Solely economic mitigation strategy suggests upward revision of nationally determined contributions

Abstract

•We review the 2030 NDCs from an economic perspective•We provide a solely economic mitigation pathway up to 2100 for 15 regions•The NDCs from India and Russia are higher to secure an economically favorable level•Countries can further reduce emissions to achieve economic efficiency globally The Paris Agreement relies on the reviewing process of national climate commitments to produce enhanced national pledges by virtue of national comparisons. At present, most reviews are based on equity principles, which countries have little agreement on. This paper provides an alternative perspective to the current discussion and treats emission reduction as a solely economic behavior motivated by avoiding future economic damages from climate change. Assuming no consensus over equity or international cooperation, we generate a solely economic mitigation pathway up to 2100. At each term, the national climate damage caused by an additional unit of carbon emission is no higher than the additional mitigation cost. This solely economic emission path can be informative for countries to control their national emissions at an economically favorable level while providing an alternative economic perspective for comparing national pledges. The use of equity principles to review the nationally determined contributions (NDCs) is critical to facilitating more ambitious climate actions. However, disagreement over the equity principles persists. We instead treat emission reduction as a solely economic behavior motivated by avoiding future economic damage from climate change. Assuming no international cooperation, we provide a solely economic mitigation pathway to review national climate pledges until 2100. Using the value in 2030 to review the NDCs, we find that the NDCs of China, the USA, and the EU are 1.5, 1.4, and 0.9 respective GtCO2eq lower than their solely economic emission levels, whereas India commits 3.8 GtCO2eq more than its solely economic emission level. We also propose an equal-effort cooperation scenario toward 2°C where each country reduces emissions by 28% of their solely economic levels in 2030. Through exploration of the economic trade-offs, our results suggest that more ambitious NDCs are urgently needed. The use of equity principles to review the nationally determined contributions (NDCs) is critical to facilitating more ambitious climate actions. However, disagreement over the equity principles persists. We instead treat emission reduction as a solely economic behavior motivated by avoiding future economic damage from climate change. Assuming no international cooperation, we provide a solely economic mitigation pathway to review national climate pledges until 2100. Using the value in 2030 to review the NDCs, we find that the NDCs of China, the USA, and the EU are 1.5, 1.4, and 0.9 respective GtCO2eq lower than their solely economic emission levels, whereas India commits 3.8 GtCO2eq more than its solely economic emission level. We also propose an equal-effort cooperation scenario toward 2°C where each country reduces emissions by 28% of their solely economic levels in 2030. Through exploration of the economic trade-offs, our results suggest that more ambitious NDCs are urgently needed. The international climate regime entered a new stage after the Paris Agreement was adopted in 2015, by which countries set their nationally determined contributions (NDCs, all abbreviations can be found at Table 1) in the context of national priorities. With no legally binding constraints in the near term, the key driver in accomplishing these goals is the “pledge and review” system, in which countries put forward a national commitment and assess their progress periodically.2Jacquet J. Jamieson D. Soft but significant power in the Paris Agreement.Nat. Clim. Chang. 2016; 6: 643-646https://doi.org/10.1038/nclimate3006Crossref Scopus (43) Google Scholar Taking stock of the collective progress is critical to producing enhanced national pledges by virtue of national comparisons.3UNFCCC (Adoption of the Paris Agreement. Report no. FCCC/CP/2015/L.9/Rev.1.2015http://unfccc.int/resource/docs/2015/cop21/eng/l09r01.pdfGoogle Scholar,4Rogelj J. Den Elzen M. Höhne N. Fransen T. Fekete H. Winkler H. Schaeffer R. Sha F. Riahi K. Meinshausen M. Paris Agreement climate proposals need a boost to keep warming well below 2°C.Nature. 