Residual emissions and carbon removal towards Japan’s net-zero goal: a multi-model analysis

We study Japan’s net-zero emissions target by 2050 in a multi-model framework, focusing on residual emissions and carbon dioxide removal (CDR). Four energy-economic and integrated assessment models show similar but stronger strategies for the net-zero target, compared to the previous, low-carbon policy target (80% emissions reduction). Results indicate that around 90% (inter-model median) of the current emissions are reduced through abatement, including improved energy efficiency and cleaner electricity and fuels. Models deploy new options such as CDR based on carbon capture and storage (CCS) (bioenergy with CCS and direct air carbon dioxide capture and storage) and hydrogen to achieve net zero. The scale of CCS-based CDR deployment reaches an inter-model median of 132Mt-CO2/yr. The median hydrogen share of final energy in 2050 increases from 0.79% to 6.9% between the low-carbon and net-zero scenarios. The CDR sensitivity analysis reveals that limiting the use of CDR significantly increases the mitigation costs for net zero. Achieving Japan’s net-zero goal will require exploring methods to reduce residual emissions, including demand-side solutions, and accelerating responsible CDR policies.


Introduction
This study presents the results of scenario analysis in the Japan Model Intercomparison Platform (JMIP) 2 Net Zero project in order to inform the policy debate on Japan's goal of net-zero greenhouse gas (GHG) emissions by 2050.Japan was the fourth largest economy in the world in 2023 (IMF 2023, Ueno and Wakabayashi 2024), and has a significant manufacturing base as one of the largest exporters of steel (World Steel Association 2023) and automobiles (ITC n.d.), for example.On the other hand, it has few energy resources, with an energy sufficiency rate of 13% in 2021 (Agency for Natural Resources and Energy 2023).Analyzing its policy pathways is instructive not only for Japanese policymakers and stakeholders, but also for other countries in a similar situation.We use four integrated assessment and energy-economic models, focusing on residual emissions and the role of carbon dioxide removal (CDR).We clarify the additional mitigation effort by shifting the target from 80% reduction to net zero.We also demonstrate the role of CDR in the net-zero scenarios.
In October 2020, Japan committed to achieving net-zero GHG emissions by 2050, raising its emissions reduction target from 80% to 100% (Suga 2020).In 2021, Japan then increased its 2030 reduction target from 26% to 46% (with an ambition to pursue 50%). 10The government has mandated the net-zero GHG emissions by 2050 under the Law on Global Warming Countermeasures as amended in 2021, and has enacted a series of policies, most recently, a package of policies dubbed Green Transformation (GX).Japan plans to issue GX economic transition bonds of up to 20 trillion yen over 10 years to facilitate public and private investment in transforming the entire socio-economic system, including the power sector and heavy manufacturing industry.As part of these GX packages, the government has begun to consider a carbon pricing scheme that would contribute to economic growth with a plan to start tariffs on fossil fuel imports in 2028 and an emissions trading auction scheme for power producers in 2033 (Agency for Natural Resources andEnergy 2023, METI 2023).It has also encouraged local initiatives by adopting the Local Decarbonization Roadmap in 2021 and providing financial support to municipalities selected as Decarbonization Leading Areas (Ministry of the Environment 2023).
Despite the numerous policies in place, the question remains as to whether they are sufficient to achieve the net-zero goal, or how they can be improved to achieve the stated goal of net zero.This is because the net-zero goal requires a complete rethinking of strategy, and thus of policy analysis (Pye et al 2021, Fankhauser et al 2022), and these goals and policies have not yet been fully analyzed.For example, the Japanese policymakers need to address key issues such as the strategy for reducing emissions from harder-to-abate sectors such as heavy industry (Davis et al 2018, Energy Transitions Commission 2018, Luderer et al 2018, IPCC 2022, Gailani et al 2024) (e.g., through the use of clean fuels such as hydrogen), the CDR needed to offset residual emissions, and the potentials, costs, and sociopolitical barriers of new strategies and CDR.However, Japan's strategy remains ambiguous about the expected residual emissions and the scale of CDR deployment (Buck et al 2023).
For CDR, the government has already initiated research and development activities, including a Moonshot Program on direct air capture and related technologies (NEDO 2022).However, the government has yet to formulate a long-term roadmap for CDR deployment (see (Schenuit et al 2021) for discussions of policy in selected OECD countries).Japan has been promoting forest sinks for years but its amount has been fairly stable.
In order to analyze residual emissions, it is critical to consider the Japan's key climate change mitigation challenges.Japan was the fifth largest emitter of carbon dioxide (CO 2 ) in 2021 (Crippa et al 2022) and is a major contributor to GHG emissions.Japan has a significant presence of harder-to-abate heavy industries, including steel, cement, and petrochemicals.In addition, the potential of variable renewables (solar and wind power) is relatively low compared to the endowments in regions such as North America and Europe (Luderer et al 2017, Schreyer et al 2020), which may act as a barrier to high penetration of renewables and deployment of affordable green hydrogen11 .In terms of non-CO 2 GHG emissions, the largest emissions are hydrofluorocarbons, followed by methane and nitrogen oxide (figure ESM 1).These characteristics have implications for Japan's residual emissions under a net-zero scenario.
Few studies have been conducted to understand the residual emissions and the role of CDR in Japan's policy (Oshiro et al 2018, Kato andKurosawa 2021) (see also Agency for Natural Resources and Energy (ANRE) (2021) for scenarios referenced in government committee deliberations).The Stanford Energy Modeling Forum (EMF) 35 JMIP 1 project (Ju et al 2021, Sakamoto et al 2021, Shiraki et al 2021, Sugiyama et al 2021) explored robust strategies for Japan's deep mitigation in a multi-model setting, but the focus of these studies was on 80% emissions reductions rather than 100% reductions or net zero.More recently, the scenario database of the 6th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) has compiled national-level scenarios, including those for Japan (see Sakamoto (2023) for an analysis of scenarios for Japan).
Model uncertainty remains a key concern for long-term climate policy analysis (Dekker et al 2023), making multi-model analysis crucial.Such studies have been conducted to analyze net-zero scenarios, residual emissions, and the role of CDR in some national contexts.For instance, the Stanford EMF 37 study used 16 models to explore decarbonization with advanced technologies and sectoral policies.Regardless of technology assumptions across the industry, buildings, transportation, and carbon management sectors, and across different models, they found that models relied on bioenergy with carbon capture and storage (BECCS) and direct air carbon capture and storage (DACCS) in addition to land carbon sinks (Browning et al 2023).China's mitigation pathways have also been analyzed in a multi-model setting (Duan et al 2021), again showing the importance of CDR for a 90% emissions reduction by 2050.However, no multi-model study has been conducted on Japan's net-zero target.
Therefore, this study poses two research questions: (1) Are the robust mitigation strategies for net zero in Japan the same as those for the 80% emissions reduction target?If not, how do they differ?
(2) What is the role of CDR in Japan's net-zero goal?How much and what kind of CDR should be deployed?

