The Obligations of States in Respect of Climate Change – A Landmark Opinion of Urgency and Hope
On 23 July 2025, the International Court of Justice (ICJ) handed down its much anticipated Advisory Opinion on the Obligations of States in respect of Climate Change. This case represents the highest example of climate litigation to date, with the Court joining numerous other international courts and tribunals which have examined states’ obligations with regard to their activities in the field of climate. This blog will review the Court’s key considerations in this extensive Advisory Opinion.
Preliminary Issues
Applicable law
The first topic addressed in the Opinion regards which rules could be considered by the Court (para 162 et seq). Throughout the proceedings, some parties claimed that since the framework applicable to climate change is the International climate change regime made up of the UNFCCC, Kyoto Protocol and the Paris Agreement (PA), other potential sources of obligations could not alter or supersede the climate framework and should not be reviewed in this context. This was justified by parties under the principle of lex specialis argument. The opposing view, based on the principle of systemic integration, was that the climate change treaties should be interpreted harmoniously with other rules of international law (para 164). The Court stated that the principle of lex specialis is a maxim of determining what rule applies when there is inconsistency of rules. The go-to objective in this sense, should be to interpret rules in a complementary fashion, not an exclusionary one. This means, that “when several rules bear on a single issue, they should, to the extent possible, be interpreted so as to give rise to a single set of compatible obligations” (para 165). The Court therefore found that the international climate change treaties do not exclude the application other rules of international law - human rights law, customary international law and international environmental law, all apply to the complex issue of climate change, and should be interpreted so as to strengthen each other.
Conference of the Parties Decisions
Early in the delivery of the Advisory Opinion, the President of the Court set out the preliminary issue, namely that of the legal value of a COP decision - an as-of-yet unsettled issue in literature. The Court observed:
“[…] that in certain circumstances the decisions of these bodies have certain legal effects. First, when the treaty so provides, the decisions of COPs may create legally binding obligations for the parties.”
This is uncontroversial - where the mother treaty says that the legal effect of a certain provision depends on a COP decision, then that decision could be binding. What happens if this is not the case?
“[D]ecisions of these bodies may constitute subsequent agreements under Article 31, paragraph 3 (a), of the Vienna Convention on the Law of Treaties, in so far as such decisions express agreement in substance between the parties regarding the interpretation of the relevant treaty, and thus are to be taken into account as means of interpreting the climate change treaties” (para 184).
Thus, should the governing treaty deem it so, or should the COP decide by consensus, these decisions indeed have normative weight, namely as either binding decisions laying out rules attached to a specific mechanism, or as interpretative mechanisms showing the development of certain obligations. The Court therefore used COP decisions as a means of determining the content of certain obligations of the climate treaties.
Obligations Under The International Climate Change Regime
The Temperature Target
The Court’s consideration of the temperature goal of the PA is notable. The Court cited UNFCC’s overall goal to avert dangerous anthropogenic climate change through international cooperation, noting that:
“the aim of ‘holding the increase in the global average temperature to well below 2 degrees Celsius’ and pursuing efforts ‘to limit the temperature increase to 1.5 degrees Celsius above pre-industrial levels’ represents an important concretization of the Framework Convention’s overall objective [...]” (para 223, emphasis added).
Furthermore, the Court identifies 1.5 degrees Celsius as “the scientifically-based consensus target under the Paris Agreement”, despite the fact that the wording of article 2 identifies it as an additional effort (para 224). Citing the Glasgow Climate Pact, a COP decision from 2021, the Court’s stated;
“these decisions express the agreement in substance between the parties regarding the interpretation of article 2 (...) and thus constitute subsequence agreements in relation to the interpretation of the Paris Agreement within the meaning of article 31(3)(a) of the Vienna Convention on the Law of Treaties” (para 224).
The Court, using the scientific and political consensus expressed in the COP decisions, recognises the normative shift in the meaning of article 2, from 2 degrees Celsius to 1.5. The determination of whether a party adheres to the obligations in the PA, and specifically in article 2, is assessed based on whether the party exercised due diligence and employed its best available efforts in the performance of its obligations (para 229).
