be found in ESM Tables S7 and S8. The range of the share of CCS is relatively wider compared with other energy sources in 2050, reflecting the different assumptions among models.
The change in the energy demand from 2010 onward and the share of low-carbon carriers are summarized in Fig. 5 and ESM Fig. S7–S9. Although the final energy consumption varies greatly across the models, the median drops to 20 and 26% in the high and low budget scenarios, respectively, by 2050. In particular, a large-scale reduction in energy demand is observed in transportation sector by 2050 (median − 41% below 2010 in low budget scenarios). In addition, the share of low-carbon carriers, such as renewables, electricity, hydrogen, and heat, is increased with the more stringent carbon budget in 2050. In the low budget scenarios, this share exceeds half of the final energy consumption by 2050. The buildings sector is characterized by a large-scale deployment of low-carbon energies; especially in the low budget scenarios this accounts for almost 80% by 2050, while energy demand reduction is moderate in this sector.
The relationship between carbon prices and emission reductions in 2050 relative to those in 2010, and the net present value (NPV) of mitigation costs expressed as a fraction of the baseline gross domestic product (GDP) for the period of 2021–2050 are shown in Fig. 6. Although carbon prices of each model should not be directly compared because the level of emission reduction in 2050 is different depending on the model, the stringency of the carbon budget has a large impact on the level of the carbon price for all models. In the high budget scenarios, the median carbon price is projected to be 110 US$/t-CO2 by 2050 (full range 65–261 US$/t-CO2) and 141 US$/t-CO2 (full rang: 36–431 US$/t-CO2) in the NPi1600 and NDC1600 scenarios, respectively. Although cumulative mitigation costs (NPV) over the period 2021–2050 are also not directly comparable as the models report different cost indicators, they range from 0.1 to 2.0% of the baseline GDP in the high budget scenarios.
In contrast, to meet the more stringent carbon budget, the low budget scenarios require an additional effort compared with the NDC and the high budget scenarios. The median carbon price in the NDC1000 scenario rises to 497 US$/t-CO2 (full range 133–2093 US$/t-CO2) in 2050, while in the NPi1000 scenario it represents 376 US$/t-CO2 (full range 151–1073 US$/t-CO2). NPV mitigation cost in the low budget scenarios also becomes higher than the high budget scenarios for most models, although the full range is similar with the high budget scenarios (0.1–2.1%).
With regard to carbon price, a large gap is observed, especially among the national and global models(ESM Fig. S10a). One plausible reason for this gap is that the national models result in a deeper emissions reduction in 2050, especially for AIM/Enduse[Japan], and they assume larger economic growth than the global models. In addition, the national model considers regional specific barriers, such as constraints on electricity interconnections across sub-regions in Japan for AIM/Enduse[Japan], which would exacerbate the challenge to integrate variable renewable energies (VREs). It should also be noted that cumulative mitigation costs for some global models, such as DNE21+ (global), IMAGE, and REMIND-MAgPIE, in the NDC scenarios are smaller than those in the NPi scenarios, especially in the low budget scenarios. This is in contrast to the national models and is generally associated with the higher carbon budgets in the NDC scenarios between 2011 and 2050, which are compensated for in the second half of this century in these global models (Fig. 1). Whereas the differences in the carbon budget between the NDC and NPi scenarios are moderate for IMAGE, this model shows a large difference in the annual mitigation costs in 2030 between the NDC1000 and NPi1000 (ESM Fig. S10b), thus the cumulative cost in NPi1000 is still higher compared with the NDC1000.