Interpretation of equity principles
Allocating national warming budgets on the basis of equity fundamentally relies on interpretating principles of equity and CBDR&RC, which underpin international consensus on climate action as captured by the Paris Agreement2. Alongside and aiding their interpretation, we also consider the do-no-harm and precautionary principles described in international environmental law22 (Fig. 1). We link these principles to related principles for equitable effort sharing discussed in the climate equity literature14. This includes the ability-to-pay, beneficiary-pays, and polluter-pays principles, which we consider in addition to the overarching principle of equality. In total, we develop three interpretations of how these equity principles can be combined (see also Methods). The grandfathering principle has been a longstanding concept in the literature, but it is not considered a basis for equitable distributions14 and is therefore not included.
Our first interpretation relies on equality and the polluter-pays principle. Here we consider all living persons having an equal right to pollute, but inheriting country-based historic emissions extending back to two distinct starting years, 1850, and 1990. 1850 is chose as a common proxy year for the beginning of the industrial revolution and from which good data is available, and 1990 is the year of the first assessment report of the Intergovernmental Panel on Climate Change (IPCC). We determine the remaining warming from this start year to our target temperature, be it 1.5°C or 2°C, allocate this to individual countries, and subtract from this allocation countries’ respective historic contributions to warming until 2021, thereby determining the remaining budget to net zero with due consideration of our interpretation of CBDR&RC.
Our second interpretation expands our first interpretation with consideration of the ability-to-pay principle, placing weight on different national circumstances. Ability to pay for mitigation is modelled as a function of GDP per capita and the government bond yield (a proxy for government capacity to invest). These indicators are used to scale the initially equal-per-capita distribution of our first interpretation (Fig. 1).
Our third interpretation replaces the ability-to-pay with the beneficiary-pays principle, placing emphasis on unequitable benefits of nations from the past emissions they caused. We use total historical fossil fuel sales in present dollars attributable to the population in each country from the year 1900 to 2021 as a proxy indicator to reflect benefits, beyond historical contributions to warming. Also here, we scale the per-capita allocations from our first interpretation with this indicator before proceed with the subtraction step.
From a normative perspective, countries with higher GDP per capita, lower government bond yields, and higher benefits from historical fossil fuel sales should be allocated lower overall budgets. To avoid extreme punishment or encouragement of certain countries, values more than two standard-deviations removed from the mean are capped to these limits (see Methods for details).
Fair national CO2 warming equivalent budgets
We explore the quantitative results through the lens of three country groups: developed, developing, and least-developed countries (LDCs) (see Method for definitions, Fig. 2). Of 51 developed countries, 44 to 45 are left with negative remaining budgets for the 2021-2050 period under our three equity interpretations. Typical countries include the United States (US), the United Kingdom (UK), Russia, and Japan, due to their early industrialization and fossil fuel-based industries. Over half of developing countries maintain positive budgets (63-66 of 102 across all three equity interpretations). These countries experienced later industrialization with lower historical emissions compared to developed countries. Several developing countries are among the ten countries with the lowest remaining CO2-we emissions budget, even below most developed nations. These countries include Aruba, Trinidad and Tobago, Qatar, Brunei Darussalam, and Bahrain, which all are nations with small populations and economies based on oil and gas extraction, resulting in extremely high historical per capita emissions. All LDCs have positive remaining CO2-we budgets above the global average. This means that, globally, a total of 82 to 85 countries had exhausted their 1.5°C budget by 2021.
This country ranking remains robust against alternative equity interpretations and indicator variations. The primary reason for the significant disparity between country groups is the wide variation in historical warming contributions, which surpasses the influence of equity interpretations because for most countries there’s a large difference between historical emissions and their total budgets.
Consumption of remaining budgets
Many developed countries have far exhausted their fair budget estimates even before 1990. Notably, the UK, France, Germany, Australia, and the US are among the first nations to exceed their budgetary limits. South Korea still had 46-71% budget left in 1990 (compared to that in 1850) depending on equity interpretation, yet exhausted it rapidly in the 1998-2010 period. Despite the adoption of the UNFCCC in 1992, the slowdown rate is moderate. Russia is an exception with a notable turning point around 1990. However this is the result of the collapse of the Soviet Union, which is an economic shock, not climate policy23. For developing countries, most of them had a large budget left in the 1990s. Yet, some of them soon exhausted it at an increasing rate after 2000. China’s budget was exhausted between 2013-2021. In 2021, India and Brazil still have 75-79%, and 8-29% of their estimated 1.5°C compatible budgets compared to the year 1850, respectively (Fig. 3, Fig. 4a).
Over 2022 to 2049, adhering to the equity interpretations, countries follow fair allocations. The framework here assumes that by 2050 unequal historical warming contributions are resolved and all countries return to zero CO2-we emissions (Fig. 4b) by the time of peak warming. After 2050, the world is assumed to further maintain zero emissions in this stylized example.
