Navigating the black box of fair national emissions targets

nature climate change Article https://doi.org/10.1038/s41558-025-02361-7 Navigating the black box of fair national emissions targets Received: 3 September 2024 Accepted: 21 May 2025 Published online: 16 June 2025 Mark M. Dekker 1,2 , Andries F. Hof 2,3, Yann du Robiou Pont 2, Nicole van den Berg2,4, Vassilis Daioglou 1,2, Michel den Elzen 1,5, Rik van Heerden 1, Elena Hooijschuur 1, Isabela Schmidt Tagomori Chantal Würschinger 1 & Detlef P. van Vuuren 1,2 , 1 Check for updates Current national emissions targets fall short of the Paris Agreement goals, prompting the need for equitable ways to close this gap. Fair emissions allowances rely on effort-sharing formulas based on fairness principles, yielding diverse outcomes. These variations, shaped by normative decisions, complicate policymaking and legal assessments of climate targets. Here we provide up-to-date numbers, comprehensively accounting for three dimensions—physical and social uncertainties, global strategies and equity—and the relative impact of them on each country’s emissions allowance. In the short run, normative considerations substantially impact fair emissions allowances—directing current discussions to this debate— while global discussions on temperature targets and non-CO2 emissions take over in the long run. We identify many countries with insufficient nationally determined contributions in light of fairness and discuss implications for increased domestic mitigation and financing emissions reductions abroad— yielding a total international finance flux of $US0.5–7.4 trillion in 2030. The recent Global Stocktake has shown that current combined action by countries is not enough to meet the Paris Agreement climate goals. With a remaining carbon budget (RCB) of roughly 5 (for 1.5 °C) to 25 (for 2.0 °C) times the annual emissions in 20241–3, global emissions have to decrease rapidly to keep these global targets within reach. Both current unconditional nationally determined contributions (NDCs) and current policies are expected to limit warming by 2100 to between 2.5 and 3 °C (refs. 4–8). There is no agreement on the required additional contribution of each country for closing the gap between agreed global temperature goals and current collective mitigation efforts. With updated NDCs due in 2025, informing policymakers on fair Paris-aligned emissions targets is crucial. Effort sharing has received attention both inside9 and outside10–12 academia, yielding a rich literature along a variety of fairness principles13–15. Collectively, these studies result in large ranges of fair emissions targets9–12,16. However, this variability of the results—stemming from both normative17 and non-normative factors—remains underexplored. Many calculations are done from only a selection of perspectives and uncertainties, leading to confusion or cherry-picking when non-academics use this ‘black box’ on fair shares15,18,19 and want to assess what levers and considerations are affecting the results. Hence, a systematic study on determining fair emissions targets and the factors impacting them is missing. In this research, we provide three advances with respect to current literature, in addition to providing up-to-date fair targets. First, we compute these targets across a wide range of impacting factors and their uncertainties, creating a more comprehensive and transparent dataset20. Second, we identify the impact of each factor on the fair targets using Sobol decomposition analysis21. Third, implications for domestic mitigation and international finance are derived by comparing the targets to NDCs and cost-optimal emissions. Three dimensions that impact fair shares The extensive literature on fair emissions allocations (or ‘fair shares’) offers many computation approaches. This study focuses on fairness PBL Netherlands Environmental Assessment Agency, The Hague, Netherlands. 2Copernicus Institute of Sustainable Development, Utrecht Universiteit, Utrecht, Netherlands. 3National Institute for Public Health and the Environment (RIVM), Bilthoven, Netherlands. 4TNO, Socio-Economic Energy Transition Studies (SES), Amsterdam, Netherlands. 5Institute for Environmental Studies (IVM), Vrije Universiteit Amsterdam, Amsterdam, Netherlands. e-mail: 1 Nature Climate Change | Volume 15 | July 2025 | 752–759 752 Article https://doi.org/10.1038/s41558-025-02361-7 Table 1 | Key dimensions of setting a fair national emissions target Physical + social uncertainties Global strategies Equity considerations Epistemic/scientific Political Normative Climate sensitivity Temperature target and overshoot Allocation principle (responsibility, capability or equality) GDP projections Global timing of action Population projections Global negative emissions Global non-CO2 reductions Detailed normative parameters (for example, discounting of historical emissions) Note that elements could be argued to be part of various dimensions, but this partitioning is merely used for presentation purposes later in the paper. in the mitigation burden, acknowledging that climate equity extends beyond mitigation22. Using global peak temperature target as a constraint (that is, aside from, although engaging with discussions on overshoot23, feasibility24 or cost optimality25) four key fair share concepts13,24,26,27 emerge, discussed in Supplementary Fig. 2. Beyond these methodological concepts, there are many choices, assumptions and uncertainties that affect fair shares and are traditionally studied separately. We bring them together and sort them into three dimensions (Table 1) based on how they are decided. The first dimension involves scientific uncertainties, both physical and social. Physical uncertainties are mainly associated with uncertainty in Earth’s temperature response to emissions (expressed in probability percentiles of reaching targets or a percentile of climate sensitivity), which greatly impacts the RCB3,28, in turn affecting global and (therefore) national emissions trajectories. In many studies, a single probability percentage of reaching a certain climate target (that is, a single percentile of the climate sensitivity) is used to calculate the remaining carbon budget (for example, reaching 1.5 °C with 50% probability14), although this exact percentage differs among studies (for example, a 50 or 67% likelihood13,26). Social uncertainties linked to projections of gross domestic product (GDP) and population are captured in Shared Socio-economic Pathways (SSPs)29 and affect emissions allocations that are based on equality and capability considerations. Most existing literature only considers the ‘middle-of-the-road’ scenario SSP213 with some exceptions30. The second dimension concerns global strategies for meeting the Paris Agreement goals, where we include temperature goals, mitigation timing and assumptions on negative and non-CO2 emissions. Most studies focus on 1.5 °C and (well below) 2.0 °C with either 50% or 67% probability, (...truncated)