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Achieving the collective goal of limiting warming to below 2 °C or 1.5 °C compared to pre-industrial levels requires a transition towards a fully decarbonized world. Annual greenhouse gas emissions on such a path in 2025 or 2030 can be allocated to individual countries using a variety of allocation schemes. We reanalyse the IPCC literature allocation database and provide country-level details for three approaches. At this stage, however, it seems utopian to assume that the international community will agree on a single allocation scheme. Here, we investigate an approach that involves a major-economy country taking the lead. In a bottom-up manner, other countries then determine what they consider a fair comparable target, for example, either a ‘per-capita convergence’ or ‘equal cumulative per-capita’ approach. For example, we find that a 2030 target of 67% below 1990 for the EU28, a 2025 target of 54% below 2005 for the USA or a 2030 target of 32% below 2010 for China could secure a likely chance of meeting the 2 °C target in our illustrative default case. Comparing those targets to post-2020 mitigation targets reveals a large gap. No major emitter can at present claim to show the necessary leadership in the concerted effort of avoiding warming of 2 °C in a diverse global context.

The international community agreed to limit warming below 2 °C or even 1.5 °C (ref. 1). Current pledges up to 2020 are not on track for that collective goal². However, new research continues to remind us about the implications of not limiting warming: for example, today’s warming of just 0.9 °C already implies 1.2 m global-mean sea-level rise over the coming centuries from ice loss in the West Antarctic Amundsen Sea sector alone³.

Country-level emission allocations are contentious within the international community, despite the multiple complementary benefits that decarbonization of the energy and transport sectors can have (such as improved local air quality⁴ and increased energy security⁵). Mitigation discussions at the United Nations Framework Convention on Climate Change (UNFCCC) are dominated by a ‘burden sharing’ debate, and disagreement in this so-called ‘equity discussion’ persists. This reflects fundamental differences regarding the allocation of future emissions following ‘common but differentiated responsibilities and respective capabilities’⁶ (CBDR&RC). Scientific literature so far provides limited guidance on appropriate quantitative national targets for 2025 or 2030 under different allocation regimes^{7, 8, 9, 10, 11}. The recent Fifth Assessment Report¹² (AR5) summarizes six distinct allocation categories, and a set of scenario categories that approximate but do not equate to global ambition. Although providing some regional disaggregation, the IPCC and the underlying literature review¹¹ stopped short of providing country-level detail. Here, we re-analyse the IPCC allocation database and develop country-level allocation pathways to address this information gap. As countries within the UNFCCC have not converged to any particular allocation category or regime, we assume a world with continued differing opinions on what constitutes a fair allocation. Our results give an indication of what might be required for a ‘leading’ country to guide the world towards a 2 °C-consistent trajectory.

First, we derive 2025 and 2030 waypoints—that is, indicative global aggregate greenhouse gas (GHG) emissions levels consistent with a carbon budget of 1010 GtCO₂. This 1010 GtCO₂ budget was found by IPCC to be the cumulative CO₂ emissions remaining after 2011 to preserve a likely chance of staying below 2 °C based on multiple lines of evidence¹³. On the basis of our analysis of the IPCC AR5 Scenario Database (see Supplementary Section 3), we choose an illustrative 2025 waypoint of 10% above 1990 emissions (15% below 2010) for world emissions to be in line with the IPCC carbon budget for 2 °C (Fig. 1). For 2030, we define our waypoint as ‘1990 levels’ (or 22% below 2010; see Supplementary Figs 10–31 for variable global waypoints). Our waypoints happen to be in line with RCP3PD, the lowest of the four main IPCC scenarios. These waypoints are more ambitious (that is, imply lower emissions) than some delayed scenarios at the high emissions end suggest (Fig. 2b, c), but less than findings of the least-cost 2010 scenarios assessed by UNEP (ref. 14; see Supplementary Section 3.2) and also less ambitious than the median of IPCC AR5 WG3 scenarios that do not assume subsequent net negative fossil and industrial CO₂ emissions (Supplementary Information). Hence, the global waypoints defined here roughly reflect upper limits for ‘middle-of-the-road’ indicators. Consequently, this same interpretation applies for the national targets that we discuss below—that is, that national targets might err on the side of too small reductions, for the reason of how we derive global waypoints, not necessarily for other reasons. Furthermore, our results should be considered conservative in two other respects: remaining within a 2 °C target with a higher level of confidence than likely (>66%), or limiting warming to 1.5 °C, imply global emissions lower than these waypoints in 2025 and 2030 (see discussion of the waypoints with regard to earlier studies and recent emission trends in the Supplementary Information).

