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.
Global greenhouse-gas emissions are still rising: when will they peak?
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 CO 2 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.
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