On 23 July 2025, the International Court of Justice delivered an advisory Opinion on the “obligations of states in respect of climate change” from 15 judges, in 130+ pages. Its ruling was unanimous, with a strong closing flourish trailed in my title. There has been a great burst of favourable responses to the Opinion which was delivered yesterday, for instance the Center for International Environmental Law.
But what does the Opinion say, and does it matter?
The questions
The request for the Opinion had come from the UN General Assembly in March 2023. After a typically baggy “chapeau” of potentially relevant climate change and human rights treaties, the UN GA sought answers to the following questions
What are the obligations of States under international law to ensure the protection of the climate system and other parts of the environment from anthropogenic emissions of greenhouse gases for States and for present and future generations?
What are the legal consequences under these obligations for States where they, by their acts and omissions, have caused significant harm to the climate system and other parts of the environment, with respect to:
States, including, in particular, small island developing States, which due to their geographical circumstances and level of development, are injured or specially affected by or are particularly vulnerable to the adverse effects of climate change?
The peoples and individuals of the present and future generations affected by the adverse effects of climate change?”
In December 2014, the ICJ heard oral submissions from 96 states and 11 international organisations (including the EU, the African Union, WHO, and various organisations representing small island states). Major emitting states such as the UK, the US, Russia, China, Saudi Arabia, and Kuwait made individual submissions. This response marks the importance that most UN states attached to this process.
Written transparency of the case is excellent: all those submissions, the written statements preceding them, and the underlying treaties and legal materials are to be found on the ICJ website here.
An ICJ advisory Opinion is just that – advisory. As the ICJ itself acknowledges, it has no binding force, but they may carry great weight and, as the ICJ hopes, “moral authority”. Such opinions are not for bruising fights between individual state parties. Their aim is that the ICJ contribute to the clarification and development of international law.
So how does this Opinion advance the sum of international law knowledge on climate change?
The answer
The answer is yes, it does contribute, to a very significant degree. People will chew over the details of this ICJ Opinion for a wee while to come, but, for the moment, the important parts appear to be these.
Question (a): State obligations
A preliminary question concerned two opposing views between States on the body of law that should be considered to amount to climate change law, namely
a narrower view focused solely on the UN Framework Convention on Climate Change (UNFCCC), the Kyoto Protocol, and the Paris Agreement, and
a broader view expanding the applicable law beyond climate treaties and absorbing principles of Customary International Law (CIL) of environmental and human rights relevance.
The ICJ adopted the broader view. They said that there was an independent CIL duty to prevent significant harm to the environment, which was applicable to climate change, and that States had a duty to exercise due diligence in this respect: [132]-[139], [272]-[300]. Similarly, there was a CIL duty on States to cooperate for the protection of the environment: [142], [301]-[308]: “cooperation is not a matter of choice for States but a pressing need and a legal obligation”. There was no lex specialis arising from the climate change treaties so as to exclude more general principles of CIL from supplementing them.
Other CIL principles were considered at [146]-[160]. The ICJ considered sustainable development, “common and differentiated” responsibilities, equity, including inter-generational equity, in fairly anodyne terms. “Common and differentiated” is the phrase which seeks to yoke developed and developing states under the same regime, with more onerous obligations on the former. The ICJ said that the precautionary principle is part of international climate change law, but it does not include the EU/domestic “polluter pays” principle as between States: [160]. For a short summary of this, see [172].
Analysis of the climate change treaties [174]-[270]
This lengthy section of the Opinion reaches many expected conclusions and some more expansive ones. The UNFCCC (into force in 1994), the Kyoto Protocol (into force in 2005) and the Paris Agreement (into force in 2016) are “mutually supportive”: [195].
There is a particularly detailed analysis of the Paris Agreement from [222] onwards, which I can only touch upon. The Agreement contains a mixture of legal obligations. Some obligations of “conduct” require due diligence in respect of both mitigation – avoiding emissions – and adaptation – minimising or avoiding their effect. But there are other obligations of “result” which require a particular outcome, but these are for the most part procedural ones.
