2.1. Reading international law harmoniously

The Advisory Opinion’s finding that many relevant bodies of international law apply simultaneously and without contradiction will be of great benefit in addressing climate change. The Court did not accept the position put forward in submissions by a number of States that the Paris Agreement and/or climate change treaties are lex specialis, essentially determining all States’ climate obligations.Footnote ¹² Further, the Court opted expressly for an approach that considered how the applicable bodies of international law may be read in light of one another: ‘it is a generally recognized principle 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’.Footnote ¹³

Generally, the Court’s harmonious reading of rules from multiple sources and fields of international law, including the UN Charter itself, shows that these rules inform one another and are each to be taken into account when applying the others. On closer analysis, harmony is present in at least three distinct forms. The first is an intra-regime harmony, the second is harmony between customary international law and treaty law and the third is harmony across different treaty regimes applicable to the problem of climate change. Further, the Court explained that key principles of international environmental law also inform the various bodies of applicable law.

As to intra-regime harmony, the Court began by establishing that the UN Framework Convention on Climate Change (UNFCCC),Footnote ¹⁴ the Kyoto ProtocolFootnote ¹⁵ and the Paris AgreementFootnote ¹⁶ complement each other.Footnote ¹⁷ They are not incompatible; and the Kyoto Protocol and Paris Agreement provide greater specification to the UNFCCC.Footnote ¹⁸ Article 2 UNFCCC sets the ultimate objective: ‘stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system …’,Footnote ¹⁹ and the Paris Agreement’s temperature goal then serves as a means for pursuing this overarching object and purpose.Footnote ²⁰

As to harmony between customary international law and treaty law, the Court put the primary stake in the ground in identifying that States’ generally agreed recognition of the customary international law duty to prevent significant harm to the environment applies equally in respect of harm to the climate system.Footnote ²¹ From here, the Court observed that the various requirements applying in relation to States’ Nationally Determined Contributions (NDCs) under the Paris Agreement are informed in part by this customary international law prevention obligation.Footnote ²² Critically, the Court considered that the concept of due diligence applies both in relation to the Paris Agreement obligations and under customary international law.Footnote ²³ This means that due diligence becomes a central and unifying feature of the international law complex that governs climate change.

As to harmony across different treaty regimes, the Advisory Opinion embraces important obligations from other multilateral environmental agreements, including the Convention on Biological Diversity (CBD),Footnote ²⁴ the United Nations Convention to Combat Desertification (UNCCD),Footnote ²⁵ the ozone regime,Footnote ²⁶ the United Nations Convention on the Law of the Sea (UNCLOS)Footnote ²⁷ and international human rights instruments,Footnote ²⁸ addressing each one and demonstrating their relationship to climate change treaties and customary international law. The ICJ’s Advisory Opinion is, of course, subsequent to the International Tribunal for the Law of the Sea (ITLOS, the Tribunal) Advisory Opinion on Climate Change and International Law (ITLOS Advisory Opinion) of 21 May 2024, and endorses ITLOS’ findings throughout, with the Court taking the view that it should ascribe great weight to the Tribunal’s interpretations of UNCLOS with the aim of achieving clarity and consistency in international law, as well as legal security.Footnote ²⁹ The Court also acknowledged the role played by the International Civil Aviation Organisation in regulating GHG emitting activities as well as the International Maritime Organization.Footnote ³⁰

2.2. The UNFCCC, the Kyoto Protocol and the Paris Agreement

At the heart of the Advisory Opinion lies the finding that limiting global average temperature rise to 1.5°C above pre-industrial levels is the agreed primary temperature goal of the parties to the Paris Agreement, consistent with the best available science.Footnote ³¹ This is significant because Article 2(1)(a) Paris Agreement incorporates two temperature goals and the Court focused clearly on 1.5°C,Footnote ³² taking into account also the commitment in Article 4(1) to the aim of reaching global peaking of GHG emissions as soon as possible so as to achieve a balance between anthropogenic emissions by sources and removals by sinks of GHGs in the second half of this century.Footnote ³³

By clarifying the role of the 1.5°C goal, the Court set the tone for worldwide efforts to combat climate change, emphasising that this target is scientifically necessary as well as legally pivotal. The Court built here directly on established science, as well as decisions taken at meetings of the parties to the Paris Agreement, viewing these as subsequent agreements in relation to its interpretation.Footnote ³⁴ The Court emphasised the Paris Agreement’s temperature goal as a means for achieving the overall objective set in Article 2 UNFCCC of averting a dangerous concentration of GHGs in the atmosphere.Footnote ³⁵ The irony is that Earth just experienced its warmest year on record, and its first year above the threshold of 1.5°C, according to the World Meteorological Organization’s State of the Global Climate report for 2024.Footnote ³⁶

Consistent with the attention devoted to the 1.5°C goal, the Court focused on the mitigation of climate change, defined as involving human intervention to reduce GHG emissions or enhance carbon sinks, such as forests.Footnote ³⁷ Perhaps the greatest surprise was the extent to which the Court explained clearly how fossil fuel activities fell within the scope of the Advisory Opinion,Footnote ³⁸ noting that licensing of exploration, production, subsidising and consuming fossil fuels all constitute conduct that could be in breach of customary international law and conventional obligations.Footnote ³⁹ This impliedly also includes the export of fossil fuels. Judges Bhandari and Cleveland considered that the Court should have taken an even more emphatic approach to ‘the reality that irreversible harm to the environment is inevitable if the current pace of fossil fuel production, licensing and subsidisation continues unchecked’.Footnote ⁴⁰ As they put it, ‘global production of fossil fuels is on a collision course with the scientific consensus put forward for combatting climate change’.Footnote ⁴¹ Emissions from existing fossil fuel infrastructure will already take the planet over 1.5°C and no new fossil fuel extraction projects can be developed if it is to stay below this temperature.Footnote ⁴² Despite this, in 2030 States intend to produce more than double the amount of fossil fuels than would be consistent with 1.5°C.Footnote ⁴³