2016; 534: 631-639https://doi.org/10.1038/nature18307Crossref PubMed Scopus (1428) Google Scholar However, no consensus has been made on the operationalization of equity following the common but differentiated responsibilities and respective capability (CBDR-RC) principles. Therefore, alternative aspects of conducting the NDC review, which ranks and compares national climate efforts, are needed. NDC review using equity principles is conducted by allocating 2°C or 1.5°C global emission budgets to countries with the use of equity criteria.5Intergovernmental Panel on Climate ChangeClimate Change 2014: Mitigation of Climate Change. Cambridge University Press, 2015Google Scholar, 6Höhne N. Den Elzen M. Escalante D. Regional GHG reduction targets based on effort sharing: a comparison of studies.Clim. Policy. 2014; 14: 122-147https://doi.org/10.1080/14693062.2014.849452Crossref Scopus (123) Google Scholar, 7Robiou du Pont Y. Jeffery M.L. Gütschow J. Rogelj J. Christoff P. Meinshausen M. Equitable mitigation to achieve the Paris Agreement goals.Nat. Clim. Chang. 2017; 7: 38-43https://doi.org/10.1038/nclimate3186Crossref Scopus (127) Google Scholar Ambitious countries are those that commit NDCs below the allocation results. Equity can be quantified into five categories: responsibility,8Wei Y.M. Wang L. Liao H. Wang K. Murty T. Yan J. Responsibility accounting in carbon allocation: a global perspective.Appl. Energ. 2014; 130: 122-133https://doi.org/10.1016/j.apenergy.2014.05.025Crossref Scopus (57) Google Scholar capability,9Den Elzen M. Höhne N. Sharing the reduction effort to limit global warming to 2°C.Clim. Policy. 2010; 10: 247-260https://doi.org/10.3763/cpol.2009.0678ACrossref Google Scholar equality,10Chakravarty S. Chikkatur A. De Coninck H. Pacala S. Socolow R. Tavoni M. Sharing global CO2 emission reductions among one billion high emitters.Proc. Natl. Acad. Sci. USA. 2009; 106: 11884-11888https://doi.org/10.1073/pnas.0905232106Crossref PubMed Scopus (227) Google Scholar responsibility-capability need,11Baer P. Fieldman G. Athanasiou T. Kartha S. Greenhouse development rights: towards an equitable framework for global climate policy.Camb. Rev. Int. Aff. 2008; 21: 649-669https://doi.org/10.1080/09557570802453050Crossref Scopus (47) Google Scholar and equal cumulative per capita emissions.12Raupach M.R. Raupach M.R. Davis S.J. Peters G.P. Andrew R.M. Canadell J.G. Ciais P. Friedlingstein P. Jotzo F. Van Vuuren D.P. et al.Sharing a quota on cumulative carbon emissions.Nat. Clim. Chang. 2014; 4: 873-879https://doi.org/10.1038/nclimate2384Crossref Scopus (209) Google Scholar,13Pan X. Teng F. Ha Y. Wang G. Equitable access to sustainable development: based on the comparative study of carbon emission rights allocation schemes.Appl. Energ. 2014; 130: 632-640https://doi.org/10.1016/j.apenergy.2014.03.072Crossref Scopus (30) Google Scholar The allocation of the emission budgets varies significantly when applying different principles, and no approach has been agreed upon by all countries.14Schroeder H. Boykoff M.T. Spiers L. Equity and state representations in climate negotiations.Nat. Clim. Chang. 2012; 2: 834-836https://doi.org/10.1038/nclimate1742Crossref Scopus (48) Google Scholar,15Keohane R.O. Victor D.G. Cooperation and discord in global climate policy.Nat. Clim. Chang. 2016; 6: 570-575https://doi.org/10.1038/nclimate2937Crossref Scopus (148) Google Scholar A suite of studies combined effort-sharing principles and proposed a hybrid allocation in which each country adopts the least stringent allocation principle to realize the Paris Agreement goal.16Meinshausen M. Jeffery L. Guetschow J. Du Pont Y.R. Rogelj J. Schaeffer M. Höhne N. Den Elzen M. Oberthür S. Meinshausen N. National post-2020 greenhouse gas targets and diversity-aware leadership.Nat. Clim. Chang. 2015; 5: 1098-1106https://doi.org/10.1038/nclimate2826Crossref Scopus (63) Google Scholar,17Robiou du Pont Y. Meinshausen M. Warming assessment of the bottom-up Paris Agreement emissions pledges.Nat. Commun. 