But the dichotomy between obligations of “conduct” and those of “result” is not absolute. For example, the content of the NCDs prepared pursuant to Art.4(2) of the Paris Agreement is not to be left entirely to the discretion of the State; indeed, the ICJ said that discretion in preparing NDCs was “limited”, [245]. It added that the standard of due diligence to be exercised in such preparation was “stringent” because of the seriousness of the threat posed by climate change: [246] – each party “has to do its utmost to ensure that the NDCs it puts represent its highest possible ambition in order to achieve the objective of the Agreement.” The ICJ emphasised that Art.4(2) also requires States to pursue domestic mitigation measures, not simply prepare NCDs.
There was a greater onus on those who had made a greater contribution to historical contributions to GreenHouse Gases (GHG) – this is the “differentiated” bit – and it would take longer for developing countries to reach peak emissions: Art.4(1).
The relationship between treaty obligations and Customary International Law
[309]-[315] casts the climate change net wider than might be thought than simply looking to which states were party to which climate change treaty. Indeed, [315] may have resonances for UN member states who have flipped in and out of specific climate change treaties over the last 20+ years:
“Customary obligations are the same for all States and exist independently regardless of whether a State is a party to the climate change treaties. On this basis, the Court considers that it is possible that a non-party State which co-operates with the community of States parties to the three climate change treaties in a way that is equivalent to that of a State party, may, in certain instances, be considered to fulfil its customary obligations through practice that comports with the required conduct of States under the climate change treaties. However, if a non-party State does not co-operate in such a way, it has the full burden of demonstrating that its policies and practices are in conformity with its customary obligations.”
So, the ICJ seeks to sweep into climate change compliance those states who are not party to the central climate change treaties. The extent to which this is enforceable will be hugely controversial.
The law of the sea
This is worthy of another post: [336]-[365]. Apart from this Opinion, there was an advisory opinion of the International Tribunal for the Law of the Sea (ITLOS) in May 2024, addressing the relationship between the UN Law of the Sea (UNCLOS) and climate change. UNCLOS is considered by the ICJ as “part of the most directly relevant applicable law” [124], and the ICJ attaches “great weight” to ITLOS’s interpretation of UNCLOS: [338]. Co-operation addressing climate change caused sea-level rise “is not a matter of choice for States but a legal obligation”: [364].
Climate change obligations under international human rights law: [369]-[404]
The issue here was whether international human rights law “imposed, in and of itself,” obligations on States in respect of climate change: [370]. The Court considered this in four stages: [371]ff.
What is the adverse effect of climate change on the enjoyment of human rights?
Is there a human right to a clean, healthy and sustainable environment?
What is its territorial scope?
What are the consequent international human rights obligations to protect the climate system and other parts of the environment?
Impacts identified under (1) included effects of sea level rise, drought, desertification and natural disasters, which would affect the enjoyment of the right to life: [376]-[377]. Notably, the ICJ observed at [378] that such conditions might lead individuals to seek safety in another country, from which there would be “obligations under the principle of non-refoulement where there are substantial grounds for believing that there is a real risk of irreparable harm to the right to life…if returned to their country of origin.” The ICJ refers to Art.6 of the International Covenant on Civil and Political Rights of 1966 (ICCPR) for that consequence, but domestic lawyers will immediately have in mind Art.2 of the ECHR for the right to life. So, immigration lawyers should cue into climate change shifts when considering non-refoulement issues.
So, as to (1), adverse effects of climate change may impair the effective enjoyment of human rights.
