[Interview] ICJ climate opinion puts 'final nail in coffin' of states' excuses, experts argue

Last week, the International Court of Justice (ICJ) delivered a "historic" Advisory Opinion on climate change obligations, calling it "an existential problem of planetary proportions." Though not legally binding, the Court's emphasis on international cooperation and law's potential to address the crisis offers new hope. Academic director Machiko Kanetake (Asser Institute) asked four legal experts – Nataša Nedeski, Kate Mackintosh, Margaretha Wewerinke-Singh, and Yusra Suedi – what this means for climate policies and litigation. An interview.

What does the Court’s Opinion mean for high-emitting states, concretely? If those states’ officials read the Opinion, what should they do as a matter of priority?

Yusra Suedi: “Let me articulate some of the key takeaways. High-emitting states' climate obligations go beyond the Paris Agreement on climate change only. They must also consider other international law sources. These include environmental agreements like the UN Convention on the Law of the Sea, human rights treaties, and customary international law. Next, even if states are not parties to climate agreements such as the Paris Agreement, they must still respect the climate obligations that the Court has laid out. Third, when countries prepare their next national climate plans (known as ‘NDCs, which are a key part of the Paris Agreement and are due in September 2025 - ed), they need to ensure these plans are the strongest and most ambitious they can create. They do not have complete freedom to set weak goals; they must aim high. Finally, countries must take the necessary regulatory and legislative measures to limit the quantity of emissions caused by corporations on their territory.”

Kate Mackintosh: “The Court made clear that states have legally binding obligations under the Paris Agreement to set ambitious emission reduction targets, and that these targets are not purely discretionary. As a matter of law, states must set their highest possible ambition due to the urgency of the crisis. They must also set a target that is capable of making the necessary contribution to limit global heating to 1.5 degrees Celsius above pre-industrial levels, and historic emissions are relevant. So the first thing states need to do now is to take a scientifically informed look at their targets and adjust them accordingly, to avoid being in violation of international law.”

Natasa Nedeski: “While all states are under strict obligations to take measures to prevent harm from climate change, the Opinion articulates that the obligations of high emitters are especially stringent and demanding, more so than those of other states. Since high emitters have contributed most to the problem and are typically in the best position to take effective action, they are expected to do more than others.

Most of those states’ officials should therefore seriously question whether they are currently complying with their obligations under international law. They should ask themselves: Are our mitigation targets currently ambitious enough in light of our capabilities, and are they capable of making an adequate contribution to the 1.5 degree Celsius temperature goal of the Paris Agreement? Can we grant that fossil fuel exploration license and still convincingly say that we are pursuing our best efforts to prevent significant harm to the climate system?

The laws and rules within a country should be set up to quickly cut down on greenhouse gas  emissions to prevent harm. These rules need to cover what both government bodies and private companies do within the country's borders, including things like producing and using fossil fuels. High-emitting states should also take their legal obligations to provide support to vulnerable states seriously, for instance in relation to adaptation.”  

Margaretha Wewerinke-Singh: “The Opinion makes it clear that states must exercise due diligence to prevent climate‑related harm. And while the Opinion did not explicitly establish violations, the Court’s reasoning implies that, because the aggregated emissions of many industrialised States have already driven the temperature increase and inflicted serious human rights and environmental damage, those states are already in breach of these obligations. The Opinion clearly states that those states must urgently realign their mitigation pathways with the 1.5 °C limit, halt any new fossil‑fuel expansion, and deliver restitution, compensation and other forms of reparation to affected countries and communities commensurate with the losses incurred.”

During the proceedings, the Paris Agreement was strategically used by some states to limit the scope of obligations. What do you think of the way the Court engaged with the Paris Agreement?

Yusra Suedi: “I was not surprised that the Court did not accept the argument that only the Paris Agreement applies, pushing aside other international legal instruments. What I found especially encouraging, though, was the level of detail the Court provided in its analysis of the Paris Agreement’s obligations. Overall, the Opinion marks a significant step forward in legally explaining the responsibilities of countries under international climate law.”

