Exceeding 1.5 °C requires rethinking accountability in climate policy

Around ten years after the Paris climate agreement was adopted, the world is again at a crucial moment. In 2015, 195 countries committed to hold global warming “well below 2 °C” and to “pursue efforts to limit warming to 1.5 °C” to prevent dangerous human interference with the climate system (see go.nature.com/4qne62f)¹. How to interpret these two temperature levels was ambiguous², but it was clear that both had not yet been reached and were being pursued from below. At the time, pathways to stay below both levels could be modelled³, but much has changed since then.

In 2024, global average temperatures exceeded 1.5 °C for the first time. Going above 1.5 °C in one year does not mean that the Paris threshold itself is technically breached — it is defined as an average over at least 20 years to account for year-to-year variations⁴ — but it indicates that the world is on track to pass it in a decade or less⁵.

In July, an advisory opinion from the International Court of Justice firmly anchored 1.5 °C as the primary limit of the Paris agreement, reducing ambiguity over its aim. Although severe negative effects of climate change materialize below 1.5 °C, this limit demarcates the minimum threshold of dangerous human interference that governments agreed as unacceptable.

In an ‘overshoot’ world — one in which global warming exceeds 1.5 °C but is later brought back below this limit — countries’ obligations to meet this temperature limit remain. However, the pursuit of the 1.5 °C limit from above poses further challenges.

Countries will need to commit not only to reach net-zero carbon dioxide emissions, but also to achieve and sustain net-negative emissions — by removing billions of tonnes of CO₂ from the atmosphere and durably storing it. They will need to confront the further loss and damage and the adaptation needs that arise as a result of exceeding the 1.5 °C limit. And governments need to ask why they failed to prevent dangerous human interference, and who is responsible.

Recognizing this failure also implies that remedial measures are needed to address the consequences of going past 1.5 °C, and countries need to be held to account for them. Establishing such measures will require careful consideration of feasibility — how much capacity a country has for cutting emissions considering social, economic and other constraints. It also requires consideration of fairness, or how responsible a nation is for an overshoot.

Although balancing fairness and feasibility has always been central to tackling the climate problem, we argue that approaches must now be extended to explicitly account for overshoot and be accompanied by a comprehensive retrospective evaluation of past inaction. Here, we outline what this process should look like.

Avoiding the feasibility trap

Energy–economy–climate models can be used to project technically feasible pathways through which countries can achieve agreed climate targets together by cutting their emissions⁶. Nations use such pathways to inform targets on the timing of achieving net-zero emissions and interim benchmarks⁷. But technical feasibility isn’t the only thing that matters when setting targets.

Fairness is as essential, because responsibilities for causing the climate problem, its impacts and the capabilities to address it are unequally distributed. Global climate agreements formalize fairness considerations through the principles of equity, and common but differentiated responsibilities and respective capabilities⁸. These principles can be translated into calculations of fair shares of emissions for countries. Such calculations rely on indicators, such as total cumulative emissions or gross domestic product per person, to distribute global emissions that are consistent with a climate target and reflect varying perspectives⁸.

When approaching a warming level from below, countries can try to align feasibility and fairness approaches to set emission-reduction targets that are consistent with a global pathway for staying within the 1.5 °C limit. In doing so, they might define their fair contribution to meeting the 1.5 °C remaining carbon budget (the amount of carbon emissions that can be emitted without exceeding a global warming level with a given probability). Analysts can benchmark whether a country’s emissions target is a fair contribution to the collective global effort or whether it falls short.

When the warming level is approached and surpassed, however, this process is faced with a lack of remaining emissions allowances to allocate. Moreover, as global emissions continue to rise, the lowest peak level of warming that the models suggest the world can stay within also creeps higher.

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In 2018, a few modelled pathways suggested⁹ it was still possible to keep warming to below 1.5 °C. Pathways with no or limited overshoot assessed by the Intergovernmental Panel on Climate Change (IPCC) in 2022⁷ required global emissions to plummet by 2025. Today, we must recognize that these reductions have not materialized, and cumulative emissions since then point to a minimum achievable peak warming level closer to 1.7 °C.

Accepting pathways that accommodate peak warming substantially above 1.5 °C as the starting point for assessing fair contributions to mitigation efforts provides an incentive to delay action. Countries might justify inaction by referring to such implicit target relaxation and rely on vague promises of future carbon removal. Meanwhile, peak warming will continue to creep up, jeopardizing the chance of ever returning to below the 1.5 °C limit.

Avoiding such backsliding requires researchers, policymakers and the public to reflect on how to adapt and expand existing approaches to hold nations to account.

Towards accountability

In an overshoot world, the priority must be to keep peak warming as low as possible. Continued exploration of feasible forward-looking pathways for doing so is necessary to benchmark ambitions for the highest possible reductions in emissions and to unlock the levels of fair international finance needed to help nations decarbonize^10,11. Fairness assessments will need to expand their scope by establishing the responsibilities of countries and other actors for exceeding 1.5 °C. They will then need to capture how those responsibilities would change with different levels of future effort to cut emissions.

The concept of ‘net zero carbon debt’ is one such approach¹⁰. The first step in calculating this is to determine how much of each nation’s fair share of the global carbon budget has already been consumed. This can be done by taking a carbon budget that is compatible with the 1.5 °C warming limit (starting from a given year, such as 1990) and assigning fair shares of the budget on the basis of different principled approaches.

Countries that have already emitted beyond this fair share are classified as ‘carbon debtors’ and each unit of subsequent emissions results in a further unit of carbon debt. Using projections of countries’ emissions, evaluations can be made regarding how much carbon debt might accrue in future.

Crucially, potential responsibility for exceeding 1.5 °C — calculated as a debtor country’s share of the sum of debt across all debtor countries — can be projected even before the global 1.5 °C remaining carbon budget is exhausted. It continues to accrue until a country achieves net-zero CO₂ emissions.

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This measure provides a simple and transparent starting point for tracking how countries contributed to exceeding the 1.5 °C limit. However, it raises a question: if two countries achieve net-zero CO₂ emissions with similar levels of carbon debt, should they be held equally accountable for remedial measures?

Answering this question requires evaluating the underlying circumstances. Over more than three decades of the United Nations climate regime, and in the decade since the Paris agreement, can a country claim to have made its best efforts to reduce emissions given the mitigation options and resources available to it? Did its government respond to key climate summits and other events in a way that furthered climate action (such as through investments and economic stimulus packages¹²)?