Question (2), whether there is a human right to a clean and healthy environment, has been a live issue in domestic and ECHR terms for the last 40 years or so. The ECtHR was reasonably clear that there was no such overarching right until confused signals came off from the twin climate change cases which got to its Grand Chamber with decisions in April 2024; Verein KlimaSeniorinnen (about old people in Switzerland: app 53600/20) and Duarte Agistonho v. Portugal and 32 other states (app. 39371/20 about children in Portugal). I tried to summarise the upshot in Episode 195 of Law Pod UK, “Human Rights embrace Climate Change”
After a review of a full set of human rights instruments, international and local, the ICJ considered that the “adverse effects of climate change may impair the effective enjoyment of human rights”: [386]. To similar effect was the conclusion at [393] that “clean, healthy and sustainable environment is a precondition of many of human rights, such as the right to life, the right to health, and the right to an adequate standard of living, including access to water, food and housing.” Many a future domestic law argument will examine this conclusion, and the steps which led to it.
Question (3) is the territoriality argument. The ICJ fudged this one, saying that it depended on the treaty in question. This is a bit odd, because the Duarte Agistonho v. Portugal case decided that a child inhabitant of Portugal could not bring a climate change case against any entity other that Portugal, and, in particular against the other Council of Europe members joined in the proceedings. This is the big outstanding question.
(4)The answer emerges at [404] from a synthesis of the above.
“The Court is of the view that international human rights law, the climate change treaties and other relevant environmental treaties, as well as the relevant obligations under customary international law, inform each other (see paragraphs 309-315 above). States must therefore take their obligations under international human rights law into account when implementing their obligations under the climate change treaties and other relevant environmental treaties and under customary international law, just as they must take their obligations under the climate change treaties and other relevant environmental treaties and under customary international law into account when implementing their human rights obligations.”
Question (b): the legal consequences of acts and omissions which cause significant harm
There was a big debate in the submissions as to what this question meant. The ICJ said what it thought it meant at [405]:
“…this question concerns the legal consequences arising for States that have breached any of the obligations identified in relation to question (a). As the Court noted above… the term “legal consequences” in question (b) is to be understood as referring to the legal consequences arising from internationally wrongful acts of States, which are to be ascertained on the basis of the primary rules and the customary rules on State responsibility.”
But this did not involve determining the individual responsibility of a State or group of States for damage; this could only be done on a case-by-case basis. More fundamentally, though this is not said in terms, this was not a conclusion which the ICJ could reach in delivering an advisory opinion.
The Opinion could therefore only indicate principles. But it sent some warning shots, e.g. at [427]: “Failure of a State to take appropriate action to protect the climate system from GHG emissions — including through fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies — may constitute an internationally wrongful act which is attributable to that State. The Court also emphasizes that the internationally wrongful act in question is not the emission of GHGs per se, but the breach of conventional and customary obligations identified under question (a) pertaining to the protection of the climate system from significant harm resulting from anthropogenic emissions of such gases.”
It is not clear how these generalised statements of international law as perceived by the ICJ mesh with domestic law authorising long-term existing fossil fuel activities. This problem is intrinsic in the seeking of an advisory Opinion. It does not actually decide any individual case (in concreto in international law jargon).
In some unrevealing observations from [436] onwards, the ICJ set out some principles which might arise if States were to be found in breach of their international law obligations. The State would remain under a duty to perform, it must cease from not performing, and it must make reparation, including restitution (if possible), compensation, and/or non-financial remedies such as satisfaction via apology or similar.
Conclusion
The tenor of the Opinion can be well judged from part of [456], its last paragraph, picked up online:
“However, the questions posed by the General Assembly represent more than a legal problem: they concern an existential problem of planetary proportions that imperils all forms of life and the very health of our planet. International law, whose authority has been invoked by the General Assembly, has an important but ultimately limited role in resolving this problem. A complete solution to this daunting, and self-inflicted, problem requires the contribution of all fields of human knowledge, whether law, science, economics or any other. Above all, a lasting and satisfactory solution 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.”
The real question is whether these views embed themselves into domestic law, not least whether domestic law is to recognise that there is a human right to a clean and healthy environment. If so, that would affect domestic decisions as to the scope of Art.8 ECHR and that ECHR right’s permeation into the law of nuisance.
The post What to do about Climate Change: “an existential problem of planetary proportions.” appeared first on UK Human Rights Blog.
