Symposium on the ICJ Climate Change Advisory Opinion: Carrying the Flag of the Progressive Development of International Law (Part I)

[Dr. Fabián Cárdenas is a Professor of international environmental law at Pontificia Universidad Javeriana]

The Advisory Opinion (AO) issued by the International Court of Justice (ICJ) on 23 July 2025 portrays the progressive, yet cautious, trend recently adopted by this traditionally orthodox tribunal. With the banners of climate change, the Court also carries the flag of the progressive development of international law, providing a fresh approach – not only to the topic itself, but to the way in which it intends to renew international legal discourses in the midst of a contemporary world characterized by political chaos and increasing legal indifference, particularly of those who boast power. Carrying the flag of climate change, the Court championed the progressive development of international law (defined by the ILC Statute as the preparation of new rules “on subjects which have not yet been regulated by international law or in regard to which the law has not yet been sufficiently developed in the practice of States”). 

We could certainly assure that, on the one hand, the world has a renewed and significantly more robust legal framework on climate change that will indeed strengthen and increase climate litigation around the world. On the other hand, it seems that we also count on a more open perspective on international law that not only smashes old theoretical ideas of fragmentation but also prepares approaches capable of reaching those who thought they could get away with it by being absent, evasive, and indifferent. In other words, the Court got rid of the idea that different branches of international law develop independently, causing overlaps and inconsistencies, as ICL studies have been arguing. Quite the opposite, the Court decisively reaffirmed the unity of international law as a whole and resolved possible conflicts through the idea of cross-pollination of distinct fields. 

To better understand the advances and implications of this historic decision, I will address three key aspects in two parts regarding the ICJ’s Advisory Opinion on international law and climate change: in part I, the conclusions reached by the Court are presented from a critical point of view;part II analyses the questions the Court declined to address or left unresolved, as well as the implications the Opinion may have for the future development of international climate law. As a conclusion, part II will present a southern perspective on the decision as well as the challenges faced for its implementation. 

The Court’s Conclusions

Given the numerous contributions of the Court, each warranting independent, detailed analysis, this text will briefly outline key statements with accompanying commentaries. Among others, it is possible to extract the following Court’s findings:

What are States’ International Obligations Regarding Climate Change?

International climate law must function in accordance with the “best available science” (para. 74). In this regard, the Court sensibly references scientific findings of the Intergovernmental Panel on Climate Change (IPCC), a technical body comprising thousands of experts participating in a professional capacity (paras. 74-76). I believe this perspective is appropriate. In fact, just two weeks ago, in the daily El Espectador, when analyzing the Inter-American Court of Human Rights’ Advisory Opinion on Climate Change, I criticized that tribunal’s attempt to conduct scientific evaluations acting ultra vires in that particular regard. Conversely, the ICJ aptly concludes at the outset that climate change consequences are proven, severe, undeniably affect human populations, and necessitate urgent action, just as the most credible scientific institution found (paras. 72-73).

Conventional climate law, according to the Court, consists of a holistic, coordinated, and systematic interpretation of relevant treaties, such as the United Nations Framework Convention on Climate Change, the Kyoto Protocol, the Paris Agreement, the Vienna Convention for the Protection of the Ozone Layer, the Montreal Protocol, the Convention on Biological Diversity, and the United Nations Convention to Combat Desertification. These treaties must be interpreted in line with customary international law and relevant principles. Although the ICJ did not entirely specify, it is deemed that it includes all customary principles of environmental law identified by the Court’s jurisprudence, such as no harm, prevention, permanent sovereignty over natural resources, due diligence, sustainable development, environmental impact assessment, inter alia. 

Under these sources of international law, essential mitigation obligations include conducting national inventories of GHE as well as assistance which should be provided by developed states, who shall cooperate with less-developed states with adaptation costs. In this regard, cooperation is no longer seen as discretionary assistance but rather as a mandatory obligation derived from historical debts as well as required by the urgency of concrete action. So, continuous neglect to cooperate will imply international legal responsibility. This new approach will certainly demand political flexibility, judicial action, and the pressure of multilateral organisations.   

According to interpretations from various Conference of the Parties decisions (some of them considered binding), the Court concludes that the agreed target is to limit the global average temperature rise to 1.5°C above pre-industrial levels, rather than the previously inferred 2.0°C from the Paris Agreement (para. 224). Such a target, according to the Court, could be attainable only if States commit themselves to the preparation and maintenance of periodic reports, known as National Determined Contributions (NDCs), which should now be considered as obligations of result and not merely of conduct (paras. 234, 245). Thus, clear, precise, and transparent environmental information regarding the state of GHG reductions is considered an essential requirement for reaching the set global targets. In this vein, not only is the target stricter and accurate, but the obligations become determinable and assessable, a situation which may not bring comfort to States aiming for flexibility but turn out to be instrumental for the fight against the climate change crisis.    