Obligations of Mitigation
When it comes to aspects of the Opinion which have more practical, operational effect, the Court’s consideration of the obligations regarding mitigation, and specifically Nationally Deteremind Contribution (NDCs) is of fundamental importance. The obligations regarding NDCs are laid out in article 4 PA, where “each Party shall prepare, communicate and maintain successive nationally determined contributions” (article 4 PA, para 234). This is the first legal obligation that the Court identifies - the obligation to prepare, communicate and maintain NDCs which are successive, representing increased ambition in each NDC (para 235 and 246).
This notwithstanding, it has been argued that the content of NDCs is self-defined, or left to the discretion of the parties (para 237). The Court disagreed with this for two reasons. Firstly, article 4 PA lays out that each successive NDC must represent a progression beyond that Party’s current NDC - it must become more ambitious and therefore demanding over time. Secondly, each NDC must represent a Party’s “highest possible ambition” (art 4, and para 242). As such, the content, and level of ambition of an NDC, have not been left entirely to the discretion of the parties (para 242). Therefore,
“The provision, when interpreted in its context and in light of its object and purpose and the customary obligation to prevent significant harm to the environment, reveals that the content of a party’s NDCs must, in fulfilment of its obligations under the Paris Agreement, be capable of making an adequate contribution to the achievement of the temperature goal.” (para 242)
In light of this, the Court established that the discretion of the parties in the formation of their NDCs is limited by the obligation to exercise due diligence, and the obligation that when taken together, NDCs are capable of meeting the temperature goal (para 245). This standard, “consistent with the varying character of due diligence and the principle of CBDR-RC” will vary on the basis of historical contributions to cumulative GHG emissions, the level of development of the party in question, and its national circumstances (para 247).
The Court therefore informed the application of the PA’s mitigation measures, and laid out that the discretion claimed by many parties is significantly limited by the need to work towards article 2’s temperature goal, a standard of due diligence and a requirement of increased ambition over time. How this will be translated into concrete action is yet to be seen, though this is a progressive reading of the Agreement, as the Court stated in clear terms that ambitious action is a legal obligation.
Customary International Law and Human Rights Law
In addition to its review of the applicable climate treaty law, the Court reviewed the application of customary international law, and presented applicable climate-related obligations.
Duty to Prevent Significant Harm
The duty to prevent significant harm was identified as a further applicable legal obligation upon states with regards to climate change (para 273). The Court disagreed with the contention, by some participations, that this duty is inapplicable in the case of climate change as it only applies in instances of cross border harm (para 134). According to the Court, the prevention duty is an obligation of conduct with a robust and stringent due diligence standard, requiring states to take “appropriate and, if necessary, precautionary measures, which take into account scientific and technological information, as well as relevant rules and international standards, and which vary depending on each States respective capabilities” (para 136). Further, states are required to take risk assessments, and notify and consult other states, as appropriate. States are thus required under customary international law to apply the standard of due diligence consistently to their conduct when acting, so as to prevent significant harm to the climate system.
Duty to Cooperate
The Court stated that inherent in international law is a duty to cooperate, specific to international climate change law, and which is complemented by a strict due diligence standard. The duty to cooperate stems from customary international law, the Friendly Relation Declaration, and article 1 of the United Nations Charter. This echoes the Advisory Opinion delivered by the Inter-American Court of Human Rights (IACtHR) earlier this month, wherein the IACtHR also read into the Inter-American Convention on Human Rights an obligation to cooperate, interpreted in line with principles of common but differentiated responsibilities and equity (IACTHR, The Climate Emergency and Human Rights, para 247 et seq). As the ICJ says clearly, “climate change is a common concern. Co-operation is not a matter of choice for states but a pressing need and a legal obligation.” (para 308).
A Human Right to a Clean Environment
The Court additionally reviewed international environmental and human rights law, noting that over 100 states have enshrined the right to a clean and healthy environment into their constitutions or domestic legal systems (para 391). Furthermore, the Court referenced regional human rights instruments doing the same, such as the African Charter on Human and Peoples’ Rights, the Arab Charter on Human Rights, and the San Salvador Protocol. The Court thereby recognised that under international law “a clean, healthy and sustainable environment is a precondition for the enjoyment of many human rights”(para 393).