Comparison between fair budgets and the deepest technically feasible mitigation
Our analysis estimates future national budgets consistent with specific equity interpretations. Yet, these fair allocations may not be reachable for certain countries even if maximum technically possible reductions are implemented. We take the C1 category in the IPCC Sixth Assessment Report (AR6) scenario database as an illustrative proxy for these deepest technically feasible mitigation pathways, which are consistent with the target of limiting warming to 1.5°C (>50%) with no or limited overshoot.
An important gap exists between many of the national fair budgets and the most ambitious mitigation as estimated by global integrated assessment models (Fig. 4, Supplementary Fig. 10). The deepest technologically feasible emissions reductions for the US, Canada, Russia and the European Union would result in about net-zero CO2-we emissions by 2050, for China around 2060. However, even these deep reductions over the next decades are insufficient for these countries to stay within their equitable remaining budget allocations as virtually all of them are negative. Brazil can roughly attain its fair carbon budget due to its abundant carbon sink resources but there are large uncertainties surrounding the estimates of its deepest technically feasible emissions reductions. The remainder of India’s equitable warming budget is about ten times the cumulative emissions under its deepest mitigation pathway. Indonesia, Pakistan, and Colombia are in a similar situation.
Influence of normative considerations
Our analysis adopts three equity interpretations (Fig. 1) that reflect different combinations of the equity principles and that are translated into quantified estimates using a set of key indicators. Figure 6b shows how national remaining CO2 budgets change with equity interpretations and the applied indicators.
In general, incorporation of the ability-to-pay or beneficiary-pays principles increases the divergence between country groups compared to a purely equality and polluter-pays approach. Developing countries and LDCs get allocated more while developed countries less. This is expected when GDP per capita is taken as indicator for the ability-to-pay principle, as this indicator closely correlates with whether countries are categorized as developed, developing or least developed. Using purchasing power parity (PPP) or market exchange rates (MER) estimates of national GDP shows negligible differences (Supplementary Note 1, Supplementary Fig. 1). Using the 10-year bond yield as indicator for the ability-to-pay principle implies a greater emphasis on a government’s credit and financial outlook. Consequently, emerging economies such as China, India, and Indonesia get smaller budget allocations, while countries like Russia, Iran, and Brazil get more given their lower economic expectations.
Using historical fossil-fuel sales as indicator for the beneficiary-pays principle also does not dramatically influence the overall ranking of countries. Yet, the pattern gets more complex than when using GDP. Several countries across the various groups get markedly lower budgets than under other interpretations, resulting from a relatively higher reliance on fossil fuel production during their capital accumulation process. Notable examples include Trinidad and Tobago, Qatar, Bahrain, Kuwait, United Arab Emirates, Libya, and Saudi Arabia among developing countries, the UK, USA, Norway, Canada, and Australia among developed countries, as well as Angola and Timor-Leste among LDCs. We consider colonial histories of empires when calculating historical fossil-fuel sales by attributing the sales from a colonized country to the colonial power during the colonization period to simulate the benefit transfer. The influence of this additional operation on the remaining budgets is only minor (Supplementary Note 2, Supplementary Fig. 2).
Another normative choice we explore is the starting year from which countries’ historical warming contribution is calculated, which is subject to debate. For example, 1850 is often taken as a proxy for the start of the Industrial Revolution and the start of the increasing consumption of fossil fuels. Also later years are sometimes considered, such as 1990 (the release of the IPCC First Assessment Report) or 1992 (the establishment of the UNFCCC), arguing that excusable ignorance before the international treaty or recognized report may reduce the moral burden of the emitters. When 1990 is taken as starting year compared to 1850, the difference in national remaining budgets decreases. Countries with positive budgets see a reduction; countries with negative remaining budgets have less ‘debt’. However, the relative change tends to be smaller for developing than for developed countries, indicating the relatively higher warming contributions of developing countries after 1990 (Supplementary Note 3, Supplementary Fig. 3-4).
Various choices can affect budget outcomes in some countries, which can lead to gaming. When countries are asked to demonstrate the equity of their long-term goals, countries may adopt choices that minimize their burden3. Typical developed countries with long fossil-fuel histories might favour later starting years for historical contributions. Developed countries like South Korea, Japan, and Switzerland, whose economies rely primarily on the tertiary sector may emphasize that they have limited extraction of fossil energy and thus limited benefit from emissions. The Organization of the Petroleum Exporting Countries may argue their economy is heavily reliant on fossil fuel production and therefore emphasise challenges associated with transitioning away from fossil fuel. Large developing countries like India may underscore their enormous and expanding population, emphasizing the significance of allocation in proportion to population size.
Influence of uncertainty due to methodological factors and aerosol masking effect
In addition to the variation of estimates due to normative choices, also uncertainty arising through model parameter settings and aerosol masking effect uncertainties have been explored. These include the degree to which a country’s allocation is changed as a function of differences in driving indicators, the desired peak global warming target, and the estimation of the aerosol masking effect at the time of peak warming (Supplementary Table 1). The influence of these uncertain factors turns out to be insignificant compared to the variations due to normative considerations (Supplementary Note 4-6, Supplementary Fig. 5-9).