IntroductionTo achieve the goal of limiting global temperature rise to under 1.5 °C, the 2015 Paris Agreement established a framework that relies on national pledges to curb increased emissions, and introduced a 5-year ratchet cycle to progressively enhance countries’ climate policy ambition1,2. The Paris Agreement’s success thus critically depends on increased ambition levels and effective policy actions3. However, climate policy ambition differs significantly across countries2,4. While a burgeoning literature on climate policy studies has begun to investigate influential factors of inter-state variation3,5,6, a limited understanding of the overall landscape and evolution of national climate ambition still exists. This gap hinders efforts to identify effective solutions for enhancing national climate ambition. Furthermore, a growing body of research indicates that current global climate ambition remains insufficient to achieve the goal established by the Paris Agreement, highlighting the urgent need for countries to set more ambitious targets and to bridge the gaps between ambition and outcomes7,8. A complementary measure of national climate ambition provides a robust comparative basis, systematically capturing the extent, scale, and changes of such implementation gaps. Hence, I ask: how can we reclaim the analytical value of climate policy ambition and turn it into a comparative measure?A small but growing literature presents various approaches for assessing climate policy efforts. For instance, some focus on the regulation stringency or goals of emission reduction declared by governments9,10,11. Some use policy density, namely, the number of climate policies, as an alternative12,13. However, the previous studies have paid insufficient attention to the combined features of climate policies, such as variations in policy intensity across sectors, and how these variations comprehensively impact ambition levels. In terms of operationalization, due to extensive contextual differences, there are typically gaps when attempting to apply these processes across a diverse range of cases and periods. A large body of state-level comparisons focuses on state actors with similar contexts, such as member states of the European Union (EU), but this neglects the benefits to be gained from comparing states with very different economic and political contexts14. As such, evaluation methodologies are urgently needed to capture a wide spectrum of perspectives on ambition, and assessments of ambition need to be ongoing, especially considering the complex processes of global climate negotiations and the Paris Agreement’s pledge-and-review system2.To mitigate this gap, the primary goal of this paper is to construct a comparative index of national climate policy ambition. Specifically, this study introduces an output-based, new measurement mode, namely the Climate Policy Ambition Index (CPAI). With 12 policy variables, the CPAI tracks six policy instruments in four areas, covering 35 countries from 1990 to 2020. To comprehensively assess national ambition levels, the dataset includes all types of public policy documents issued at the national level by the 35 countries during this period (N = 1578).Compared to the existing literature, the new measurement model first explicitly distinguishes ambitious policy outputs from outcome variables, aiming to provide a clearer comparative basis and to enable more precise analyses of climate policy processes across multiple stages. I argue that, although many other factors can drive policy outcomes to deviate from their original designs, ambition still clearly captures the apparent intent behind national policies. Policy outputs thus uncover the key features shaping climate ambition, as outputs are causally prior to policy outcomes and impacts, representing the initial governmental decision to act15,16,17. Second, I derive two major comparative dimensions—depth and breadth from the pertinent theoretical work on policy outputs. Specifically, depth refers to the intensity levels of policy instruments, while breadth captures the diverse policy domains related to climate change. By delving into the depth of instruments in these areas, the index provides composite evaluations for national climate ambition, but also compiles results for specific climate policy sectors and instruments. The resulting panel data provides a valuable analytical tool for addressing a series of critical questions within the climate policy literature. For instance, what determines the ambition levels? Does the Paris Agreement fail to achieve its goal? To what extent does ambition diverge from actual outcomes? Third, the study develops a transparent and replicable assessment process, carefully designed to avoid overlapping indicators. This not only makes the structured database flexibly extensible, but also reduces the high research intensity required by the maintenance of large datasets.ResultsThis section presents the main descriptive findings from the CPAI, focusing on: (1) the variation and evolution of climate ambition across the 35 countries from 1990 to 2020; (2) cross-sectoral differences in the intensity of instruments; and (3) the divergence between climate ambition and actual policy performance.Evolving national ambition levelsFigure 1 delineates the cross-national differences in climate ambition at three time points—1994, 2005, and 2016 (the years for the entry into force of the UNFCC Convention, Kyoto Protocol, and Paris Agreement). These