Models
We employ four energy systems and integrated assessment models (table 1): AIM-Hub-Japan, AIM/ Technology-Japan, IEEJ-NE_Japan, and TIMES-Japan.While this is not an exhaustive list of models whose work has been published in the peer-reviewed literature, this set of models covers diverse modeling approaches and perspectives that can be found in Japan.AIM/Hub-Japan (Silva Herran et al 2019) belongs to the Asia-Pacific Integrated Assessment Model (AIM) family of models, which has been used extensively for climate policy assessment.It is a national model that is part of the global model.It is a recursive-dynamic general equilibrium model with a one-year time step and 42 industrial classifications.In addition to CDR in managed land, it has implemented BECCS.
AIM/Technology-Japan is a recursive-dynamic, partial equilibrium model with detailed representation of end-use technologies and energy supply sectors (Oshiro et al 2021).It is a linear programming model that minimizes the total system cost under various exogenous parameters, including service demand, energy prices, and technology parameters.For CDR, it includes both DACCS and BECCS.IEEJ_NE-Japan (Matsuo et al 2020, Otsuki et al 2023) consists of two soft-linked sub-models.A macroeconomic sub-model is used to compute macroeconomic indicators, which are then used to as inputs to an energy technology model that optimizes energy technology deployment using linear programming.The power generation module has an hourly time resolution, allowing detailed calculations of variable renewable energy deployment.Both BECCS and DACCS are implemented for CDR.
TIMES-Japan (Kato and Kurosawa 2019) is a partial equilibrium model using the TIMES modeling framework developed by IEA ETSAP.The model is similar to MARKAL-Japan but with an additional energy carrier (imported hydrogen).The model minimizes the discounted, total system cost per under exogenous service demand assumptions.It has BECCS and DACCS as CDR options.
These four models have different gas coverage and different solution concepts.The Agriculture, Forestry, and Other Land Use (AFOLU) sector is only included in AIM-Hub.AIM/Technology and TIMES-Japan cover energy and industrial processes while IEEJ-NE_Japan covers only the energy sector.Non-CO 2 GHGs are only covered by AIM-Hub and AIM-Technology.Only one of the four models is based on general equilibrium.
The CDR options included in each model vary (table 1).All participating models have BECCS as a CDR option.All except AIM/Hub also have DACCS.AIM/Hub includes AFOLU CDR, although it is not analyzed in detail in this study.See the ESM for the details on each model.