a, Historical GHG emissions and harmonized future scenarios from the IPCC AR5 scenario database (thin lines) and RCP scenarios (thick grey lines). Our default waypoints are indicated as well as a 50% reduction compared to 1990 by 2050 (60% reduction compared to 2010). b, 2030 GHG emission waypoints derived by quantile regression of GHG emissions in 2030 versus the scenarios cumulative emissions from 2012–2100—distinguishing between scenarios that imply negative fossil CO₂ emissions (orange circles) or not (blue circles). c, Same as b, but for 2025 GHG emissions.

Much of the equity debate within the UNFCCC centres on the operationalization of the CBDR&RC principle instated in the 1992 Framework Convention. At the time, CBDR&RC was primarily addressed by creating the dichotomy between industrialized Annex I and developing Non-Annex I countries¹⁵ as the main indicator for mitigation responsibilities. With only 22% of the global population¹⁶, Annex I countries emitted approximately 46% of global GHG emissions (incl. land use) in 1992 (Supplementary Fig. 3). This binary differentiation remains strongly influential on the negotiations¹⁷. However, given a decreasing global share of Annex I countries’ direct GHG emissions (~31% in 2014, Supplementary Section 6) and given that China’s territorial emissions share has risen to almost the same level as all Annex I emissions together (~26% in 2014, Supplementary Section 6), negotiations are moving towards a more complex self-differentiation within an agreement that shall be ‘applicable to all’¹⁸. Hence, the more than decade old effort-sharing debate^{19, 20, 21, 22} for a more gradual differentiation gained momentum again: How much should individual countries contribute to the collective mitigation effort in the coming decades? In the academic literature, a host of effort-sharing approaches has been developed on this question, and the answer is fundamentally dependent on a series of value judgements^{6, 11, 12, 20, 21, 23, 24, 25, 26, 27, 28, 29, 30, 31}.

For our study, a simplification of the political debate is useful. In essence, countries’ positions predominantly follow a logic of either distributive or corrective justice³². Thus, an almost binary view has surfaced about what type of gradual differentiation can be considered fair. We capture the range of proposals with two illustrative allocation approaches: ‘Common but differentiated convergence’²⁴ (CDC), which is a modified per-capita convergence approach, and the ‘equal cumulative per-capita’ approach (ECPC) (compare Fig. 2c and Supplementary Sections 8.1 and 8.2). The CDC approach essentially postulates that it is fair to converge to equal per-capita emission allocations (distributive justice). The ECPC approach implies that a country A with higher per-capita emissions than country B in the past will have lower per-capita emission allocations in the future (corrective justice)^{28, 29}. We model two variants with different starting years from when per-capita emissions are counted, either 1950 (ECPC50) or 1990 (ECPC90) (Supplementary Information). Per-capita convergence is implied in some government’s submissions. For example, the indicative previous US 2050 goal of −83% below 2005 (ref. 33), confirmed recently³⁴ with a tentative ‘−80% or more’ goal by 2050, is only somewhat short (4 to 7%) of an equal per-capita allocation by 2050. Similarly, the EU’s ‘intended nationally determined contribution’ (INDC; ref. 35) is presented in the context of a per-capita convergence approach (Supplementary Information). On the other hand, ‘cumulative equal per-capita emissions’ are mentioned as an equity principle in presentations by China^{15, 36}. Although India proposed a regime similar to the CDC (ref. 24) in 2007 by stating its ‘per-capita emissions will remain lower than those of the developed countries’³⁷, recent Indian negotiator and expert positions seem to favour approaches closer to China’s cumulative per-capita proposal³⁸.