2018; 9: 4810https://doi.org/10.1038/s41467-018-07223-9Crossref PubMed Scopus (45) Google Scholar However, these equity principles do not consider the economic efficiency of mitigation. Comparatively, the cost-effectiveness approach, which is usually realized via integrated assessment models (IAMs), has been developed.18Aldy J. Pizer W. Tavoni M. Reis L.A. Akimoto K. Blanford G. Carraro C. Clarke L.E. Edmonds J. Iyer G.C. et al.Economic tools to promote transparency and comparability in the Paris Agreement.Nat. Clim. Chang. 2016; 6: 1000-1004https://doi.org/10.1038/nclimate3106Crossref Scopus (77) Google Scholar, 19Tavoni M. Kriegler E. Aboumahboub T. Calvin K. de Maere G. Wise M. Klein D. Jewell J. Kober T. Lucas P. et al.The distribution of the major economies' effort in the Durban Platform scenarios.Clim. Change Econ. 2013; 4: 1340009https://doi.org/10.1142/S2010007813400095Crossref Scopus (59) Google Scholar, 20Wei Y.M. Han R. Liang Q.M. Yu B.Y. Yao Y.F. Xue M.M. Zhang K. Liu L.J. Peng J. Yang P. et al.An integrated assessment of INDCs under Shared Socioeconomic Pathways: an implementation of C3IAM.Nat. Hazards. 2018; 92: 585-618https://doi.org/10.1007/s11069-018-3297-9Crossref Scopus (34) Google Scholar It can be calculated by total cost minimization or equalizing the marginal abatement cost among all countries depending on the model structure.21Mi Z. Liao H. Coffman D.M. Wei Y.M. Assessment of equity principles for international climate policy based on an integrated assessment model.Nat. Hazards. 2019; 95: 309-323https://doi.org/10.1007/s11069-018-3408-7Crossref Scopus (8) Google Scholar Robiou du Pont et al. combined cost-effective global emissions with the equality principles and allocated the global cost-optimal emissions from IAMs under the effort-sharing approach.7Robiou du Pont Y. Jeffery M.L. Gütschow J. Rogelj J. Christoff P. Meinshausen M. Equitable mitigation to achieve the Paris Agreement goals.Nat. Clim. Chang. 2017; 7: 38-43https://doi.org/10.1038/nclimate3186Crossref Scopus (127) Google Scholar,22Robiou du Pont Y. Jeffery M.L. Gütschow J. Christoff P. Meinshausen M. National contributions for decarbonizing the world economy in line with the G7 Agreement.Environ. Res. Lett. 2016; 11: 054005Crossref Scopus (28) Google Scholar However, the benefits associated with decarbonization (i.e., avoided climate damage) were usually masked23Ringius L. Torvanger A. Underdal A. Burden sharing and fairness principles in international climate policy.Int. Environ. Agreem-P. 2002; 2: 1-22https://doi.org/10.1023/A:1015041613785Crossref Google Scholar,24Zenghelis D. Equity and national mitigation.Nat. Clim. Chang. 2017; 7: 9-10https://doi.org/10.1038/nclimate3192Crossref Scopus (4) Google Scholar and thus could reduce the incentive to achieve optimum action toward reducing climate change.25Iyer G. Calvin K. Clarke L. Edmonds J. Hultman N. Hartin C. McJeon H. Aldy J. Pizer W. Implications of sustainable development considerations for comparability across nationally determined contributions.Nat. Clim. Chang. 2018; 8: 124-129https://doi.org/10.1038/s41558-017-0039-zCrossref Scopus (30) Google Scholar A recent paper introduced the mitigation benefits into the reviewing process and proposed a self-preservation strategy.26Wei Y.M. Han R. Wang C. Yu B. Liang Q.M. Yuan X.C. Chang J. Zhao Q. Liao H. Tang B. et al.Self-preservation strategy for approaching global warming targets in the post-Paris Agreement era.Nat. Commun. 2020; 11: 1624https://doi.org/10.1038/s41467-020-15453-zCrossref PubMed Scopus (31) Google Scholar This self-preservation strategy offers a higher cumulative net benefit for all countries within this century than that represented by their current NDC pledges. The strategy is built on an assumption of international cooperation, and all four equity principles are used in the calculation. To facilitate further discussion of the NDC review, we have extended the self-preservation strategies from a noncooperative perspective. Our review approaches fairness by providing an economic perspective, which balances national mitigation costs and benefits. Countries invest in mitigation to reduce future climate damage. The avoided damage is regarded as the benefit of emission reduction, whereas the damage is the monetized aggregation of both market and nonmarket loss related to climate change. Market