Scenarios
We analyze scenarios up to 2050, the target year of Japan's long-term strategy.We have three main scenarios (Baseline, Low Carbon (80%), and Net Zero) and additional scenarios for sensitivity analysis.For the sensitivity analysis, we systematically vary two dimensions: (1) policy stringency in terms of emission caps (Fujimori et al 2021), and (2) the availability of CDR (table 2).
Japan does not yet have a strong carbon pricing scheme.Therefore, we do not impose any price on carbon in the Baseline scenario.The Japanese government plans to introduce tariffs on fossil fuel imports in 2028 and an emissions trading auction scheme for power producers in 2033, but this is not fully reflected in this scenario as there are many uncertainties about the details of the proposed emissions trading scheme at the time of writing.
The Net Zero scenario is our main mitigation scenario.While Japan's target concerns GHG emissions, this study focuses on CO 2 emissions.The scale of CDR deployment would be greater if non-CO 2 GHG emissions were considered.We will discuss the quantitative impact of residual emissions from non-CO 2 emissions later.We contrast this with the Low Carbon (80%) scenario, which is intended to represent the previous target.As gas coverage varies between models, our constraint is applied to CO 2 from fossil fuels and industry.For models with broader gas coverage, we apply consistent carbon prices across gases and sectors.In addition, international emissions trading is not allowed to achieve the net-zero target.Note that this is only an approximation of the government's goal of full GHG emissions reduction, a limitation of this study.We also examine the 46by30+Xby50 (without exogenously imposed CDR constraints) and 46by30 +Xby50_CDRy scenarios, where X and y have been varied to imply different levels of emissions reduction and CDR deployment constraints.Specifically, the scenario names imply an emissions reduction of X % by 2050 and a CDR deployment constraint of y Mt-CO 2 /yr.Although Japan is targeting a 100% emissions reduction by 2050, it is instructive to consider the implications of overachieving or underachieving this target.An American study (Browning et al 2023) analyzed different emission ceiling targets for 2050.
For CDR constraints, we change the availability from the model default value to 100, 50, and 0 Mt-CO2/yr.While the role of CDR is acknowledged in the long-term policy discussion, no clear quantitative target has been set.As noted above, in May 2023, the government indicated a possible carbon storage of 120-240Mt-CO 2 /yr by 2050 (CCS Long-Term Roadmap Review Committee 2023).Although the focus of the document is on the role of CCS in reducing emissions from power plants and industrial facilities, it also gives an indication of the potential scale of CDR.Another report by a METI's expert committee presented a range of 50-240 Mt-CO 2 /yr for CDR deployment, based on the analysis of IPCC scenarios (Investigative Committe for Creation of Negative Emissions Markets (negatibu emisshon shijo soshutsu ni muketa kento kai) 2023).However, these estimates are based on geophysical and techno-economic considerations, and social acceptance must also be taken into account.This is the justification for the 3 levels of CDR constraints above (in addition to the default case).Note that the maximum CDR deployment scale in the models is ∼ 200Mt-CO 2 /yr (see below), which corresponds roughly to the upper limit of the range considered by the government.
Following our previous work (Sugiyama et al 2021), we have harmonized population and economic growth rates.We used the growth rates consistent with the Shared Socioeconomic Pathway (SSP) 2.

