Avoiding a grandfathering influence from continuous fair share trajectories
The literature quantifying emissions allocations based on diverse principles of distributive justice, including the Paris Agreement's CBDR-RC, agrees on the insufficiency of the NDCs of most of the largest emitting countries. Although there are divergences on the modeling choices of equity concepts, this literature focused on a 'continuous' allocation of emissions trajectories starting at current emissions levels. In this context, 'continuous' refers to trajectories starting at current emissions levels, rather than immediately at equitable levels. Effort-sharing formulas can achieve such continuity by design or through a transition period added to ensure continuity towards allocations only based on fairness considerations. This continuity is also commonly assumed when allocating national carbon budgets over time into emissions trajectories.
The legacy influence of current emissions levels on near-term emissions allocations is described here as a 'grandfathering' effect. This grandfathering influence on equity-based emissions allocation is strongest in the near term and increasingly affects the ambition assessment of NDCs in 2030. As we near 2030, a given NDC's emissions target will be closer and closer to a continuous emissions allocation that is iteratively updated (Fig. 1). The grandfathering allocation is criticized for its lack of ethical basis and has been shown to penalize the poorest countries as it preserves a status-quo, including current inequalities. Prior to the Paris Agreement, a study highlighted the value of a moderate grandfathering, from a political theory perspective, with a realist justification for negotiations and a utilitarian justification. Indeed, the pledges of many high-emitters only align with a grandfathering allocation. However, the IPCC has highlighted the need for a fair distribution of mitigation efforts, excluding grandfathering, in order to achieve an effective global agreement on emissions reductions. Likewise, recent reports of scientific advisory bodies have disapplied grandfathering when presenting fair-share emissions allocation. The Paris Agreement now requires NDCs of the highest possible ambition that reflect equity. A recent study described grandfathering allocations as not in line with international law. It identified that all continuous allocations entail elements of grandfathering but did not offer a solution.
The key motivation for allocating continuous emissions scenarios is to address the need for emissions trajectories that countries can implement domestically. However, different from emissions scenarios from Integrated Assessment Models (IAM), equity allocations do not engage with feasibility concerns but solely focus on effort allocation irrespective of where emissions reductions take place. Past delays of emissions reduction progressively leads to steeper fair share trajectories that are unlikely to be considered politically, technically or economically realistic for any country. Recent studies on budget allocations have shown already depleted budgets for high-emitting countries. Modeling trajectories from current levels in such instances leads to countries accumulating further excess depletion to be compensated by substantial negative emissions in the future. However, equity-based allocations serve as a proxy to distribute mitigation efforts and do not need to be met through domestic mitigation exclusively. Instead, countries can achieve their equity-based emissions allocations through a combination of domestic effort and international cooperation. As such, emissions allocations do not require to be continuous, and countries can immediately provide an equitable share of the global mitigation effort, beyond the limitations of what is feasible to implement within their borders. Immediately contributing to mitigation efforts outside of their own territories would be better able to remedy current inequities caused by past emissions as well as address the current emissions gap between current NDCs and the Paris Agreement targets.
To inform this extraterritorial contribution, the IPCC sixth assessment report calls for research extending equity frameworks to quantify equitable international support as the difference between equity-based national emissions scenarios and national domestic emissions scenarios. Such frameworks would enable assessing the ambition of countries' total contribution to global mitigation through domestic and international cooperation jointly. International cooperation to mitigate global emissions through bilateral agreements (as in the Swiss NDC), financial support or trading of Internationally Transferred Mitigation Outcomes (ITMOs), is now facilitated with the agreement of trading rules under Article 6 of the Paris Agreement at UNFCCC COP29. Such mechanisms offer a solution to progress towards an equitable distribution of the mitigation effort while contributing to the funding of mitigation measures, in line with cost-optimal implementation of mitigation measures across countries. The utilitarian justification for a moderate grandfathering relies domestic mitigation costs and is no longer relevant when allocations can be traded to achieve a globally cost-effective pathway.
Emissions scenarios from IAM assume the implementation of the cheapest mitigation option in each region without specifying which country should fund these measures. Implementing these scenarios without international cooperation - assuming that each country should fund the measures modeled for their territory - would represent a much greater fraction of Gross Domestic Product (GDP) in regions with lower GDP per capita. While much of the mitigation potential is in countries with low GDP per capita, countries with the greatest financial capacity fail to provide sufficient unconditional finance or meet their own pledges. Additionally, accounting for the higher cost of access to capital in poorer countries would impact mitigation costs and imply greater domestic mitigation in richer countries than currently found in IAMs. The implementation of the Paris goals, or of IAM scenarios, thus requires significant and immediate international support. Equitably implementing a global IAM trajectory can imply that countries' domestic emissions trajectories follow cost-optimal scenarios - that can be downscaled at the national level - and provide (or receive) the climate finance needed to mitigate overseas the difference with their equitable allocation. Compared to the domestic measures aligned with IAM trajectories, countries could pursue greater domestic mitigation to reap important co-benefits not accounted for, which can cover a substantial share of the mitigation costs. In practice, countries with high responsibility and capability can align with the Paris Agreement with an NDC within their fair shares, met through a combination of domestic mitigation of the highest possible ambition, and funding to mitigate global emissions overseas. As a novel mechanism, the international trading of mitigation outcomes raises implementation issues regarding the additionality of the finance and of the funded mitigation measures. Scrutiny will be needed to ensure the integrity of mitigation measures under Article 6 whose implementation rules were just adopted at COP29, with safeguards on human rights and the additionality of emissions reductions.