periods capture the essential transitions in global climate negotiations.Fig. 1: Cross-national variation in climate ambition levels in 1994, 2005, and 2016.Each panel displays the spatial distribution of national climate policy ambition in a given year: a 1994, b 2005, c 2016; differences in color shades illustrate the levels of climate ambition, in which darker shades refer to higher levels.Full size imageAt the starting stage, global climate policy ambition was generally low, and the highest level was below 0.13. Following the enforcement of the Kyoto Protocol, many countries have developed, designed, and adopted climate strategies for the build-up of the first commitment period. However, it is notable that the increase appeared to be levelling off over this timeframe. In 2005, Germany exhibited the highest levels (about 0.19), whereas developing countries generally lagged behind the average level. This may be because the Kyoto framework failed to adequately confront the inherent distributional conflicts around mandatory emission reduction18.A more pronounced increase is evident after the adoption of the Paris Agreement, with average ambition levels rising to ~0.35 by 2016. This reflects a strategic shift in international climate governance by embedding mitigation goals into national commitments, which emphasize the importance of global cooperation and collaboration in sustaining national progress. Simultaneously, it is noted that part of the observed rise is structurally embedded in the CPAI’s design, as certain indicators (e.g., the submission of (Intended) NDCs) were not applicable prior to the Paris Agreement.Focusing on the cross-national variations, most of the European countries have been frontrunners in climate policy efforts in terms of ambition over these periods, especially Germany, France, and the UK. From 2005 to 2016, countries in Asia showed marked improvements, for instance, China (from 0.1 to 0.42), India (from 0.09 to 0.34), and Japan (from 0.13 to 0.45). By contrast, countries in the Middle East, North America, and partially Africa showed relatively few changes. While these patterns reflect broader geopolitical dynamics, the trajectories of climate ambition for early movers in the international cooperation domain may appear flattened once they fulfill the relevant indicator.Sectoral variation in policy intensity levelsAlthough the overall ambition levels increased significantly in the post-Paris era, intensity levels of climate policy outputs (the depth of ambition) diverge significantly across policy fields (the breadth of ambition) and over time (see Fig. 2). The evolution of intensity levels in international cooperation correlates with key events in global climate negotiations, primarily reflecting the influence of time-specific developments on ambition levels rather than a continuous, purely policy-driven increase.Fig. 2: Changes in climate ambition depth, disaggregated by the breadth (2005–2020).The figure displays the evolution of policy intensity across four climate policy fields (economy and industry, energy paradigm, GHG emissions, and international cooperation), highlighting how each dimension has contributed to overall ambition over time.Full size imageIn contrast, economy-wide sectors present a continuous increase in the depth of climate ambition. This reflects that state actors have gradually recognized the great potential of climate actions in stimulating socio-economic developments and transformation. By contrast, the intensity levels across policies in the energy and emission fields increased modestly. The cross-field variations partially reflect that countries tend to adopt economic fixes to climate issues, but with great difficulties putting quantified targets for emission mitigation and energy transformation in place, especially given the huge costs of relevant policies.The growing gaps between ambition and outcomesFigure 3 depicts the temporal evolution of changes in the CPAI, with a comparison to the results of an alternative—the Climate Change Performance Index (CCPI) (which highlights a comprehensive assessment of policy outputs and outcomes)19. In addition, the CCPI provides data from 2005 to 2022, but some measurement criteria were changed since 200719. Therefore, to ensure the availability of data, the empirical comparison between the two indices limits 26 countries and 12 years (2008–2020). Moreover, as the CCPI index scores countries’ climate protection performances from 0 to 100, it is re-coded to the range of [0–1] to facilitate the comparison with the new CPAI index.Fig. 3: Temporal changes of CPAI and CCPI from 2008 to 2020.This figure uses boxplots to compare the annual distribution of a Climate Policy Ambition Index (CPAI) and b climate Change Performance Index (CCPI) scores; each box represents the interquartile range (IQR), with the median shown as a central line, and black dots represent outliers.Full size imageThe major difference lies in their median fluctuations. The median evolutions of the CPAI give a stronger sign that there is an upward tendency in the ambition scores of countries, especially since 2015. Comparatively, the median changes of CCPI remain less, which suggests that most countries have achieved rather limited progress in their policy outcomes. This difference originates from the variance between the comparative focuses of the two indices. As noted above, in its assessment process, the CCPI gives higher weights to countries’ performances on emissions