Countries’ preferences in international negotiations can often be understood as primarily motivated by self-interest: ‘general principles of fairness are invoked only to promote or defend one’s own interests’²⁰. This narrow self-interest is the underlying cause for the ‘tragedy of the commons’³⁹. The existence of an international regime to address climate change, however, is evidence of a limited extent of cooperative behaviour, when ‘rational choice, prisoner’s dilemma, collective goods and global commons theories would predict non-cooperation with a worse outcome overall’⁴⁰. By its very nature, a regime can provide soft boundaries and incentives for acceptable behavioural norms or principles that guide the development of country positions. Within those soft boundaries of a regime, countries are guided by their interests, and engage in a deliberative discourse, positioning themselves with sometimes fluid moral justifications to match their interests⁴¹. Within the soft boundaries of a climate change regime, self-interest of a country serves as a first-order explanation of why the least-ambitious emission allocation seems preferable. Indeed, in the UNFCCC negotiations and countries’ submissions, it seems—not unexpectedly—that countries explicitly or implicitly align with principles and notions of equity that match their interests (Supplementary Sections 4 and 5 with examples USA, China and EU28). This poses a fundamental problem, because ‘notions of fairness can provide a basis for an international regime only if there is a certain minimum of consensus among its members about what is fair and what is unfair’²⁰.

However, it is not uncommon in negotiation settings for different actors to agree on an outcome while subscribing to very different, possibly incompatible principles. Such settings necessarily call for agreements that focus on the final outcome, while not attempting to reach agreement on principles. Thus, a consensus on the principles is circumvented by ‘political consensus’^{41, 42, 43} of what a fair distribution of the burden would be. Those negotiation outcomes are what Sunstein called ‘incompletely theorized agreements’⁴⁴, or one could also refer to them as ‘incompletely principled agreements’. The likely alternative would be continued disagreement on principles and no agreement. This could lead to ensuing mitigation delay and a very inequitable outcome by exposing those with a low adaptive capacity to high climate change impacts. Therefore, contemporary environmental ethics suggests pragmatism may be a promising guiding principle for achieving fairer outcomes^{43, 45}.

Whereas some countries favour or imply fairness principles, others negate the value of considering such principles as guidelines for target setting⁴⁶. An ‘un-principled’ approach is not unrealistic, but is outside the scope of this study. In fact, the international negotiations are not ‘un-principled’. Arguably, the pledges under the Copenhagen Accord and Cancun Agreements were made within the aforementioned soft guiding of a regime discussion, although they failed to bring the world much closer to its collective goal of keeping temperature increase below 2 °C (ref. 2). In line with the current agreed process, we analyse how self-differentiation can take place within the spectrum of discussed allocation approaches.

By calling for ‘nationally determined contributions’, the international community departed from an ‘ideal’ (and possibly unrealistic) scenario of a national emission allocation following a single common allocation approach: countries are asked to provide their own reasons for why they consider their contribution fair and commensurate with the joint target. Neither an un-principled approach nor a single globally applied allocation approach is taken. For the time being, countries apply the logic of ‘self-differentiation’. Such a bottom-up architecture avoids one problem—the possibly utopian attempt to agree on a singular set of guiding principles—and might accomplish collective agreement on the outcome. However, self-differentiation creates another problem: the outcome might be insufficient compared to the ultimate collective goal in the absence of additional ambition-enhancing coordinated measures or mechanisms.

That failure to achieve the collective goal is due to the supposed general tendency for a country to choose the allocation approach that offers the higher emission allowance from various options that are consistent with the collective goal. Suppose each country selects the lower ambition approach consistent with 2 °C, then the sum of all individual actions is not going to be consistent with 2 °C (Fig. 3a). One solution could be that countries enhance their collective nominal target (for example, from 2 to 1.5 °C) to offset the effect of self-differentiation—so that the original collective target (2 °C) is still met (Fig. 3b).