loss includes the value of physical damage from climate hazards (such as droughts and cyclones), whereas nonmarket losses are those that are hard to reflect in market values, such as climate impact on mortality and crime. We equalize the marginal mitigation cost with marginal climate damage in the long term for each region by using the noncooperative scenario of the Regional Integrated Climate-Economy (RICE) model.27Nordhaus W.D. Yang Z. A regional dynamic general-equilibrium model of alternative climate-change strategies.Am. Econ. Rev. 1996; 86: 741-765http://www.jstor.org/stable/2118303Google Scholar,28Nordhaus W.D. Economic aspects of global warming in a post-Copenhagen environment.Proc. Natl. Acad. Sci. USA. 2010; 107: 11721-11726https://doi.org/10.1073/pnas.1005985107Crossref PubMed Scopus (297) Google Scholar In this noncooperative scenario, marginal mitigation cost and its demerit of reducing economic output are optimized regarding the related marginal climate damage over time. National emission decisions will jointly affect the global temperature; therefore, we use a Nash equilibrium decision-making theorem to calculate the Pareto optimality, which means that no country could have a welfare gain when changing its emission strategies. The resulting emission trajectory suggests a ceiling emission where the marginal mitigation cost is equalized with marginal mitigation benefits (the damage costs of an additional ton of carbon emission) at each point in time. Exceeding the ceiling emission will lead to more significant marginal damage than marginal abatement costs. The RICE model has been widely employed to investigate Nash noncooperative equilibria and the social cost of carbon.29Nordhaus W. Climate clubs: overcoming free-riding in international climate policy.Am. Econ. Rev. 2015; 105: 1339-1370https://doi.org/10.1257/aer.15000001Crossref Scopus (355) Google Scholar, 30Kotchen M.J. Which social cost of carbon? A theoretical perspective.J. Assoc. Environ. Resour. Econ. 2018; 5: 673-694https://doi.org/10.1086/697241Crossref Scopus (12) Google Scholar, 31Chen C. Zeckhauser R. Collective action in an asymmetric world.J. Public Econ. 2018; 158: 103-112https://doi.org/10.1016/j.jpubeco.2017.12.009Crossref Scopus (6) Google Scholar The global version of RICE (i.e., the Dynamic Integrated Climate-Economy [DICE] model) is one of three models that have been used to value the benefits of reducing CO2 emissions from an array of energy, air pollution, and climate change regulations in the US.32California Assembly Bill 197 (AB-197) (2016).Google Scholar, 33Schlatter L. Findings of Fact, Conclusions, and Recommendations: Carbon Dioxide Values State of Minnesota. Office of Administrative Hearings, 2016Google Scholar, 34Larson A. Subsidies proposed for New York's upstate power plants.https://www.powermag.com/subsidies-proposed-for-new-yorks-upstate-nuclear-power-plants/Date: 2016Google Scholar, 35Yang P. Yao Y.F. Mi Z. Cao Y.F. Liao H. Yu B.Y. Liang Q.M. Coffman D.M. Wei Y.,M. Social cost of carbon under shared socioeconomic pathways.Glob. Environ. Chang. 2018; 53: 225-232https://doi.org/10.1016/j.gloenvcha.2018.10.001Crossref Scopus (19) Google Scholar With a rigorous and transparent modeling framework, we conduct the review from two aspects. “Mitigation” indicates the absolute amount of emission reduction, defined as the gap between the economically optimal emission and the national commitment. “Effort” is defined as the percentage of emission reduction compared with national cost-benefit optimal emission. The effort index standardizes national mitigation and makes the review comparable among countries. The optimal level is reached when the marginal mitigation cost is equal to the marginal climate damage avoided at a national level. Although the national emission reductions under the solely economic scenario are theoretically optimal, they are subject to the assumption of Nash equilibrium without any national cooperation. International cooperation, which allows high-mitigation-cost countries to transfer their mitigation effort, can reduce the general mitigation cost, thus having a Pareto improvement for all. However, the international cooperation equilibrium can be challenged by free-rider intentions, which is to say that countries benefit from the carbon reductions of others without themselves contributing to reduction efforts that would impose costs on their citizens.36Lattanzio R.K. International Climate Change Financing: The Green Climate Fund (GCF). Congressional Research Service, 2014https://www.everycrsreport.com/files/20140505_R41889_9d0d7f034c21dca729f78a95d52371abed94f678.pdfGoogle Scholar,37Green Climate Fund Annex IV: Status of pledges and contributions made to the Green Climate Fund.in: Seventh Report of the Green Climate Fund to the Conference of the Parties to the United Nations Framework Convention on Climate Change. 