and energy status, while policy evaluations only account for 20% of the final score. It would be expected that the practical effects of climate mitigation policy are undulated over time, as there are a range of complex factors affecting the policy implementations. This also explains its median changes. By contrast, the CPAI index exclusively highlights the climate policy outputs of countries. Comparing the two indices further indicates that global climate policy ambition has progressed extraordinarily over the past decades, while practical results, to some extent, may not correspond with the strategies.More crucially, the limited outcomes alongside the high rise of ambition reveal an increasingly widening gap, providing important insights into the lasting climate policy discussion in the post-Paris era. Since its adoption in 2015, the effectiveness and prospects of the Paris Agreement have triggered extensive debates1,20,21. Some are optimistic, pointing out that the agreement’s pledge-and-review structure and ratchet mechanism can be increasingly strengthened through improved transparency, accountability, and peer pressure22. However, others indicate that its small, steady steps, while better than stagnation, are not keeping up with the rapidly closing window to prevent severe climate disruption23. The systematic measure of national climate ambition, thus, provides a valuable lens to critically review the post-Paris climate governance. The Paris logic has secured the rise of ambitions and promoted international climate cooperation, but it has not put in place the structure for a long-term technological and socio-economic transformation. This tension between ambition and outcome highlights the need for great caution in celebrating the Paris framework as a grand success and calls for a careful reconsideration of the agreement’s fundamental design to translate ambitious targets into actual implementations.Notably, this increasingly widening gap between climate ambition and actual performance is particularly driven by domestic policy outputs. Due to their bounded and cumulative characteristics, the indicators for international climate cooperation appear to exhibit limited variance over time and therefore contribute less to this observed divergence. This highlights the importance of considering the potential decoupling between international and domestic climate policy ambition. For instance, some countries may exhibit strong international ambition through early treaty ratification or net-zero commitments, while maintaining relatively low levels of domestic policy ambition. For future studies, the CPAI framework could be used to assess alignment or divergence between international commitment and the follow-up domestic actions. For example, by analyzing the temporal and structural correlation between scores across the two domains, future research could examine whether international commitments serve as credible signals of domestic action or merely as symbolic gestures.DiscussionThis article makes an original contribution to climate policy studies by structuring the output basis of ambition to introduce the Climate Policy Ambition Index (CPAI). Based on the conceptual focus of outputs, I integrate depth and breadth to interpret ambition, expanding the avenues for comparing national climate policies. This structure has the advantage of only focusing on outputs, which contrary to performance are directly under the control of climate policymakers. Moreover, this research rigorously follows a transparent and replicable process, which enhances its flexibility for future expansions, for instance, the inclusion of more national cases or disaggregation at finer levels (e.g., sub-national climate policies). By doing so, it enables a wide range of comparative studies relevant to climate policies, offering different avenues to compare previous findings.Besides that, the index seeks to convey complicated policy output information related to climate change in a parsimonious way. Comparatively assessing the levels of countries’ ambition presents a great challenge, especially considering national diversity in capacities, policy types, or goals2. Most of these composite indices with respect to climate policy efforts are maintained by large organizations, such as Japan’s National Institute for Environmental Studies, OECD, or NewClimate Institute24,25,26. This reflects the high research intensity required to create, manage, or update large policy datasets. Based on an output approach, the study develops a series of easily available metrics, and the measurement process is less research-intensive than what is currently necessary to keep large policy databases up to date. Future researchers or other users can draw on this process or synthesize the original data to create their metrics to facilitate independent comparisons of an array of climate policy topics.The CPAI also provides several potential applications for future climate policy research. For instance, it can serve as a standardized benchmark for cross-national comparisons by quantifying national climate ambition levels. In addition, the index facilitates the empirical analysis of the alignment or divergence between ambitious policy outputs and actual performance, enabling assessments of whether stated ambitions are translated into concrete policy actions. The index also allows researchers to generate disaggregated results for specific climate policy sectors and instrument types, supporting