By Jennifer Bansard, Team Leader for the Earth Negotiations Bulletin

The year 2025 marks the tenth anniversary of the adoption of the Paris Agreement on climate change. It may now also be remembered as the year in which the International Court of Justice (ICJ) released its Advisory Opinion on States’ obligations with respect to climate change and the legal consequences for failing to live up to them. Similarly to the Paris Agreement, which was hailed as a “breakthrough of multilateralism,” many consider the ICJ Advisory Opinion a “landmark achievement for climate justice” and a “watershed moment” that ushers in a “new era of accountability” for those who are driving the climate crisis.

As the only court with both general and universal jurisdiction, the ICJ issued an authoritative clarification of the substance of international law, which will provide guidance to other courts around the world and influence the rising number of climate litigation cases.

But what could be the Advisory Opinion’s implications for the climate change negotiations, especially at the upcoming UN Climate Change Conference (UNFCCC COP 30) in Belém, Brazil?

Nationally determined contributions and the transition away from fossil fuels

Among the key elements of the Advisory Opinion is the clarification that the content of nationally determined contributions (NDCs) under the Paris Agreement is not entirely up to parties’ discretion: parties have to do their utmost to ensure that the NDCs represent their respective highest possible ambition. NDCs must be progressively ambitious, informed by the outcomes of the Global Stocktake (GST), and, when taken together, capable of achieving the Agreement’s 1.5°C temperature goal.

The Advisory Opinion could fuel some parties’ calls to reflect on this year’s new round of NDCs and inspire further action at the upcoming negotiation session in Belém in November. While the European Union, the Environmental Integrity Group, and the Umbrella Group issued prominent calls to that effect in their closing statements at the June Climate Meetings, the Like-Minded Developing Countries were not favorable to such discussions. Whether the matter makes it onto the agenda remains to be seen.

A crucial question is the extent to which the new NDCs respond to the calls formulated in the decision on the first GST, specifically regarding energy transition. The Advisory Opinion explicitly points to fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licenses, and the provision of fossil fuel subsidies as examples of acts for which States may be held responsible under international law, considering they represent a failure to take appropriate action to protect the climate system from greenhouse gas (GHG) emissions.

For a long time, the climate negotiations remained largely silent on the issue of fossil fuels. This started to change in 2021 with the cover decision adopted in Glasgow, UK. The matter became more prominent with the adoption of the GST decision in 2023. The Advisory Opinion will further intensify such discussions, including in the context of the negotiations on just transition, where even references to the socioeconomic opportunities associated with transitioning away from fossil fuels remain subject to opposition.

Common but differentiated responsibilities and respective capabilities

Throughout its Advisory Opinion, the Court emphasized the principle of common but differentiated responsibilities and respective capabilities (CBDR-RC) as a key tenet for assessing States’ compliance with their obligations, especially in terms of due diligence. Although CBDR-RC has always been important in the climate negotiations, the ICJ’s work may provide new impetus to advance the principle’s practical operationalization, as the determination of what constitutes “highest possible ambition” and “best efforts” varies accordingly.

Such operationalization would be needed to assess NDCs. The Court specified that the standard to be applied in determining the adequacy of NDCs depends on, inter alia, countries’ historical contributions to cumulative GHG emissions and their level of development and national circumstances, and that as States develop economically and their capacity increases, the requirements of diligence heighten. There is abundant academic research to draw on in advancing these considerations, including with regard to the notion of “fair share” climate action and assessments of countries’ climate plans against global warming pathways (see, for example, the Climate Action Tracker).

Means of implementation and the specter of climate litigation

Operationalization of CBDR-RC not only relates to assessing compliance with mitigation obligations, but it also ties into various aspects of climate action. While parties are not displaying much appetite for pinning down quantitative expectations for NDCs, negotiations on means of implementation are by their very nature more amenable to such discussions. In this respect, the ICJ specified that parties are to implement their finance commitments under the Paris Agreement (Article 9) in a manner and at a level that allows for the achievement of the Agreement’s objectives (Article 2), and that this level can be evaluated on the basis of factors such as developed countries’ capacity and developing countries’ needs.

This point may be brought up in the context of discussions on the Like-Minded Developing Countries’ proposal to address the implementation of Paris Agreement Article 9.1 (developed countries’ finance provision) in Belém – a proposal that already spurred much debate at the June Climate Meetings. In this regard, the Advisory Opinion may be brought up by those pushing for enhanced scrutiny of developed countries’ delivery of climate finance, as well as by those who are calling for countries “in a position to do so” to step up their finance-related efforts. For now, although some countries are using fair share metrics as internal benchmarks for their provision of climate finance, there is strong opposition from all types of provider countries against defining burden-sharing arrangements at the multilateral level.