2015: 80-83Google Scholar Because cooperation is hard to achieve, we assume no international cooperation or consensus over equity. The results provide an economically rational mitigation approach, thereby enhancing national climate pledges in light of vulnerability and national risk aversion toward climate change. Our calculation offers a ceiling emission value for countries rather than a 1.5°C or 2°C consistent national emission at the NDC commitment year. If one country’s emissions are higher than its ceiling emissions, it is regarded as economically inefficient. The review facilitates further ambitious commitment through an identification process by using both absolute and relative quantities. We compare the NDCs with the economically optimal emission. Mitigation is the absolute amount of emission reduction, and effort is the relative quantities of emission reduction. We also use the National Effort Index (NEI) to denote this percentage change, which is defined as the quantity of emission reduction divided by the noncooperative economic emissions. The index makes the review comparable among countries and helps to recognize the effort made by small emitters. Balancing the national cost and benefit in mitigation without cooperation assumption, this paper reviews the NDCs through their solely economic aspects, which can supplement the current equity principles. We divided the world into 15 regions according to the international climate regime. Countries with formally stated emission-reduction targets under the Kyoto Protocol (also known as the Annex-B countries) were grouped into six. The US, the EU (including the UK), Russia, Canada, and Japan were taken separately, and the rest were taken as other Annex-B countries. Four developing countries taking the lead in climate negotiations (also known as the BASIC group), namely China, India, Brazil, and South Africa, were also analyzed separately. To ease computational difficulties, we divided the rest of the world into five regions according to geographical locations, namely the Middle East and Africa (MAF; with the exception of South Africa), other Asia (OAS; with the exception of the Middle East, China, India, Japan, and former Soviet Union states), Latin America and the Caribbean (LAM; with the exception of Brazil), the reforming economies of Eastern Europe and the former Soviet Union (REF), and other Europe (OEU). The above regional definitions will be used for MAF, OAS, and LAM in the following context. Detailed information is provided in the supplemental experimental procedures. We account for the uncertainty regarding socioeconomic development and social preferences to generate the economically optimal emission path for regions (Figure 1, range in gray) and then compare our results with five equitable 2°C allocations (capability, constant emission ratio, equal cumulative per capita, equal per capita, and greenhouse development right) from Robiou du Pont et al.7Robiou du Pont Y. Jeffery M.L. Gütschow J. Rogelj J. Christoff P. Meinshausen M. Equitable mitigation to achieve the Paris Agreement goals.Nat. Clim. Chang. 