more targeted analyses of sectoral or instrumental drivers of ambition. Finally, the CPAI enables comparative analysis of the relationship between international commitments and follow-up domestic outputs, assessing whether there is a decoupling between the two realms.Descriptively, the CPAI finds that countries notably improve their climate ambition levels with the Paris enforcement, which reflects the importance of international processes in promoting national policy progress. Second, it demonstrates a regional disparity in ambition, emphasizing an urgent need for solutions to advance the incorporation of less ambitious districts, such as the Middle East. Third, the intensity levels of outputs diverge significantly across policy fields, in which energy and emission sectors present a very modest increase. Fourth, the gaps between the outputs and outcomes are prominent and require further attention.However, as the paper exclusively focuses on climate policy outputs of countries, it deliberately excludes non-central or sub-national policy initiatives. More insights remain to be expected by the extension of the examination scope. In addition, given its output basis, the CPAI data are naturally less informative about the extent of policy implementation or climate policy effectiveness. Future studies should be cautious when using or interpreting the results, as a higher ambition level does not necessarily indicate a better outcome.Simultaneously, it is worth noting that, from an abstract perspective on policy outputs, the index assigns equal weight to the intensity of various policy instruments. The empirical significance or robustness of certain instruments in the given sectors may change at the complex implementation stage. Using the CPAI data as a basis, future studies can further weight different variables according to various criteria, for instance, the relationships between sectoral ambition levels and emission pathways.The design of the index also raises an important empirical consideration. As discussed, the bounded and cumulative characteristics of indicators in the international cooperation domain can limit the CPAI’s temporal sensitivity and introduce confounding effects when assessing cross-country progress in ambition levels. To mitigate these challenges, future research could consider restructuring the index into dynamic and bonded sub-indices to better differentiate between event-based achievement and continuous policy ambition. Incorporating temporal weighting (e.g., assigning greater emphasis to recent policy developments) could further improve the temporal sensitivity and hence enhance the robustness of longitudinal and cross-country comparisons.MethodsAn overview of existing studiesWith the intensification of climate change and the rapid growth of national climate actions, climate policy literature has increased significantly in recent years27. A burgeoning strand of research seeks to add a comparative perspective by mapping climate policy actions across different entities and over time. This kind of research typically includes two categories. The first group provides descriptive statistics relevant to climate policies, such as policy types, contents, or sources of publication28,29,30. While these descriptive data are highly aggregated, such global assessments can serve as useful collection tools for the second group—composite indices19,24,25. These indices seek to rate the climate policy performances of countries and produce a high level of granularity on climate policy. However, these efforts vary across their measurement focus, criteria, and scope. Table 1 provides a comparative description of the key indices assessing climate policy on the national level.Table 1 An overview of the established assessment frameworks for national climate policyFull size tableThe shortlist of frameworks is developed through a keyword-based search conducted on Google Scholar, Scopus, and Web of Science. Specific keywords include combinations of “climate policy”, “climate policy ambition/efforts/commitments/outputs”, “climate policy comparisons”, “climate policy evaluation”, and “climate policy index”. Frameworks are included if they explicitly focus on ambition and provide disaggregated, indicator-based assessments. Those that focus exclusively on policy outcomes (e.g., emissions trends) or lack a structured approach to comparing policy ambition were excluded from the comparison.These indices reveal diverse perspectives on climate policy efforts, adopting a variety of measurement approaches. Some seek to rate countries based on a set of defined criteria24,26,31. Others, in which this research is included, aim to illustrate nuances and continuous changes in national climate policies by developing specific index scores. For instance, based on the combination of outcome indicators (e.g., emissions) and output variables (e.g., experts’ policy evaluations), the CCPI19attempts to assess the overall climate performance of countries. Others12 use the density perspective to analyze the levels of national climate ambition. The Climate Actions and Policies Measurement Framework (CAPMF) maintained by the Organisation for Economic Co-operation and Development (OECD) leverages 130 variables to evaluate how climate policy adoption and stringency vary across countries, requiring a high level of research intensity to maintain the dataset25.With its selective focus, each approach has its pros and cons, and may lead to a particular outcome or understanding