Countries’ carbon budget math is broken

Climate action is falling behind on the goals as stated in the Paris Agreement. To meet those goals, countries must act according to their ‘fair share’ targets. However, researchers from Utrecht University found a bias in how ambition and fairness assessments were calculated until now: “previous studies assessing countries climate ambition share a feature that rewards high emitters at the expense of the most vulnerable ones.” This finding influences climate change mitigations globally. The research, led by Yann Robiou du Pont, was published on 3 September in Nature Communications.

The researchers argue that previous fairness and ambition assessments were biased, as they start from shifting goalposts of rising emissions. Their proposed method avoids delaying the obligation to reduce emissions and calculates the immediate ambition gap that can be filled by climate measures and international finance. As negotiated climate targets are still insufficient, this work underscores the growing role of courts in ensuring that climate and human rights obligations are met. The study highlights that high-emitting countries, most notably G7 countries, Russia, and China, need to do more given the very different historical responsibility and financial capability of countries.

An approach based on historical responsibility needed

Fair-share emissions allocations distribute the global carbon budget among countries based on principles like historical responsibility, capability, and development needs, aiming to assign each country a ‘fair share’ of allowable emissions. Under the Paris Agreement, these allocations indicate what each country should commit to in order to collectively limit global warming to 1.5°C and staying well below 2°C. 

By calculating each ambition and fairness assessment from the present situation, we increasingly let major polluting countries off the hook. This pushes a heavier burden onto countries that have done the least to cause the crisis, or more realistically brings the world towards catastrophic levels of global warming. Therefore, the authors propose calculating fair-share emissions allocations immediately based on each country’s historical contributions to climate change and their capacity to act. Accounting for immediate responsibilities sets a new baseline. It would cause some countries’ emission paths to suddenly and drastically change instead of following a smooth decline. This approach would demand steep, immediate cuts mostly from wealthier, high-emitting countries. Since the cuts needed from these countries are too large to achieve locally, it requires substantial financial support for additional mitigation in poorer countries. Importantly, removing the systemic reward for inaction affects the ranking of countries’ gap between their current pledges and fair emissions allocations, even within the group of high-income countries. Then, the USA, Australia, Canada, the UAE and Saudi Arabia have the greatest gap, requiring the most additional effort and finance. Much of equity discussions is about developed versus developing countries, but this paper is particularly relevant for developed countries being rewarded for inaction compared to other and more ambitious developed countries.

Role in climate litigation

Fair-share studies like this one are increasingly used in climate litigation, such as the KlimaSeniorinnen case before the European Court of Human Rights. The court recognised that insufficient national climate action constitutes a breach of human rights and that countries must justify how their climate pledges are a fair and ambitious contribution to the global objectives. Courts rely on these assessments to evaluate whether national emissions targets are sufficient and equitable. Biases in the assessments therefore have real-world impact: they can shape legal rulings, influence policy commitments, and inform public opinions. Courts are thus emerging as a key force in ensuring accountability and indirectly promoting cooperation when political and diplomatic negotiations fall short. In a landmark advisory opinion issued on July 23, 2025, the International Court of Justice affirmed that countries have a legal obligation under international law to prevent significant harm to the climate system, emphasising the duty to act collectively and urgently. “This strengthens and underscores the growing role of courts in enforcing climate justice,” says Robiou du Pont.

Paying the debt Solving the climate crisis is a moral imperative long identified by climate justice activists and scholars. Practically, we are observing that the lack of fair efforts by countries with the greatest capacity and responsibility to act and provide finance, results in insufficient action globally. A fairer allocation of effort is likely to results in more ambitious outcomes globally. This study explains how immediate climate efforts and finance are key to align with international agreements to limiting global warming.

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