2017; 7: 38-43https://doi.org/10.1038/nclimate3186Crossref Scopus (127) Google Scholar (Figure 1, colored range). Capability emphasizes the availability of resources to mitigate and allocates carbon budgets by per capita gross domestic product (GDP). The constant emission ratio (also known as the “grandfathering law”) maintains the base year’s national emissions rate. Equal cumulative per capita strengthens historical responsibility and allocates populations with high historical emissions with low-carbon budgets. Equal per capita emphasizes per capita equity and suggests a per capita emission convergence after the convergence period. The greenhouse development right preserves a “right to development” and allocates carbon budgets by weighted capability index and equity index. Further details on competing methodologies for equity allocation can also be found at Paris Equity Check (http://paris-equity-check.org/) and in Table S3. Under our assumptions, national emission without equality or temperature consideration (Figure 1, gray range) will generally increase at the beginning and slow down as a result of the severe climate damage at the end of this century. Greenhouse gas (GHG) emissions in China, India, and Russia show a declining trend after the peak. However, as a result of conservative estimations of the climate damage and consideration of fossil-fueled development (SSP5), the solely economic emission without cooperation in many regions will increase throughout the period. According to the GDP projections in the shared socioeconomic pathway (SSP) database, MAF and OAS are expected to have a fast economic development throughout the century. Under a noncooperative assumption considering only national mitigation costs and benefits, the solely economic emission path is increasing throughout the time period. The solely economic mitigation will lead the average surface temperature to 3.5°C–4°C, which is much higher than the 2°C goal suggested; therefore, in most cases, the results are higher than the equitable allocation. The inconsistency between equality allocation can be shown; for example, the constant-emission-ratio principle maintains the ratio of national emissions in the base year. The allocation under this principle will provide more carbon quotas to the developed countries that emit higher today. In contrast, the equal-cumulative-per-capita principle will allocate more emission space for developing countries because they have less historical emission. In some cases, countries can receive emission quotas much higher than their needs. Following the capability allocations, which concern the liability for climate damage with different abilities to pay, MAF and OAS countries will have a high possibility of emitting higher than their needs. Similar results are also applied for greenhouse development rights for the REF when equitable 2°C allocations under these principles are much higher than the solely economic emission. According to their national mitigation cost and estimated climate damage, if the countries emit as the highest 2°C allocations have suggested, the additional climate damage will be much higher than the mitigation cost. The pioneers and laggards identified by the equitable allocation and noncooperative economic aspects are mostly similar, although there are a few exceptions. The NDCs of India, Latin America, and REF countries satisfied the equity principles but are not economically efficient. India's NDC is lower than the equal cumulative per capita allocation by 0.8 GtCO2eq. However, India's cost-benefit emission suggests committing an additional 3 GtCO2eq emission reduction than the current NDC. Although the REF commitment conforms to the greenhouse development rights, it is higher than the average level of purely economic emissions, indicating a possibility of facing more significant future damage than the current mitigation cost. We conducted a range of sensitivity tests, including the socioeconomic uncertainty, the sensitivity test of the social discount rate, the climate impact on productivity, and the equilibrium climate sensitivity (see Tables S1 and S2). The socioeconomic path has the dominant effect on the emission path. The effect of the social discount rate, which represents governments’ attitude toward climate change, is also considerable. Combating climate change is a long-term process involving multiple generations, and the social discount rate presents the generational preference in the decision making. Therefore, valuing the social discount rate is crucial to determining the solely economic emission trajectory given that climate change concerns the decision maker only if they value the future. Within the Ramsey framework, the social discount rate can be estimated by the social time preference (STP) and the elasticity of