of climate policy2. A simple count of the number of policies, for example, neglects differences in regulatory stringency and would therefore lead to misleading results. Assessments based on the combination of policy outputs and outcomes may blur the comparable basis of empirical studies. Consequently, a comprehensive and precise evaluation of climate ambition is still lacking. Achieving effective assessments of climate policy ambition necessitates active engagement with diverse analytical frameworks and comparative methods, which greatly enhances the reliability of findings2,31.An output approach: combining depth and breadthThis study seeks to complement the existing evaluative efforts by employing an output approach to clarify the comparative basis, highlighting the intensity levels of policy tools across diverse fields. Namely, I identify ambition as the overall intensity of instruments across all policy fields concerning climate change, which captures different kinds of climate policy outputs. Here, intensity refers to the degree to which resources are allocated to achieve a given objective, reflecting the substantive stringency of the policy tools employed. Figure 4 illustrates the process of using the output approach to track the climate ambition of countries.Fig. 4: The output-based evaluation framework is used to construct the Climate Policy Ambition Index (CPAI).This original figure illustrates the output-based approach used in constructing the Climate Policy Ambition Index (CPAI), where “Goals” refer to overarching climate objectives, “Intensity levels” indicate the degree to which resources are allocated to achieve a given objective, and “Domestic policy fields” and ‘International commitments’ represent the two key domains captured by the index in assessing national climate policy efforts.Full size imageFirst, the output approach concretizes the comparative focus of national climate ambition, representing the starting point for evaluating the climate policies of countries. Focusing on outputs is analytically independent from outcome-based or mixed approaches, as it represents the immediate, tangible commitments made by countries, rather than the effects that might be influenced by external factors15,16,17. This makes policy outputs an appropriate basis for assessing climate ambition. Namely, an output-based perspective highlights specific policies and targets the government aims to adopt, whereas an outcome-based approach focuses on the extent to which such actions mitigate climate issues.It is important to use outputs to situate ambition in a proper causal relationship, which allows researchers to recognize the policy efforts themselves, even if outcomes have yet to be achieved17,32. For instance, a country might pass an ambitious renewable energy policy, but actual increases in green energy may be delayed due to international fuel price drops. Therefore, an output-based perspective provides a standardized baseline for cross-national comparisons, helping to control for contextual differences, such as development levels or historical emissions. For example, a highly industrialized country with high emissions can still set ambitious emission reduction targets. In this way, the output-based approach provides a consistent framework for assessing how far states are willing to go in addressing climate change and for identifying potential gaps between ambitious policies and implementations.Two comparative dimensions, depth and breadth, are derived from the theoretical background of evaluating policy outputs. Changes in depth rest upon the intensity levels of different types of instruments, such as regulations or economic incentives. The breadth reflects the changes in outputs across different sectors. These two dimensions, together, provide a structured basis for comparing climate policy ambition across countries.The first dimension, depth, focuses on the nature of policy instruments. Specifically, it considers how the content of these instruments is calibrated to achieve intended goals, thereby ensuring the comparability of policy outputs. Policy outputs generally include ends and means at levels of abstraction, operationalization, and specific measures33. Policy means do not necessarily imply the achievement of designed policy goals, as loose assumptions about the relationships between ends and means may exist, but these design flaws are typically identified only during the implementation process34. Therefore, at the output stage, policies end are integrated into specific means in a bottom-up way by transferring more general norms and goals into substantive requirements32,35. In doing so, policy goals or objectives are tailored to exact settings. Namely, in the output process, on-the-ground requirements are usually embedded in the contents of designed means or tools (hereafter “instruments” for consistency)33.To monitor changes in policy instruments, previous studies13 introduce density and intensity as two basic directions, in which density examines the number of adopted instruments, while intensity focuses on their stringency levels, such as limits on fossil fuel usage. However, only considering density can easily produce errors in measuring policy output levels, while intensity specifies the substance of instruments2,32. Hence, evaluative frameworks usually identify intensity measures as comparative focus, although they may employ different terminology, such as stringency or levels of support19,25,26,36. Specific