Strategies for net zero
Figure 1 shows the time series of CO 2 emissions in the baseline scenario, the Low Carbon (80%) scenario (previous policy target), and the Net Zero scenario (current policy target).Although Japan's economy continues to grow in our scenarios, population decline and energy efficiency improvements lead to substantial reductions in final energy and CO 2 emissions even in the Baseline scenario, except for the AIM/Hub model, which is the only general equilibrium model among our participating models (figure ESM 2).All sectors contribute to emissions reductions in the Low Carbon (80%) scenario, and more reductions are observed in the Net Zero scenario.Additional emissions reductions occur in all sectors, and the reductions are particularly large in the energy supply sector (Panel C).The amount and composition of residual emissions in 2050 differ between models, which are compensated by CDR in 2050.The residual emissions and corresponding CDR levels are in the range of 100-200 Mt-CO 2 /yr.
The actual type of CDR depends on the model (figure 2); while IEEJ_NE-Japan gives a priority to DACCS, the other three models show a dominance of BECCS.The scale of CCS-based CDR deployment reaches an intermodel median of 132Mt-CO 2 /yr (with a minimum of 107 and a maximum of 231Mt-CO 2 /yr).The total carbon capture and storage is even larger, as there is a significant deployment of CCS for fossil fuels (figure ESM 3).As for land-based CDR, it remains fairly constant until 2050 (figure ESM 4).
Although CDR deployment is significant, conventional options contribute the majority of emissions reduction.For example, the 132Mt-CO 2 /yr is about 10% of the CO 2 emissions in 2013, the reference year for Japan's mitigation target.In other words, 90% of the emissions are abated through energy efficiency, fuel switching, and cleaner electricity and fuels, etc.The same overall decarbonization strategies remain important, despite the new addition of CDR to the mitigation strategies.Figure 3 depicts four key indicators of mitigation strategies across models for the Low Carbon (80%) and Net Zero scenarios and the difference between them.Improvement in energy intensity of GDP (or improvement in energy productivity), decarbonization of electric power sources, end-use electrification, and reduced use of fossil fuels are all important.
However, there are some nuanced differences.Focusing on the Net Zero scenario, the CO 2 intensity of electricity goes to zero in the IEEJ-NE model, whereas it becomes negative in the other three models, with a median in the net-zero time of 2044.In contrast, in the Low Carbon (80%) scenario, three models show nonzero power-sector emissions in 2050.The median electrification rate increases from 31% to 33% in 2030 between the Low Carbon (80%) and Net Zero scenarios, and from 45% to 52% in 2050.The electrification rate increases much faster in the AIM/Hub model compared to other models, which have limited mitigation options in energy savings and hydrogen compared to other models.This trend was also observed in the previous study (Sugiyama et al 2021).The share of fossil fuels in primary energy decreases to ∼ 40% or below, but the level is higher for the IEEJ-NE model than for others.
The last row of figure 3 shows how the differences between the two scenarios evolve over time.The models tend to show U-shaped (or inverted U-shaped) developments over time.The additional effort required to reach net zero peaks before 2050, between 2030 and 2040 for most models and indicators.(The trend is somewhat weaker for the fossil fuel share.)In other words, achieving the Net Zero scenario requires earlier and stronger policy responses than the Low Carbon (80%) scenario.
Next, we look at the energy mix and power generation mix (figure 4) (for 2030, see figure ESM 5).Without explicit climate policy, three models imply a primary energy mix dominated by fossil fuels, while TIMES-Japan shows a moderate penetration of clean energy.Total primary energy increases in three out of the four models, while it decreases in one between the Low Carbon (80%) and Net Zero scenarios; the share of fossil fuels in primary energy also decreases significantly in 2050.Three models (AIM/Hub, AIM/Technology, and TIMES- Japan) show an increased use of BECCS in the Net Zero scenario compared to the Low Carbon (80%) scenario.In models with clean secondary energy trade (hydrogen and ammonia), it increases significantly from the Low Carbon (80%) scenario to the Net Zero scenario.Even in the Net Zero scenario, the primary energy mix in all models includes some fossil fuels, especially oil.
In 2030, when the transition is in the intermediate phase, fossil fuels are still a significant component, especially for primary energy.On the other hand, the decline of coal in the Net Zero scenario is significant in all models (AIM/Hub and TIMES retain some coal-fired power generation in 2030, but the decline is appreciable).The exact timing of the phase-out of coal power generation without CCS varies across models, ranging from 2030 to after 2050 (although it is virtually eliminated in all models by that year) (see figure ESM 6).Coal without CCS is zero or negligible in all models.
In all models, the total electricity generation increases from the Baseline to the Low Carbon (80%) to the Net Zero scenario while fossil fuels decrease significantly.The exact composition varies considerably.For instance, the share of variable renewable energy (VRE) sources ranges from 38% in the IEEJ_NE model to 72% in the AIM/Technology model.
On the demand side, final energy decreases slightly between the Low Carbon (80%) and Net Zero scenarios.In the AIM/Hub and IEEJ models, there is a significant drop in final energy compared to the Baseline scenario.The models show the dominance of electricity by 2050 (figure 5).Hydrogen makes some inroads in three models, especially for the Net Zero scenario.(Note that the AIM/Hub model used in this study does not represent hydrogen as a final energy carrier.)Among the three models (except AIM/Hub), the median share of hydrogen in final energy in 2050 is 0.79% for the Low Carbon (80%) scenario and jumps up to 6.9% in the Net Zero scenario.
Next, we present the policy costs.Figure 6 describes the consumption loss per GDP (in the AIM/Hub model), the energy system cost per GDP (in other models), and the marginal cost of abatement (or carbon price) for the Low Carbon (80%) and Net Zero scenarios, compared to the Baseline scenario, as well as their differences between the Low Carbon (80%) and Net Zero scenarios.By 2050, under the Low Carbon (80%) scenario, the consumption loss reaches about 3%, while the energy system cost increases up to ∼1% (the TIMES-Japan model shows a 0.3% for the cost).These magnitudes are roughly in agreement with our previous work (Sugiyama et al 2021).Under the Net Zero scenario, the consumption loss increases up to ∼ 5% and the energy system cost increases up to > 2% in two models.
A notable difference from the previous study is a sharp decrease in the marginal cost of abatement due to model updating.In 2050, the median value of the marginal cost of abatement across models is ∼ 400 USD2010/t-CO 2 for the Low Carbon (80%) scenario while that of our previous work was ∼ 800 USD2010/t-CO 2 (Sugiyama et al 2021) for the same scenario setting.This is consistent with rapid cost reductions in a number of key mitigation technologies (IPCC 2022), which have been difficult to incorporate into models (Shiraki and Sugiyama 2020).For the Net Zero scenario, the median value is ∼ 650 USD2010/t-CO 2 .

