Thomas Hahn is associate professor at the Stockholm Resilience Centre and Stockholm University research leader for FAIRTRANS. Robert Höglund is a carbon removal advisor with Marginal Carbon. Mikael Karlsson is associate professor at Uppsala University and research leader for FAIRTRANS.

Recent news articles, in the Guardian and from Bloomberg among others, have proclaimed the death of the 1.5°C target given how emissions and lack of sharper commitments are developing. In a new study, in Nature Communications we examine how the responsibility to limit long-term warming to 1.5°C could be allocated after a temporary overshoot, if countries are held accountable in line with the principle of Common But Differentiated Responsibility and Respective Capabilities (CBDR-RC) as established under the UNFCCC.

This principle, a cornerstone of climate diplomacy, recognizes that while all countries share responsibility for addressing climate change, their obligations differ based on historical emissions and capacity to act. By comparing countries’ past emissions and future emission claims with their equal cumulative per capita emissions, we establish a new indicator – “additional carbon accountability” – showing countries’ responsibilities beyond current climate targets.

If existing national climate targets are met, the 1.5°C fossil carbon budget (for a 50% probability of meeting that goal) will be exceeded with 576 billion tonnes of carbon dioxide (GtCO₂). To meet the 1.5°C target, our indicator shows that the EU, China, the US, and 15 other countries must sharpen their own current targets with faster mitigation and more carbon dioxide removal (CDR), such as afforestation and technical solutions like direct air capture and biochar. All other countries must stick to their national climate plans (nationally determined contributions – NDCs) and net-zero targets.

Counting future emissions

As an example, the EU would need to mitigate or remove an additional 48 GtCO₂ or finance additional reductions beyond current targets in other countries, on top of reaching its own targets in terms of the NDC for 2030 and net zero emissions by 2050. For China, the additional carbon accountability is 150 GtCO₂ and for the USA 167 GtCO₂.

While there is no international agreement on how to operationalize the fairness principles of the Paris Agreement, the new indicator – based on the CBDR-RC – provides an important tool to clarify what different countries are accountable for in relation to the mitigation (emissions) gap. This information feeds directly into the contested annual negotiations on climate financing within the UNFCCC COP meetings.

In general, high-income countries have large carbon debts, while several of the BRICS and upper-middle-income countries have high planned future emissions. Much focus of climate advocacy has been on getting high-income countries to reduce their emissions. Of planned future emissions, 26% come from high-income countries, but as much as 38% comes from upper-middle-income countries with an additional carbon accountability.

China and Iran, for example, have plans for large future emissions and could theoretically meet a large part of their accountability by achieving stricter reduction targets. But countries that have most of their emissions in the past, like the US and the EU, need to turn to not only stricter targets but also CDR, ensuring net negative outcomes.

National responsibility

Unlike the COP29 outcome on financing, which hinges on the division between developed and developing countries, our indicator assigns responsibility to individual countries based on their additional carbon accountability. This approach bypasses the debate over whether only developed countries should collectively finance emissions reductions in developing countries, instead focusing solely on national accountability for emissions.

In the article, we also calculate what it would cost to meet the accountability with CDR or reductions if the cost is $150 per tonne of carbon dioxide. For Iran, the total additional carbon accountability cost would be 1,200% of its GDP in 2021, Russia 530%, China 130%, the US 110%, and the EU 41%.

Our article points to how the responsibility to stay under 1.5°C in the long term could be allocated in line with the CBDR-RC principle. But will countries shoulder this responsibility?

Political will in short supply

For most high-income countries, the focus in practice is more on meeting existing targets, with the topic of paying for historic carbon debt absent. There is also no agreement among countries on how to operationalize the CBDR-RC principle. And few would expect countries like Russia and Iran to make radical emissions reduction cuts, not to mention taking any responsibility for historic emissions.

Hence, it does not seem very likely from the current political situation that countries would increase the ambitions of their national climate plans and net zero targets enough to meet their additional carbon accountability. However, what seems unlikely today can change.

Moreover, every fraction of a degree matters, and our indicator can be used also for other temperature targets, such as 1.7°C or 2.0°C. Our results can be used to put higher pressure on individual countries with plans for large future emissions and to start holding countries with large carbon debts accountable for their historic emissions.

Dr. Ingo Fetzer, a researcher on global systems analyses with the Stockholm Resilience Centre at Stockholm University and Dr. Johannes Morfeldt, a researcher in climate and energy systems at Chalmers University of Technology also contributed to this article.

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How to resuscitate 1.5°C by identifying accountable countries