interpretations of intensity may vary when incorporating this concept into a broad measurement-driven approach. For instance, the CAPMF define it as “the degree to which climate actions and policies incentivise or enable GHG emissions mitigation at home or abroad”25. Others clarify it as the levels of resources invested by organizations32. In general, intensity levels indicate the extent to which resources are allocated to achieve a given goal, referring to the depth of the instruments in policy outputs.Second, corresponding policy fields define the scope of policy outputs for different issues, which refers to the breadth. Due to the complexity of climate change, state actors integrate a wide range of policy fields to mitigate this issue37. This includes not only areas at the domestic level (e.g., energy) but also international policy efforts. International policy outputs of countries emphasize their commitments to multilateral agreements, usually revealing which nations are leading the way in global climate policy and which are lagging behind36. Therefore, when assessing the outputs of climate policies, the breadth needs to be extended to encompass both domestic and international policy domains to provide a holistic perspective.The two comparative dimensions (namely, the depth and breadth of climate policy outputs) measure separate phenomena. For this reason, we suggest that researchers need to consider both dimensions in order to more accurately assess levels of ambition. This structure enables future studies to build their analysis focus, for instance, by comparing ambition levels of various policy mixes or across sectors. By measuring the collective intensity of instruments across various policy fields, their combination explores the changes in climate ambition of countries over time, from an output perspective.Structure, operationalization, and scopeDrawing on the output focus, depth (intensity levels of policy instruments) and breadth (a variety of corresponding policy fields) structure our assessment framework of national climate ambition—the Climate Policy Ambition Index (CPAI). Therefore, policy variables in the CPAI are categorized in two ways, namely instrument types and policy fields.Table 2 presents the index’s structure, which includes four major policy areas and three distinct types of policy instruments within each area. The selection of the four policy areas is based on a synthesis of classification criteria identified by leading international organizations (e.g., International Energy Agency), reflecting functionally distinct yet interrelated domains of national climate strategies. Within each policy area, I primarily draw on the taxonomy of the Intergovernmental Panel on Climate Change (IPCC), classifying climate policy instruments into three broad categories. For the international dimension, the categorization of policy instruments is further informed by established frameworks in international climate policy analysis36, thereby aligning domestic and international components under a structured analytical framework.Table 2 The structure of the Climate Policy Ambition Index (CPAI)Full size tableMeasuring the depth of ambitionTo specify, first, while institutions propose different typologies of climate policy instruments, this study adopts the IPCC’s classification as its primary framework. For instance, the OECD’s Policy Instruments for the Environment dataset summarizes five types—deposit refunds, beneficial subsidies, taxes and fees, tradable permits, and voluntary approaches38. To align with the OECD’s standards, the CAPMF further classifies the types as market-based instruments (e.g., carbon pricing) and non-market-based instruments (e.g., plans for renewables)25. The European Environment Agency identifies regulatory approaches, market-based instruments, and awareness-raising as three major types of instruments39. Based on a comprehensive literature review, the IPCC generalizes the wide variety of instruments as seven types—regulations and standards, taxes and charges, tradable permits, voluntary agreements, subsidies and incentives, research and development, and information instruments (Gupta and Tirpak,37). This taxonomy not only includes the classifications commonly used, for instance, the OECD’s approach, but also goes beyond their coverage to provide a comprehensive comparison. Accordingly, the study adopts the IPCC’s classification as the primary basis for identifying domestic climate policy instruments.To enhance the clarity and make policy information more accessible, the IPCC’s seven types of instruments are grouped into three broader categories. Specifically, according to the definition of subsidies and incentives, it refers to “direct payments, tax reductions, price supports or the equivalent thereof from a government” to support a specific goal37. Therefore, it has included the tax instrument. Tradable permits (namely the emission trading methods) are included under the general type of carbon pricing which is specifically developed for emission mitigation, and also are a part of economic incentives. Therefore, I use “economic incentives” to generalize the three types of instruments, namely tax and charges, tradable permits, and subsidies. Moreover, I exclude Information instruments, as those reveal less about the levels of policy intensity, and, thus, less about the differences in states’ climate policy outputs. Moreover, considering the disputes over the validity of voluntary agreements, I also