Sensitivity to emissions cap and CDR constraint
We conduct a sensitivity analysis with respect to the emissions cap and the CDR constraint.Changing the reduction target requires more CDR deployment (except for the change from Net Zero (46by30+100by50) to 46by30+105by50 in the TIMES-Japan model) (figure 8).While the Low Carbon (80%) scenario (46by30 +80by50) requires negligible CDR deployment in three out of the four models, the models suggest much greater deployment for Net Zero and beyond.The results above are for the full CDR deployment scenarios.Figure 8 describes the response of the models to the CDR deployment constraints.The leftmost scenario for each model (each panel) corresponds to the full CDR case described in figure 7.As the maximum CDR is reduced, the models reduce the total residual emissions.How the residual CO 2 emissions vary with the CDR constraint differs from model to model.For example, transportation is elastic in the three models except for IEEJ-NE_Japan, which has inelastic emissions in transportation.The industry sector experiences a reduction between the scenarios with and without the CDR constraint in all models, although the magnitude of the reduction differs across models.(The different strategies adopted by the models are described in figures ESM 7 and 8 in terms of the difference in emissions and final energy between the Net Zero and 46by30+100by50_CDR100 scenarios).In all four models, there is no model solution with the zero CDR availability, and the AIM/Hub shows no solution for a CDR constraint of 50 Mt-CO 2 /yr.
Finally, we examine the results of simultaneously changing the CDR constraint and the emissions reduction target.We show how the carbon price (marginal cost of abatement) responds to the changing CDR constraint and emissions cap (figure 9).As expected, the carbon price increases with the stringency of the policy (color) and the CDR constraint (horizontal axis in each panel).We find that it is critical to deploy CDR at a scale to achieve economically efficient transition to net zero.For example, under the Net Zero scenario, the median marginal cost increases from 650 USD2010/t-CO 2 with the full CDR availability to 1950 USD2010/t-CO 2 with a CDR constraint of 50 Mt-CO 2 /yr.The models also do not show solutions for deep decarbonization without significant CDR deployment.