Here we quantify two sets of emissions trajectories immediately based on equity principles and that do not start at current emissions levels (see "Methods"). The two methods combine the equity principles of capability and responsibility to reflect the principles of the UNFCCC and the Paris Agreement, notably CBDR-RC. The literature suggests several approaches, conceptual or statistical, for combining different equity principles into a single allocation method (see Discussion). Here we apply each of the two equity principles to allocate global positive or negative emissions separately. This differentiated treatment of negative emissions extends a study from Fyson et al. that allocated negative emissions only, based on responsibility or capability. Fyson et al. explain that obligations to deliver negative emissions require uncertain technologies made necessary because of insufficient global emissions reductions to date. That study alone could not be used to inform economy-wide emissions targets, and thus not assess the ambition of NDCs, as it only allocated negative emissions and assumed that positive emissions follow least-cost pathways (that is, no equity principle is applied to gross emissions). Here, Approach 1 first allocates global negative emissions across countries based on their capability, assessed through GDP per capita, and then allocates global positive emissions to equalize historical responsibilities over the total net emissions (positive + negative, see "Methods"). Under this approach, rich countries are required to fund most of the negative emissions that require important research and development costs without local co-benefits. Approach 1 also ensures equal cumulative per capita emissions over the 1990-2100 period. The present results are therefore favorable for high historical emitters compared to accounting historical emissions since 1950 (Supplementary Data). Under Approach 2, all countries contribute to positive emissions reductions based on their wealth. Negative emissions, needed because of the world's important historical emissions, are then allocated proportionally to countries' individual historical responsibilities. There, countries' cumulative emissions allocations are not only based on their historical responsibility. Looking at the global emissions scenarios, the positive emissions refer here to the projected physical emissions (e.g., fossil fuels, agriculture). The negative emissions here refer to emissions captured through Carbon Dioxide Removal, excluding those from Direct Air Capture and Land Use, Land-Use Change, and Forestry (LULUCF) unlike Fyson et al..
In addition to representing CBDR-RC, the modeling of responsibility and capability also reflects considerations present in national policies. The European Union (EU) used a capability approach, based on GDP per capita, to allocate across its member states the mitigation effort of its first NDC target and to negotiate effort-sharing under the new Fit for 55 package. However, this capability criterion is not used to determine the emissions objectives of the EU NDC itself, which are not based or justified by equity considerations applicable to all. The capability principle reflects notions of progressive income taxation that many countries have implemented. Responsibility can be related to the 'polluter-pays' principle that many countries recognized in the Rio 1992 declaration and use in national law.
Here, countries' responsibility is studied solely based on territorial emissions accounted under UN frameworks. Other emissions frameworks account for emissions linked to consumption, fossil fuel extraction, or carbon intensity of countries' income. Such accounting could lead to more stringent allocations for countries with higher responsibilities, compared to territorial emissions, regarding their consumption footprint (the EU, Switzerland, Japan, Singapore), income footprint (Norway, Switzerland, Saudi Arabia, Australia) and extraction-based emissions (Canada, Saudi Arabia, Norway, Australia). Importantly, top-down effort-sharing formulas, such as those used here, may lose relevance for countries with very small and isolated populations (e.g., small island states). Such countries may have limited technical options to mitigate emissions and limited access to some options given the small size of their economies.
Under both approaches, emissions allocations start at levels that only depend on the global emissions scenario and countries' historical responsibilities and capabilities (Fig. 1, see country-level results in SI). Over time, emissions allocations for all countries follow the trends of the underlying global scenario (methods), a rapid decrease and plateau in the second half of the century. The USA, Canada and Australia have immediate negative allocations before 2035 under both approaches. Approach 2 is less stringent than Approach 1 for low-income countries (sub-Saharan African countries) and for countries with high historical responsibility (USA, Russia, Qatar and other fossil fuel extracting countries, see Fig. 2 and Supplementary Data). These different stringencies of emissions allocations do not always change the warming assessment of countries' NDCs (Fig. 3). Emissions objectives, such as NDCs or net-zero targets, should only be considered aligned with the present allocations if earlier emissions match the discontinuous allocation as well, which implies immediate contribution to global mitigation. The near-term allocation of some countries, mostly sub-Saharan countries, may exceed their current emissions and business-as-usual trajectory beyond 2030, implying mitigation efforts only later. However, staying within such decreasing allocations beyond 2030 implies immediate investments, possibly with international support. International support can enable recipient countries to implement mitigation measures in line with the underlying global socio-economic scenario in the near term. Approach 2 uses allocations inversely proportional to GDP per capita (see "methods"), resulting in high emissions allocations compared to current emissions and allocations based on business-as-usual trajectories for low-income countries (e.g., Ethiopia, Democratic Republic of Congo). These allocations theoretically imply financial transfers that may go beyond needs-based considerations and contribute to poverty reduction through climate action. The absence of continuity criteria highlights important sensitivities in emissions allocation across equity-based formulas that are otherwise dampened by the need for continuity in the near term and the declining global emissions space in the longer term.