exclude this instrument40,41,42. As such, I identify three general types of national policy instruments— regulations and standards (R&S), research and development (R&D), and economic incentives (EI).Moreover, given the global nature of climate change, I also aim to incorporate international policy outputs in the assessment framework. International climate policies usually include cooperative arrangements, international financial contributions, and reporting25,36,37. However, a state’s financial contributions to the UNFCCC are not under the policy output frameworks of countries but can be determined by many other factors, such as the national economic development level, country size, or natural disasters. I exclude this indicator from consideration. I additionally add the carbon neutrality target as one of the indicators, as increasing numbers of countries have joined in the race to Net Zero (e.g., most of the European countries, China, Canada, and Japan).Measuring the breadth of ambitionSecond, in terms of the typology of climate-related policy fields, the classification criteria also vary. The policy sectors relevant to climate issues generally include economy and industry, energy, GHG emissions, transport, building, electricity, agriculture and Land-Use, Land-Use Change and Forestry (LULUCF), and international cooperation (also see Supplementary Table 1 in the Supplementary Information for a review of the established categories). Although emission pathways or target groups of transport and building differ, the climate responses of the two sectors essentially surround emission limits and energy savings25. Therefore, based on an output perspective, the two sectors usually share the mechanisms of policy instruments, and hence can be regarded as sub-categories, falling under GHG emissions or energy. The electricity sector typically responds to climate concerns by phasing out fossil fuel-based power plants and increasing the utilization of renewable energy sources. The first is involved in industrial upgrading, and thus, is under the industry sector. The latter is reflected by the general output of the energy sector. Furthermore, due to the disparate natural resource endowments across nations, policy outputs in the sector of agriculture and LULUCF may not universally serve as a reliable indicator of climate policy ambition43. The existing classifications of climate policy fields tend to exclude this sector from their analysis25,28,44. As such, the index codes national climate policies from fields of economy and industry, GHG emissions, energy, and international cooperation, in order to avoid redundant or overlapping assessments. In the Supplementary Information, Supplementary Table 2 provides more detailed descriptions of the instrument types and policy fields, rationales for their involvements, and original data sources.In doing so, the CPAI codes the six types of instruments found in climate policy outputs across four areas, consisting of a total of 12 variables. To measure and normalize the intensity levels of these instruments, I draw on two different operationalization methods. Given the nature of intensity, the straight way of measuring its levels is to identify the invested resources or efforts to reach a particular goal. However, in practice, these efforts usually include quantitative facts (e.g., targeted emission limits) and qualitative information (e.g., bans on coal power plants). Quantitative policy outputs directly map onto levels of intensity. To specify the intensity of qualitative output information, I adopt the “Leximetric” data coding method, which assigns numerical values to measure the stringency or weakness of laws (Adams and Deakin,45). Hence, qualitative outputs are measured on an ordinal scale (e.g., 0, 0.5, 1; 0, 0.33, 0.67,1). Table 3 provides the operational coding rules for the 12 indicators introduced in Table 2, delineating the specific steps, rationale, and process of measuring each indicator. Each indicator ranges from values between [0] and [1], where [0] stands for no policy ambition, and [1] stands for the highest intensity.Table 3 The coding bookFull size tableI construct this index according to the following steps:(1) Identification of a general unit of analysis (‘climate policy outputs’)(2) Development of a high level of granularity on the unit of analysis, identifying indicators or variables which, separately or jointly, consist of the climate policy intensity.(3) Development of a coding rule which arranges the steps to be used in assigning dummy/numerical values to the primary sources.(4) Identification of a scale of measurement (e.g., 0–1), which is embedded in the coding rule.(5) Assignment of values to the individual variables or indicators, according to the coding rule.