the marginal utility of consumption (EMUC). The STP describes the consumption preference over time, reflecting the preference to consume earlier or later, which can also be seen as the generational welfare discount rate. The EMUC measures the utility change with consumption regardless of its timing. Following previous discussions and Intergovernmental Panel on Climate Change (IPCC) suggestions, we discuss a range of STP from 0.5% to 2.5% with an EMUC from 0.5 to 2.5.5Intergovernmental Panel on Climate ChangeClimate Change 2014: Mitigation of Climate Change. Cambridge University Press, 2015Google Scholar,38Glanemann N. Willner S.N. Levermann A. Paris Climate Agreement passes the cost-benefit test.Nat. Commun. 2020; 11: 110https://doi.org/10.1038/s41467-019-13961-1Crossref PubMed Scopus (42) Google Scholar Apart from the two parameters above, we also discuss the uncertainty caused by the alternative path of socioeconomic development, which is reflected by the SSPs with a set of variables. The SSP scenario is driven by a harmonized projection over GDP and population with a descriptive narrative for diverse fields. Because the RICE model is too simplified to reflect the sectoral and policy characteristics, we use the model result from the Global Change Analysis Model (GCAM) and the Integrated Model to Assess the Global Environment (IMAGE) to characterize the SSP in RICE. The purely economic emission with uncertainty is shown in Figure 2. The socioeconomic development path is the dominant source of uncertainty because it makes the national solely economic emission path remarkably diverse. The range within the SSP trajectory denotes the uncertainty from the STP and EMUC. The nine sets of STP and EMUC are illustrated in Figure 2, which covers a range of STP (0.5%, 1.5%, and 2.5%) and EMUC (0.5, 1.5, and 2.5). With the highest speed of economic development and relatively median decline in carbon intensity, SSP5 has the highest solely economic emission with the widest uncertainty range for most countries. With a greater prospect of economic development, countries have higher mitigation capability while facing a temperature increase. Besides SSP5, the regional rivalry (SSP3) scenario depicts an international fragmentation narrative with a low economic growth and the highest emission intensity at the end of this century. With limited power of global institutions and strong policy orientation toward security, this scenario will lead to a rapidly increasing trend for emission in Latin America, OEU countries, and the REF. On the basis of our analysis of the solely economic emission above, we conduct the national commitment review with regard to the amount of emission reduction and the relative percentage change. We build the NEI to provide an intuitive score of the national climate ambition. The index denotes the percentage of emission reduction reflected by the NDCs compared with the solely economic emission level in the committed year. We define the index as the relative percentage change compared with the cost-benefit emissions (see the experimental procedures). Higher NEI denotes a more ambitious climate pledge accordingly. Following previous calibration,39Nordhaus W.D. Revisiting the social cost of carbon.Proc. Natl. Acad. Sci. USA. 2017; 114: 1518-1523https://doi.org/10.1073/pnas.1609244114Crossref PubMed Scopus (341) Google Scholar we choose the value of 1.5% social preference with EMUC = 1.5 to conduct the review below because it is most commonly used by government analysis and literature. The review results could slightly differ under the five SSPs because the cost-benefit emission is sensitive to alternative socioeconomic development paths (Figure 3, pie chart). Socioeconomic factors such as economic growth, population, international relationship, and technological diffusion will lead to different mitigation and adaptation challenges. Therefore, the emission from a solely economic aspect could be different under alternative social development paths. The black ring border denotes a pie chart for global purely economic emissions in 2030 under five SSPs. The global economically optimal emission in 2030 denoted by the pie chart could differ with different socioeconomic assumptions. From SSP1 (sustainable development) to SSP