Discussion and conclusions
This study employed four energy systems models and integrated assessment models to examine the pathways for Japan to achieve the net-zero goal by 2050.The study analyzed mitigation strategies, primary and electricity generation mixes, and policy costs, and CDR deployment across models and scenarios.We examined how the mitigation strategies change from the previous target (Low Carbon (80%)) to the current one (Net Zero).We find that the strategies for the 80% target also work for the 100% reduction target as well, although each strategy needs to be strengthened in terms of both intensity and speed.Economy-wide energy efficiency improvements, power-sector decarbonization, and electrification need to be accelerated beyond the linear interpolation from the previous to the current goal.We also find that CCS-based CDR is an essential element in our study.
The present analysis suggests that while the main strategies remain the same, Japan should significantly enhance the current policy mix to facilitate energy conservation, power-sector decarbonization, and demandside electrification.To address residual emissions, Japan needs ∼ 100MtCO 2 /yr of CDR for a cost-effective netzero policy, on the order of 10% of the current emissions.This is not so different from policy discussions in other countries; Sweden's net-zero target for 2045 includes a 15% contribution from so-called 'supplementary measures' such as BECCS and international credits (Fuss and Johnsson 2021).A multi-model analysis of the long-term strategy of the United States also implies CDR deployment albeit with a wide range of ∼ 1000Mt-CO 2 /yr-∼ 3000Mt/CO 2 /yr (Browning et al 2023), which roughly translates into one-sixth (∼16%) or half (50%) of the current emissions of ∼ 6Gt-CO 2 /yr.
This finding presents a challenge for Japan, a country with a high population density and (relatively) limited carbon storage capacity.The challenge of carbon storage capacity is formidable.Geophysically, Japan has a large, total CO 2 storage capacity of up to 240 Gt-CO 2 (CCS Long-Term Roadmap Review Committee 2022).However, not all of the capacity may not be developed because of social acceptance issues (Tanaka 2020) and economic considerations.In addition, the annual (not cumulative) storage scale may be limited for the same reasons.As noted above, the government recently announced a roadmap for CCS, but it is not clear how the storage capacity will be used in the future.Storage capacity can be used for a variety of purposes, including CDR and powersector and industrial decarbonization (e.g., CCS for coal-fired power, steel, and cement).The prioritization or market selection of carbon sources for CCS (e.g., from industry or direct capture) will be an important issue to consider in the future.
Given the scarcity of domestic carbon storage, Japan is promoting the Asia CCUS Network, a network of member states from the Association of Southeast Asian Nations (ASEAN), Australia, the United States, and Japan).It intends to study the implications of exporting CO 2 across borders.Such issues need to be thoroughly analyzed in terms of international (e.g., Article 6 of the Paris Agreement) and national law, economics, technical feasibility, and environmental impacts.
Another key issue is how to divide the expected CCS storage between fossil CCS and CCS-based CDR options (i.e., BECCS and DACCS).The Ministry of Economy, Trade, and Industry published a roadmap for carbon capture and storage, indicating an annual storage rate of 120-240Mt-CO 2 /yr by 2050 (CCS Long-Term Roadmap Review Committee 2023).The roadmap covered several use cases for CCS, including power-sector decarbonization and reduction of emissions from the steel sector.In June 2022, the Japan Organization for Metals and Energy Security (an agency under the Ministry of Economy, Trade, and Industry) selected 7 projects for CO 2 storage (Japan Organization for Metals and Energy Security (JOGMEC) 2023), but most of them covered industrial CO 2 sources and power stations, with the project summaries indicating only one project with BECCS.
All in all, the current findings highlight the need for of long-term, responsible CDR policies (Bellamy 2018) given the long lead time for technology development and diffusion.
Figure 9. Carbon price as a function of the maximum CDR deployment by model and scenario (The carbon price in TIMES-Japan for the 46by30+105by50 with a CDR constraint of 100 Mt-CO 2 /yr exceeds 10,000 USD/t-CO 2 and is not reported.).The Net Zero scenario is shown as 46by30+100by50.'Full' on the horizontal axis implies no exogenous constraint on the scale of CDR deployment aside from model default assumptions.
This study has several limitations.First, although we systematically examined CDR availability and emissions reduction targets, we did not examine how residual emissions could be reduced by further progress in each demand sector.It should also be noted that the CDR deployment levels suggested in this study do not represent the absolute requirements.Rather, they should be interpreted as an economically efficient option, given the current modeling framework.There are many areas where further modeling improvements are needed, including demand-side solutions (Creutzig et al 2022, Sugiyama et al 2024), chemical feedstocks and products, and so forth.These also represent opportunities for further research.
Second, while we have demonstrated the importance of CDR, our study has not delved into the details of where and how such large storage can be accommodated, including the possibility of CCS export.Regional implications can be substantial, as in the case of the United States (Fauvel et al 2023).We have not considered CDR options other than BECCS and DACCS.Other options such as ocean alkalinity enhancement and blue carbon do not require geological storage sites, and expanding the CDR options in the models is an important research priority.Furthermore, potential international trading of CDR credits could help achieve Japan's netzero goal (Oshiro and Fujimori 2024).
Finally, we have not explicitly addressed the issue of non-CO 2 GHGs; addressing them would likely increase the CDR requirement (Harmsen et al 2023).(Our preliminary analysis suggests that including non-CO 2 GHGs would increase the CO 2 reduction target by up to 105%; see figure ESM 9).Such issues are left for future research.