In summation, the Court identified two major obligations under customary international law, the duty to prevent significant harm, and the duty to cooperate. It further recognised obligations from international human rights law and international environmental law, which also regulate behaviour of states with regards to the climate system. The Court weaved these obligations together, interpreting them as a mosaic of obligations, all existing independently from, yet strengthening, each other. It identifies the principle of systemic integration as particularly important for multilateral environmental treaties (para 311), and uses it to strengthen the sources it raises.
Consequences for Violations
The second question posed to the Court concerned the legal consequences under states’ obligations where states, by their acts and omissions, have caused significant harm to the climate system and other parts of the environment (see a review of this question by Paddeu and Jackson, here). The Court treated this question as an exercise of determining what framework is applicable for breaches of climate change obligations, rather than determining whether a breach had actually occurred and what the consequences of said breach were. The Court found that the primary rules and the customary rules on state responsibility are applicable to this issue (para 405-407), and that the ILC Articles on State Responsibility (ARSIWA) are reflective of this custom. This responsibility attaches to breaches of the Paris Agreement, Kyoto Protocol and UNFCCC. For the obligation to prevent significant harm, a “State does not incur responsibility simply because the desired result is not achieved, rather, responsibility is incurred if the State fails to take all measures which were within its power to prevent the significant harm”, an assessment of due diligence assessed in concreto, on a case-by-case basis.
Here again, an argument was raised by certain participants that the customary rules on state responsibility do not apply, as lex specialis dictates that the questions are governed solely by the climate change regime. This argument was supposedly supported by the inclusion of a compliance mechanism in the Paris Agreement, as well as a mechanism for loss and damage (paras 412-418). The Court found, however, that there existed no support from the proposition that the parties intended to exclude the customary, general rules on the responsibility of states in this context, and therefore that it indeed applies (para 418).
The chief aspects of the customary rules on state responsibility concern attribution and causation. Should a breach of an international obligation exist, this act or omission must be attributable to a state. When laying out how attribution in relation to the failure to protect the climate system works, the Court found that:
“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.” (para 427).
The Court seemingly states that the production, consumption, and granting of licences for the exploration of fossil fuels would constitute a breach of international law. This is a groundbreaking claim, and the Court does not leave much room for interpretation. It is, however, a bit of a mystery as to why the Court decided to include this claim here, attached to state responsibility, rather than analyse it in the context of obligations applicable upon states.
Conclusion
The past 2 years have heralded landmark developments in international climate litigation. Over this period, a historic judgement from the European Court of Human Rights, and advisory opinions - from the ITLOS, IACtHR and now the ICJ - were delivered. These cases clearly recognise the extreme threat that the climate system and the environment are under, as well as the impacts on human rights. A further notable feature of this advisory opinion is that it focuses on climate change mitigation, unequivocally laying out that states must act decisively to mitigate the effects of climate change, before it is too late.
The Court also highlights the importance of mobilising all actors of society, and that its own impact is limited by its function as a Court:
“[...] the Court, as a court of law, can do no more than address the questions put to it through and within the limits of its judicial function; this is the Court’s assigned role in the international legal order. 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.”
In other words, every aspect of human society must work together to make the changes necessary to avoid catastrophic failure of the climate system.
It is inspirational that what began in a classroom of a law school in Vanuatu, a country facing existential threats due to climate change, made it to the General Assembly in New York, and finally, the Peace Palace in the Hague. In this Advisory Opinion the Court unanimously laid out that States have a duty to act ambitiously, and in line with the best available science, to protect the climate system. Hopefully, the Opinion will permeate the walls of the international legal order, to foster more ambitious action. In the meantime, as the world prepares for COP30 in Belém in November, the Court’s expression of obligations and push for ambition will certainly be hotly debated.
L.R.M. Siry
Liam Siry holds an LLB in European Law from Maastricht University and an LLM in Public International Law from Leiden University. He is currently undertaking his PhD under the supervision of Prof. Marjan Peeters and Dr. Marijn van der Sluis. His research focuses on International Climate Change Law, specifically on the Paris Agreement and climate change litigation.

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