The effect on the countries' near-term tradeable emissions of adding a transition phase to ensure the continuity of an emissions allocation schematized in Fig. 1. As highlighted in previous studies, adding a transition period greatly influences near-term allocation through a grandfathering influence. Here we show an additional effect of continuous allocation where their updates reward inaction by closing the ambition gap between the updated equity-based allocations and an insufficient NDC (exemplified in Fig. 1).
Looking at the geography of reduction rates, Fig. 2 shows the date when allocations are half of 2020 levels based on a 1.5 °C trajectory with no or limited overshoot (C1 scenarios, excluding bunkers and LULUCF emissions; see "Methods"). Under both approaches 1 and 2, the allocations of most countries reach half of 2020-levels by 2030. The allocations of some sub-Saharan and South Asian countries remain positive over the century.
The 'warming maps' (Fig. 3) show the warming alignment of countries' NDCs, that is, the warming associated with the most ambitious global scenario underlying the allocation that is above their NDC in 2030. This 'warming alignment' of a country also reflects the expected global warming level when all other countries follow a similar level of ambition. The differences in terms of emissions allocations across the two approaches do not translate into important differences for countries' ambition assessments. Most countries have an ambition assessment either 1.5 °C-aligned or not even 4 °C-aligned. The main reason is that the effect of the current inequities across countries' 2030 allocations overwhelms the relative spread in numerical targets across countries. This polarization of results reflects the extreme disparities of the current situation, considering countries' responsibilities and capabilities. This effect increases as we delay climate action and as we near 2030. Many countries have committed to NDCs much more ambitious than their 1.5 °C-allocations, mostly sub-Saharan countries. Countries with NDCs within their fair-shares could sell emissions space (possibly through conditional NDCs), possibly under Article 6 of the Paris Agreement, which could fund the implementation of the mitigation measures implied by the cost-optimal scenarios. Most high-income countries have largely insufficient NDCs, even when historical emissions accounted only since 1990. Approach 2 also yields a more stringent assessment for the NDCs of the UK, Switzerland and multiple countries, mostly in Latin America and South Asia.
The absence of continuity also changes the ranking of countries in terms of additional mitigation effort needed to align with their allocation, which potentially affects their share of the climate finance to be provided globally. Figure 4 provides an illustrative case of the influence of adding a 20-year transition period on the gap between the emissions allocations of G20 countries and their respective NDCs. In theory, each country's relative contribution to total international climate finance can be proportional to how much each country's NDC deviates from its fair-share trajectories. Compared to a traditional continuous approach, applying a discontinuous approach implies here a much higher obligation to contribute to global mitigation and possibly international finance for all G20 countries except India. In terms of the ranking of the emissions gap between NDC and allocation, assuming a transition period benefits Canada and Australia (moving down 9 positions), the USA and South Korea (each 8 positions). This shows that continuous pathways reward such countries for their history of comparably low mitigation efforts, lowering their implied contribution to international climate finance. Other countries, including China, Türkiye, South Africa and even the EU move down in the ranking of the ambition gaps when removing the transition period.
Discussions about climate finance in the context of the UNFCCC have often referred to the perceived obligations of, for example, high versus low-income countries. The comparison between continuous and discontinuous fairness allocations highlights the difference amongst high-income countries when considering the amount of finance needed to meet their fair shares.
In addition to capability and responsibility, equality is the third equity principle described in the IPCC AR5. IPCC reports do not present equity-based emissions allocations since AR5, despite available studies and its importance for courts of law. The egalitarian approach modeled as equal per capita emissions is not explicitly mentioned in the Paris Agreement or international environmental law but it can reveal the inequalities of emissions spaces claimed through NDCs. Figure 5 shows the equal per capita allocation where each country's share of global emissions is proportional to its population projection at every point in time. Even discarding countries' CBDR-RC, as modeled in approaches 1 and 2, this equality-based assessment yields similar warming assessments for most large emitters. In other words, the NDCs of many sub-Saharan countries are below the equal per capita levels of a 1.5 °C scenario. However, the NDCs of many countries with high responsibility and capability, which are meant to reflect their 'highest possible ambition' and account for CBDR-RC do not even reflect an equal per capita share of a business-as-usual trajectory, itself yielding warming impacts that hit some countries much harder than others. Establishing a country's alignment with the CBDR-RC principle depends on the methods used to quantify the responsibility and capability principles. However, the misalignment of an emissions target with a simple equal per capita allocation can be used to characterize a misalignment with the CBDR-RC principle, for countries with higher-than-average capability and responsibility. In a recent ruling, the European Court of Human Rights used a simple equal per capita allocation to comment on Switzerland's inadequate emissions levels while recognizing the need to account for its CBDR-RC.