Figure 1 .
Figure 1.Time series of CO 2 emissions (excluding AFOLU) by sector and model for (a) the baseline scenario, (b) the Low Carbon (80%) scenario, and (c) the Net Zero scenario.The other in the AIM/Technology and TIMES-Japan represents DACCS.AFOFI signifies CO 2 emissions from fuel combustion in agriculture, forestry, and fishing.

Figure 2 .
Figure 2. Time series of CDR deployment (excluding land-based CDR, which is described in figure ESM 4) by model for the Net Zero scenario.

Figure 3 .
Figure3.Four indicators of climate change mitigation strategies by model.Columns represent, from the left to the right, the energy intensity of GDP, the CO 2 intensity of electricity, the share of electricity in final energy consumption, and the share of fossil fuels in primary energy.Rows represent, from top to bottom, the Low Carbon (80%) scenario, the Net Zero scenario, and the difference between the two.

Figure 4 .
Figure 4. (a) Primary energy mix and (b) power generation mix for 2050 by model and scenario.

Figure 5 .
Figure 5.The breakdown of final energy in 2050 by model and scenario.

Figure 6 .
Figure6.Three measures of policy costs by model and scenario, relative to the Baseline scenario (left column: Low Carbon (80%), middle column: Net Zero, right column: difference between the two scenarios).Shown are the consumption of loss per GDP (top row), the energy system cost per GDP (middle row), and the marginal cost of abatement (bottom row).

Table 1 .
Models participating in this study.Abbreviations are as follows: U: University, NIES: National Institute for Environmental Studies, IEEJ: Institute of Energy Economics, Japan, and IAE: Institute of Applied Energy.

Table 2 .
List of scenarios analyzed in this study.