Regarding the Law of the Sea, the Court concludes that although the United Nations Convention on the Law of the Sea explicitly requires the protection of the marine environment, it must be understood that the global climate system falls within the scope of this marine environment. Consequently, states party to UNCLOS are obliged to comply with the applicable international climate law identified in this decision if they wish to fulfill their obligations to protect the marine environment (para. 340).

Furthermore, besides referring to treaty law, the Court also deals with customary international law and its interaction with the foregoing treaties. Moreover, the Court builds upon previously recognized rules to announce the identification of new ones. Allow me to elaborate on that. 

Regarding customary law as a formal source of international law, the Court does not abandon its traditional view deployed in the 1969 North Sea Continental Shelf Cases, according to which, from the reading of Article 38 of the ICJ Statute, this nonwritten source is comprised of two elements, namely, opinio juris and state practice (paras. 71-77). Notwithstanding, as I have elaborated previously, the ICJ, being faithful to its historical actual methodology of custom identification, continues to create custom by mere assertion and not by the two formally said elements. In this case, it presents the existence of new and refurbished customary rules that are now directly concerned with climate change law. 

One of the new highlights is the newly born obligation to avoid and prevent significant harm to the climate system and other parts of the environment (para. 409), which is affirmed as custom. Under this obligation, States do 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” (para. 409). In this vein, it should be highlighted that while the existence of the no-harm, prevention, Environmental Impact Assessment and sustainable development principles were previously accepted as custom by the Court as I have previously demonstrated, there had not been an explicit acknowledgment of a general provision aiming to protect the environment as a whole. Until now, of course.  

Similarly, it is recognized as customary law the obligation to cooperate, particularly from developed countries towards others, interpreted under a strict due diligence standard (para. 364). The Court emphasized that this is not discretionary assistance but a necessary and mandatory action towards the fulfillment of climate change objectives. In the context of sea level rise, for instance, the Court considers that the obligation requires States to work together with a view to achieving equitable solutions, taking into account the rights of affected States and their peoples (para. 365). With regards to the climate system as a whole, the obligation to co-operate is “a paramount principle in solving global problems and it is by co-operating that the States concerned can jointly manage the risks of damage to the environment” (para. 261).     

Consistent with the above, the Court explicitly acknowledges the principle of environmental due diligence as customary international law (paras. 138, 246, 343, 347). Nevertheless, I affirm that this principle has been “refurbished” as the Court emphasizes that it must now be understood as requiring stringency, demanding a higher level of vigilance and proactive measures. Going even beyond of what was expected, the Court provides a checklist for assessing state compliance with this principle (para. 136, 138), including:

a. Existence of appropriate national measures and regulations to achieve mitigation and adaptation aligned with reducing anthropogenic greenhouse gas emissions.

b. Availability of scientific and technological information.

c. Measurement standards derived from both binding and non-binding instruments, including COPs decisions, which are (sometimes) mandatory due to their status as subsequent agreements.

d. Recognition of states’ capacities under the principle of Common but Differentiated Responsibilities and Respective Capabilities.

e. Scientific information evaluating the likelihood of harm occurrence that justifies the precautionary approach.

f. Inclusion of procedural requirements such as Preliminary Risk Assessments, Environmental Impact Assessments, Notification, and Consultation, among others.

Likewise, the ICJ makes clear that customary international law and treaty law maintain autonomous existence but are mutually interrelated and interpretative. Indeed, the Court asserts that compliance with customary norms can be evaluated through adherence to treaty norms and vice versa. It means that no State can excuse its disregard for the protection of the environment if it is bound either by a treaty or a custom that obliges it to do so. It also means that, as a legal system, a treaty rule can complement a customary provision and the other way around. Parties failing to cooperate under treaty obligations—even if they have not ratified these treaties—will inevitably bear a significantly heavier evidentiary burden to demonstrate compliance with customary international law. 

In the case of reluctant States such as the US or China, what I believe the Court concludes is that it does not matter if they are not parties to certain climate treaties. As long as they remain obliged by customary rules for the protection of the environment, and certain related HR treaties, as well as LOS treaties, they can be held internationally responsible for the violation of interconnected customary and treaty rules still binding their conduct. 