Natasa Nedeski: “The Court is absolutely correct in affirming that the Paris Agreement does not, for instance, set aside the rules on responsibility under customary international law, and actually contains quite demanding obligations itself! States must now prepare and communicate Nationally Determined Contributions (NDCs) that reflect each state’s highest possible ambition. Taken together, these measures should be enough to realise the objective of the Paris Agreement: holding the global average temperature to below 1.5 degrees Celsius. This aims to prevent dangerous human interference with the climate system. States must subsequently do all they can to implement their NDC domestically.”

The Opinion focuses heavily on the obligations of individual states to prevent climate change. Yet climate change stems from many sources, including states, private companies and individuals. Given that most individual state contributions seem insignificant globally, what does the ICJ have to say about this?

Natasa Nedeski: “The ICJ puts the final nail in the coffin of the “drop in the ocean” argument that has resurfaced time and time again in climate litigation. Some states and big polluters have contended that they cannot be held responsible in light of their small contribution to the cumulative problem of climate change. This argument had already been rejected by a number of domestic courts, but also at the international level, for instance by the European Court of Human Rights in the KlimaSeniorinnen case.

The ICJ now confirms that the “drop in the ocean” argument cannot serve as an excuse to evade responsibility. Individual states are under an obligation to use all means at their disposal to prevent significant transboundary harm resulting from climate change, even if their activity in isolation could be considered insignificant. What is more, the ICJ even seems to turn the argument around: precisely because climate change is caused by the accumulation of so many drops in the ocean, this actually places an additional layer upon individual states’ obligations. It requires them to cooperate with one another and take account of the situation in other states when fulfilling their obligations.”

What does the opinion mean for existing and future strategic litigation on climate change? Does it pave the way for litigation claiming compensation or reparations for climate-related harm?

Natasa Nedeski: “This Opinion without a doubt paves the way for climate litigation claiming compensation or other forms of reparation. This could be done by individuals before domestic courts, but also by states before the International Court of Justice. Or by international human rights bodies, where this Opinion will carry weight as an authoritative interpretation of international law. I would be surprised if there is no state willing to take up such a case somewhere in the near future.

Importantly, the ICJ explicitly rejects the argument that a causal link between a state’s contribution to climate change and harm resulting from climate change is impossible to establish due to the diffuse nature of climate change. This removes a major potential obstacle for reparations claims for climate harms. Still, this does not guarantee the success of any future legal compensation claim. Success will depend on a case-by-case assessment of causation between the specific harm for which compensation is claimed – like lives lost from a heatwave, or the forced relocation of inhabitants due to sea level rise - and the wrongful act of the state from whom compensation is claimed. This also depends on robust scientific evidence.”

Yusra Suedi: “I expect that this Opinion will re-energise efforts to take legal action on climate change, both within countries and across the globe. While an Advisory Opinion is not legally binding, it carries substantial persuasive authority, as it clarifies the interpretation of legally binding norms such as treaties and customary international law.”

What do you think of the way the Court engaged with the issues relating to sea-level rise, statehood and self-determination?

Margaretha Wewerinke-Singh: “The Opinion links sea‑level rise to the territorial integrity, permanent sovereignty over natural resources and thus the self‑determination of small‑island and low‑lying coastal states. It stresses that these principles are “closely connected” and are endangered as land is lost. By situating climate impacts within one of international law’s most fundamental rights, the Court signals that protecting land‑dependent cultural and political identities is not just  a humanitarian concern, it is a legal obligation.”

Yusra Suedi: “It was brilliant to see the Court clearly affirm that “once a State is established, the disappearance of one of its constituent elements would not necessarily entail the loss of its statehood”. This is a question that scholars have debated for years. This conclusion also reinforces the position adopted by the International Law Commission in May 2025, adding further weight to the principle. The Opinion could have gone further, though, by explicitly affirming that such states retain their right to self-determination over their marine territories and will not forfeit their maritime entitlements, even in the face of existential climate threats.”

Kate Mackintosh: “The Court finds that the principle of non-refoulement (meaning that a state cannot expel or return a person to a place where their life or freedom would be threatened – ed) applies to persons whose countries are rendered uninhabitable by climate change. This will be significant as the number of climate refugees increases.