Another question is how the ICJ’s clarifications regarding the legal consequences of breaches of obligations by States will affect the finance negotiations more generally. The Court emphasized that such breaches may give rise to “the entire panoply” of legal consequences provided for under the law of State responsibility – including the duty to make reparation for damage caused. With this, some see a wave of climate litigation cases looming, especially considering developing countries’ frustration over the new collective quantified goal on climate finance (NCQG), set at USD 300 billion per year – significantly below what is needed – and the fact that the Fund for Responding to Loss and Damage has yet to define disbursement modalities and be meaningfully capitalized (as of June 2025, it had USD 348 million available). While it is clear that the Advisory Opinion ups the stakes, it remains to be seen whether it ends up exacerbating tensions in the already fraught negotiations – or inspires new resolve to avoid courtrooms and ensure progress toward the mobilization of USD 1.3 trillion through the multilateral process. At the June Climate Meetings, the least developed countries (LDCs) specifically called for ensuring that the ‘Roadmap to 1.3 T,’ which the Presidency will present in Belém, brings about a tripling of adaptation finance – a quantitative demand broadly supported among developing countries.

Seizing momentum

The ICJ’s Advisory Opinion, more than 100 pages long, is a treasure trove for advocates seeking to lobby for increased engagement on the variety of issues touched upon by the Court.

For example, the Court emphasized that States incur climate-related obligations not just in the context of the UNFCCC and the Paris Agreement but also under other multilateral agreements, such as the UN Convention on the Law of the Sea (UNCLOS) and the Convention on Biological Diversity (CBD). This could provide further impetus to reflect on possible synergies, including with regard to challenges such as ocean warming and acidification that have so far received little attention in the international climate process. Fittingly, parties are going to discuss the UNFCCC’s cooperation with other international organizations in Belém.

The Advisory Opinion also bolsters the rights-based perspective to climate action. The Court considered that the full enjoyment of human rights cannot be ensured without the protection of the climate system (and other parts of the environment) and that States must therefore take their obligations under international human rights law into account when implementing their climate-related obligations. This gives further legitimacy to discussions related to gender equity, Indigenous Peoples, and youth within the climate negotiations. The ICJ also recalled that the preamble to the Paris Agreement provides that parties should, when taking action to address climate change, respect, promote, and consider the rights of persons with disabilities, which, in turn, supports civil society organizations’ (CSOs) recent demand for the recognition of a disability constituency.

Cooperation

With the US’ second withdrawal from the Paris Agreement, the dynamics of the climate change negotiations are changing. Other developed countries have to find a new footing now that they increasingly find themselves under the spotlight, and China’s stance in the process is drawing a lot of attention. In this regard, it is worth noting that, when questioned about the ICJ’s Advisory Opinion, China’s Foreign Ministry highlighted the country’s intention to make “the world’s biggest cut in carbon emission intensity and move from carbon peak to carbon neutrality in the shortest time frame ever seen in history.” They underscored that “however the world may change, China will not slow down its climate actions, will not reduce its support for international cooperation.”

The ICJ clarified that cooperation in addressing climate change is “not a matter of choice for States but a pressing need and a legal obligation.” In a nod to the US and other countries that may seek to evade climate commitments, the Court emphasized that customary international law obligations, such as the duty to prevent significant harm to the environment and the duty to cooperate for the protection of the environment, exist “regardless of whether a State is a party to the climate change treaties,” and that States are required to implement a collective climate policy that is based on an equitable distribution of burdens and in accordance with the principle of CBDR-RC.

The ICJ’s Advisory Opinion provides a benchmark against which to assess States’ good faith, which, as the Court recalled, is a key principle governing the duty to cooperate. Now that the Court has clarified States’ climate-related obligations under international law, it is clear that some arguments often heard from those seeking to evade accountability cannot be considered to be made in good faith. All States are required to undertake best efforts in their mitigation and adaptation action, and States’ engagement in cooperative efforts needs to be commensurate with their capacity.

As the Court noted, climate change is a “daunting and self-inflicted problem,” which “requires human will and wisdom – at the individual, social, and political levels – to change our habits, comforts and current way of life in order to secure a future for ourselves and those who are yet to come.” If this will does not adequately manifest in the climate negotiations and domestic implementation of climate action, any State may invoke the responsibility of those not in compliance with international law to enforce obligations owed to all through courts.

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This Policy Brief is part of a project that seeks to raise awareness and build momentum and knowledge around the ICJ advisory opinion on obligations of States in respect of climate change and to promote a better understanding of the implications of the advisory opinion among sustainable development decision makers.