In this vein, regarding applicable International Human Rights Law, it is concluded that fulfilling all international obligations related to combating climate change is a precondition for compliance with human rights obligations and vice versa (para. 393). In other words, States that are bound by human rights instruments can only comply with such obligations if they previously comply with those demanding the law of climate change, even if they are not a party to the latter. 

This is how I understand having environmental law as a precondition of human rights law. Does a State want to fulfil its obligation (both conventional and customary) to respect and ensure the right to life of its people? Well, it cannot do it unless it proves that it has previously fulfilled its obligations to combat climate change. A person cannot live unless the world it lives in continues to exist. Does a State want to fulfil its obligation to respect and ensure the right to liberty of movement? Again, such a State must combat climate change beforehand, because a person cannot freely choose their residence if the desired destination is now sunken due to sea level rise. And so on and so forth. 

In this context, the principles of Common but Differentiated Responsibilities and Respective Capabilities, Intergenerational Equity, and the Precautionary Approach serve as guiding principles in interpreting the entire corpus juris of climate change. By giving them this status to the foregoing principles, I believe that what the Court is saying is that they are not customary law themselves (yet); however, they are constituent parts of other rules which are indeed custom, such as that of Stringent Due Diligence. So, they aren’t, but they are. This part is a matter of controversy because while the whole world -with the exception of the US- had been waiting for the Court to recognize the Precautionary Principle as customary law, the Global South had been pushing for the Common but Differentiated Responsibilities principle also to be mandatory. However, it seems that the Court did not grant either wish. Notwithstanding, it left such principles in a stage of emergence, a better position than before, by including them as requisites for the compliance of Stringent Due
Diligence, which is indeed customary international law. Even if the Court has not done such an explicit recognition of those principles as custom, this new approach does give international lawyers renewed argumentative tools to argue, as of July 2025, that they actually exist and are thus binding.   

For small island states, which played a leading role in requesting this decision and whose territories are sinking -therefore disappearing- due to rising sea levels, the Court points out that they do not need to update official maps to reflect territory loss resulting from climate change, and such territorial loss will not affect their recognition as states (para. 363). Their statehood remains intact despite these changes. I wonder, therefore, if what the Court is implying is that defined territory is no longer considered a statehood requisite. That debate would, of course, have enormous relevance in contexts different from climate change. However, the Court does not elaborate on that as it only points out that States losing their territory due to climate change shall not be afraid of losing their statehood (para. 363).   

What are the Legal Consequences of Violating a Climate Change International Obligation?

Regarding the second question of the Advisory Opinion, which addresses the consequences of breaching a climate-related obligation, the Court applies, mutatis mutandis, the existing international law of State Responsibility for Internationally Wrongful Acts. Thus, the climate law corpus juris will not invoke a distinct regime of international responsibility. Existing norms remain applicable, including obligations of compliance, cessation of unlawful acts or omissions, guarantees of non-repetition, and reparations such as restitution, compensation, and satisfaction, where required by international responsibility law. This essentially means that responsibility could be argued once an internationally wrongful act is identified through the establishment of a violation of one of the obligations referred above besides the proof of attribution to the act or omission causing such breach to a State.

In this context, the ICJ affirmed that the obligations described in the opinion are erga omnes, meaning they are owed to all treaty parties or all members of the international community concerning customary norms (para. 439). Consequently, direct damage allegations are not required to initiate responsibility claims. Any violation can be alleged by any party. However, environmental damage resulting from acts not contrary to international law is excluded from this responsibility analysis. So, imagine there is an accelerated increase of GHG emissions in Siberia due to ice melting and the lack of action by Russia. Although only Russia and let’s say China are experiencing direct immediate effects of this situation, a State located on the other side of the world, like Costa Rica, could initiate a legal action with a responsibility claim, precisely because climate change obligations are owed to the international community as a whole.

On the other hand, it must be recalled that while international law has been traditionally characterized by its state centrism, recent developments such as the Business and Human Rights framework -particularly the UN Guiding Principles- have called for expansion, so also corporations can be bearers of both international rights and obligations. On this vein, the Court also confirms that corporations are also obligated to pursue established climate mitigation goals. Although the Court does not extensively elaborate on corporate obligations, it emphasizes that states must enact legislation regulating corporate activities in line with these stated objectives and require companies to report and maintain inventories of greenhouse gas emissions (para. 282). As I have stated in the Daily El Espectador, legislators must act throughout the world to make this possible.

Part II will tackle the questions left unanswered and matters that are missing from the decision, as well as the specific implications of what was decided, particularly from a critical southern perspective.