The Paris Agreement aims to limit global warming to well below 2°C above pre-industrial levels, pursuing efforts to limit it to 1.5°C. Success hinges on national-level climate policy implementation, organized through Nationally Determined Contributions (NDCs). While NDCs aren't legally binding, they are crucial for the 2023 global stocktake. Previous research indicates that current NDCs and policies fall short of the Paris Agreement's ambition. This study addresses the lack of peer-reviewed research assessing the global and country-specific impact of national climate policies using a comprehensive policy inventory and a suite of integrated assessment models. This multi-model approach enhances robustness. The aim is to analyze the impact of national policies compared to emission pathways consistent with NDCs and the Paris Agreement's overall goals, dividing the emissions gap into an implementation gap (difference between policy impact and NDCs) and an ambition gap (difference between NDCs and the well-below 2°C pathways). The analysis focuses on seven large economies and the world, using a detailed open-access policy database, translating high-impact policies into integrated assessment model inputs to assess their direct and interactive impacts, and presenting results using the Kaya identity.

Existing literature highlights the inadequacy of current NDCs and national policies in meeting the Paris Agreement's goals. Studies have shown significant shortfalls in collective efforts. However, previous assessments often lacked comprehensive policy inventories and multi-model approaches, limiting their ability to provide robust country-specific insights. Most model analyses focused on cost-optimal strategies with simplified policy representations, often using carbon prices rather than detailed policy instruments. While some studies incorporated explicit policy formulations, they were typically single-model exercises or focused solely on NDCs. This study fills this gap by utilizing a multi-model approach and a comprehensive policy inventory.

The study utilized a two-stage approach. First, a detailed open-access policy database containing implemented and planned national policies up to 2017 was created and curated with the help of national experts. A selection of approximately ten high-impact policies per G20 country was made, focusing on policies with high emission impact, government adoption through legislation or executive orders, and a lack of significant implementation barriers. These policies were translated into input parameters for nine integrated assessment models (IAMs) and ten national models. These IAMs have all contributed data to IPCC reports. Second, using these IAMs, five scenarios were evaluated: a 'no new policies' scenario (baseline), a 'national policies' scenario (impact of implemented policies), an 'NDCs' scenario (full implementation of conditional NDCs), and cost-optimal 2°C and 1.5°C scenarios. The models' projections were compared against existing national models to ensure coherence. The Kaya identity was used to decompose the emission reductions into factors such as changes in energy intensity, low-carbon energy share, and carbon intensity. Uncertainty ranges were analyzed by examining the contribution of historical emission differences, socio-economic factors, and differences in policy impact across models. The seven largest G20 economies were examined individually, along with global estimates. Furthermore, mid-century impacts were projected by assuming constant emission reduction rates relative to the 'no new policies' scenario between 2030 and 2050. Effort-sharing approaches were considered to evaluate the cost-optimal allocation of emission reductions, recognizing the potential inequities of cost-optimal implementation.

The results show that without further action beyond current policies, greenhouse gas emissions will increase by 2030, although at a slower rate than in a 'no-policy' scenario. Current policies result in a median global emissions gap of 22.4 GtCO2eq for 2°C and 28.2 GtCO2eq for 1.5°C by 2030. Full NDC implementation would reduce the gap by one-third, leaving a substantial ambition gap. National policy implementation leads to varied emission reductions across countries (0–9%), with some nations (China, India, Japan, Russia) close to meeting NDCs, while others (EU, US, Brazil) have significant implementation gaps. Analysis through the Kaya identity reveals that closing the emission gap requires substantial increases in low-carbon energy share and energy intensity improvements. The largest emission reductions are typically driven by policies targeting CO2 emissions, with a significant portion coming from OECD countries. Significant emissions gaps exist for all nations examined, requiring substantial emission reductions to stay within the 2°C and 1.5°C limits. Mid-century projections indicate that even with continued policy efforts at 2030 rates, substantial reductions will be needed to stay on track with long-term targets, though some nations' NDCs for 2050 are relatively closer to 2°C pathways. The analysis also details a decomposition of the emission uncertainties, identifying that socio-economic assumptions contribute most significantly, with model structure and implementation of policies contributing more modestly.

The findings indicate a significant shortfall in collective policy action toward achieving the Paris Agreement's goals. The existing implementation and ambition gaps underscore the need for increased ambition and accelerated policy implementation. Inadequate near-term actions necessitate greater transformations later and lead to increased costs and potentially stranded assets. The cost-optimal implementation considered in the study might not be equitable, potentially leading to disproportionately high costs in low-income countries. Effort-sharing approaches based on equity principles should be considered, potentially involving emissions trading or international financial mechanisms to ensure cost-effectiveness while addressing fairness concerns. The study also suggests areas for policy improvement, such as creating more coherent and comprehensive policy mixes and drawing on best practices from successful mitigation measures. A redesign of policy mixes may help to address current weaknesses in areas such as industry and freight transportation. International cooperation can help avoid carbon leakage and stranded assets while strengthening government regulatory power.

This study reveals significant implementation and ambition gaps across countries in achieving the Paris Agreement goals. Increased ambition and accelerated policy implementation are crucial, especially in renewable technologies and energy efficiency improvements for emerging and fossil fuel-dependent economies. The global stocktake should provide not only emissions gaps but also guidance on closing these gaps, with IAMs playing a key role in providing effective mitigation options.

The analysis is limited to G20 economies, representing approximately 75% of 2010 greenhouse gas emissions, excluding some countries with high-impact policies. The policy inventory’s cut-off date of 2017 means that some recent policy updates aren't included. Model limitations such as the assumption of a central planner and simplification of policy instruments affect the results' precision. Uncertainty exists in the models' projections, stemming from factors such as socio-economic assumptions and model structures.