(6) Aggregation of individual variables or indicators into an index specifying a measure of climate policy ambition, which can be used in comparative examinations and other statistical analyses.A country could publish multiple policies for a given indicator in a year. The index could take multiple measurements during that year. However, because the purpose is to measure the most ambitious intensity, the index gives the final score to each indicator according to the highest intensity level in the annual measurement.It is important to note that several indicators in the CPAI (namely, UNFCCC climate treaties, (Intended) NDCs, and Targets for carbon neutrality) are cumulative and bounded. Due to their linkage to time-specific international events (e.g., Cop21 in Paris), most countries may fulfill these indicators within a narrow time window. This design captures key milestones in international climate governance but may also reduce the temporal sensitivity of the index. Once fulfilled, these indicators can maintain a fixed score, making it difficult to reflect subsequent policy developments (e.g., enhanced NDCs). As a result, the CPAI may understate continued policy progress by early movers while overstating the pace of progress among laggards. To mitigate this issue, we include a supplementary table (see Supplementary Table 4 in the Supplementary Information) that identifies which indicators are bounded and cumulative, and reports the year in which each country reaches the upper bound. This enables a clearer understanding of when certain indicators may lose temporal sensitivity and contribute disproportionately to national climate ambition scores.Regarding missing policy data, the CPAI assigns a zero score to their intensity levels and labels them as “missing” in the original database. This is to make a careful handle so as to alleviate the possibility of producing biased estimates or inferences46.The CPAI assigns equal weights to the underlying policy indicators. Given the diverse and complex nature of real-world climate policies, some instruments in given policy sectors might be more robust or effective than others under a particular socio-economic context, such as the carbon taxes and trading system in top emitters with high burdens in emission reductions47,48. However, the effectiveness of policy instruments or the importance of certain sectors is rarely identified at the design stage but usually emerges during the implementation process34. As the CPAI draws on the output perspective to evaluate climate ambition, I adopt a conservative approach by assigning equal weight to the importance of each indicator.To validate the allocation approach, I conduct a second-order confirmatory factor analysis (CFA) to examine the fit of the hypothesized factor structure by assessing relationships between proposed indicators and the latent factors49. Figure 5 reports the result, which shows that the constructed measurement model provides a reasonable representation of the relationships between the latent factor and observed indicators (with a Comparative Fit Index (CFI) of 0.923 and RMSEA of 0.09).Fig. 5: Second-order confirmatory factor analysis of the Climate Policy Ambition Index (CPAI) measurement model.This structural model demonstrates how individual policy indicators (represented as rectangles and defined in Table 2) load onto first-order latent constructs, which in turn load onto the second-order latent variable “Amb” (ambition); solid one-headed arrows represent factor loadings, while dotted arrows indicate standard errors.Full size imageRegarding the scope, the CPAI evaluates the 35 largest emitters whose contributions to global carbon emissions are greater than or equal to 0.5% (see Supplementary Table 3 in the Supplementary Information). These countries have contributed the most to anthropogenic global warming, and hence not only hold more responsibility to respond to climate change but also heavily influence the future global emission trajectories. Moreover, this group includes distinct classes of countries, such as the “climate leaders” (e.g., Germany) and “laggards” (e.g., the US and China), highly industrialized and new emerging economies (e.g., the UK and South Africa), OECD and non-OECD countries (e.g., Australia and Russia). In addition to climate-related variables, the dataset includes standard country-level covariates associated with political regimes, energy paradigms, and broad domestic differences.To systematically capture longitudinal changes, the period coded is from 1990 to 2020, given that climate change was generally accepted as a policy issue by the international community since the 1990s50.Data availabilityAll data produced and analyzed in this study are available in the article and its supplementary information. The Dataset file, including raw policy data with associated climate policy ambition scores, is available upon request.ReferencesFalkner, R. The Paris Agreement and the new logic of international climate politics. Int. Aff. 92, 1107–1125 (2016).Google Scholar Höhne, N., Fekete, H., den Elzen, M. G. J., Hof, A. F. & Kuramochi, T. Assessing the ambition of post-2020 climate targets: a comprehensive framework. Clim. 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Finally, I thank the two anonymous reviewers for their insightful comments, which significantly strengthened the quality of this work.Author informationAuthors and AffiliationsDepartment of Social and Policy Sciences, University of Bath, Bath, UKYitong YeSchool of Political and Social Sciences, University of Glasgow, Glasgow, UKYitong YeAuthorsYitong YeView author publicationsSearch author on:PubMed Google ScholarContributionsYitong Ye is solely responsible for all aspects of the research, including conceptualization, data collection, analysis, and writing of the manuscript.Corresponding authorCorrespondence to Yitong Ye.Ethics declarationsCompeting interestsThe author declares no competing interests.Additional informationPublisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.Supplementary informationSupplementary informationRights and permissionsOpen Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. 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