A problem that has often been overlooked, the massive and under-reported emissions created by armed conflict, was addressed by the US Judge Cleveland in her Declaration, although it was not the main Opinion agreed upon by all judges. I anticipate this issue will become more prominent as countries start looking closely at how well their shared climate goals are working, something the Opinion also calls for.”

How does the ICJ handle the question of legal obligations toward future generations?

Margaretha Wewerink-Singh: “Throughout the Opinion the Court grounds states’ climate duties in inter‑generational equity, recalling that parties must “respect, promote and consider” generational justice when acting on climate change and that protecting the environment is essential for “present and future generations”. It therefore reads treaty and customary norms - especially the duty to prevent significant harm - in a forward-looking way, requiring action today to safeguard the rights and welfare of tomorrow’s people.”

Yusra Suedi: “In my view, the Court could have gone further here. While it rightly emphasised the importance of the principle of intergenerational equity, it stopped short of explicitly affirming the rights of future generations. This is somewhat disappointing, given the growing international recognition of such rights. This recognition is reflected, for instance, in the adoption of the 2023 Maastricht Principles on the Human Rights of Future Generations, and in the Declaration on Future Generations adopted at the United Nations Summit of the Future in 2024. A more direct acknowledgment would have solidified this emerging trend and offered stronger legal grounding for future-oriented climate action.”

How does the ICJ Advisory Opinion relate to recent human rights decisions and opinions by regional human rights courts, such as the Inter-American and European Courts of Human Rights?

Margaretha Wewerinke-Singh: “Aligning its own jurisprudence with the trajectories set by Strasbourg, San Jose and UN Treaty organs, the ICJ recognises that a safe climate is a component of the right to a clean, healthy and sustainable environment. In doing so, the Court gives those regional decisions additional weight in future litigation and policy debates.

Kate Mackintosh: “In affirming that the right to a clean, healthy and sustainable environment is essential for the enjoyment of other human rights, the Court provides a basis for claiming it as an inherent right. While this has already been established in the Inter-American system, no such right has been established at the European Court of Human Rights. It will be interesting to see how the European Court deals with this development in future cases.”

What does this Advisory Opinion reveal about the relevance - or fragility - of international law and multilateralism in a time of geopolitical fragmentation?

Margaretha Wewerinke-Singh: “By weaving together treaty law, custom, principles such as common but differentiated responsibilities, and human rights norms, the Court demonstrates that international law already contains a coherent, mutually reinforcing framework for addressing the climate crisis. The Opinion shows that multilateral rules can evolve and guide collective action. Their actual impact will now be tested by the willingness of countries to implement the due diligence and co‑operation duties the Court has so clearly articulated, and the actions taken in response to apparent breaches.”

Natasa Nedeski: “The Court is realistic about the limited role of (international) law in resolving a problem which it explicitly designates as “an existential problem of planetary proportions that imperils all forms of life and the very health of our planet (italics by ed).” Still, the Opinion expresses hope about the potential of international law to positively influence the action needed to address the climate crisis. Considering all this, the ICJ’s Opinion gives a crucial role to the international duty to cooperate. It also provides clear instructions on how this duty should be put into practice when dealing with climate issues. This is a brave and important step, especially now, in these times of geopolitical fragmentation.”

Nataša Nedeski is Assistant Professor of Public International Law, focusing on the interface between international law and (climate) science at the University of Amsterdam.

Kate Mackintosh is Executive Director of the UCLA Law Promise Institute Europe. She works on the interrelationship of the protection of the environment with human rights and international criminal justice.

Margaretha Wewerinke-Singh is an Associate Professor of Sustainability Law at the University of Amsterdam and co-founder of SEVEN, the university’s climate institute. She served as lead counsel for Vanuatu in the proceedings, and spoke to the Asser Institute in a personal capacity. 

Yusra Suedi is a Lecturer in International Law at the University of Manchester. She is an expert on international courts and tribunals; international environmental law; international climate change law and international human rights law.

Machiko Kanetake is Academic Director and the Chair of the Executive Board of the T.M.C. Asser Instituut. She is also Professor of International Law and Security Governance at the University of Amsterdam.

With thanks to Asser Institute research trainee  Eylül Kabakci LL.M.