Mitigation of climate change through emissions reduction is so central in the Advisory Opinion that the Paris Agreement’s other core objective of adaptation is defined in relationship with mitigation and as complementing mitigation obligations.Footnote ⁴⁴ Further, the Court determined the main mitigation obligations to be obligations erga omnes partes among all parties to the UNFCCC and the Paris Agreement.Footnote ⁴⁵ Their counterpart customary international law obligations were also determined to be obligations erga omnes that are of concern to all States of the world under general international law.Footnote ⁴⁶ This means that all States have a legal interest in their performance and any State may invoke responsibility for breaches of customary international law climate change mitigation obligations.Footnote ⁴⁷ Scholars have observed that this latter determination is significant, as it is the first time that the Court has acknowledged that the status of obligations erga omnes is indeed such that any State may invoke responsibility for breach. This cements the position expressed in the International Law Commission’s (ILC) 2001 Articles on Responsibility of States for Internationally Wrongful Acts (ARSIWA).Footnote ⁴⁸

Arguably, the real key to global mitigation efforts lies in Parties’ Paris Agreement obligations to prepare, communicate and maintain NDCs to international mitigation efforts every five years.Footnote ⁴⁹ These are obligations of result, meaning that parties must actually implement these measures.Footnote ⁵⁰ In contrast, the Court considered that requirements relating to the preparation of NDCs are obligations of conduct, or best effort obligations, to be executed to a standard of due diligence.Footnote ⁵¹ In this case, the standard is ‘stringent’ due diligence.Footnote ⁵² Stringent due diligence is an elevated standard of due diligence. The Court told us that this means each party has to do its utmost in ensuring its NDCs represent its highest possible ambition.Footnote ⁵³ The Court’s most vital contribution here was the authoritative statement that the content of an NDC is not a matter that is simply at the discretion of a State Party, as some participants had argued.Footnote ⁵⁴

The Court went on helpfully to clarify the status of the provision in Article 4(3) Paris Agreement that successive NDCs will represent a ‘progression’ beyond the party’s then current NDC and reflect its ‘highest possible ambition’.Footnote ⁵⁵ The Court determined that these statements are ‘prescriptive’, meaning that they are requirements of NDCs. The Court also advised that the requirement for progression means that a State’s NDC ‘must become more demanding over time’;Footnote ⁵⁶ and that ‘highest possible ambition’ requires that the content of an NDC must be ‘be capable of making an adequate contribution to the achievement of the temperature goal’.Footnote ⁵⁷

The scholarly literature has previously emphasised that the expectation that each party’s NDC will represent a progression, when linked with a State’s highest possible ambition, creates a ‘potent normative tool’.Footnote ⁵⁸ Looking forward, it will be interesting to see how successive NDCs are scrutinised to determine whether they incorporate ‘progression’ and ‘highest possible ambition’.Footnote ⁵⁹ This may involve a range of reference points, including scope, coverage and inclusion of supplementary objectives as well as ‘headline’ numerical targets.Footnote ⁶⁰ Certainly the concept of ambition is central,Footnote ⁶¹ and the Court tied individual State ambition specifically to the Paris Agreement’s ambitious and collective 1.5°C goal.Footnote ⁶²

The Paris Agreement also specifies that NDCs must be informed by the outcomes of five-yearly Global Stocktakes, including the first Global Stocktake concluded by the parties in 2023.Footnote ⁶³ The 2023 Global Stocktake recognised that ‘despite overall progress on mitigation, adaptation and means of implementation and support, Parties are not yet collectively on track towards achieving the purpose of the Paris Agreement and its long-term goals’.Footnote ⁶⁴ The significant contribution from the Court here was to add that States’ NDCs, when taken together, must be capable of achieving the temperature goal of limiting global warming to 1.5°C.Footnote ⁶⁵ This obligation applies for all parties to the Paris Agreement, although the Court added that the due diligence it considers is required in setting NDCs varies in light of national circumstances.Footnote ⁶⁶

As well as setting NDCs, States have a critically important obligation to pursue domestic measures with the aim of achieving their objectives.Footnote ⁶⁷ There is no explicit obligation in the Paris Agreement for States to implement and achieve their NDCs although there is a good faith obligation that parties will aim and intend to do so.Footnote ⁶⁸ The Court held that the obligation to pursue domestic measures with the aim of achieving the objectives of an NDC was also an obligation of conduct, requiring stringent due diligence.Footnote ⁶⁹ This requires States to be proactive and pursue measures that are reasonably capable of achieving their NDCs.Footnote ⁷⁰

The Court was attentive also to the Kyoto Protocol, both as an interpretive aid in relation to the UNFCCC and the Paris Agreement and in its own right. Specifically, the Court observed that non-compliance with Kyoto Protocol emission reduction targets by an Annex B party might constitute a legally wrongful act.Footnote ⁷¹

These various findings are fascinating, and important. It is expected that parties will advance arguments in domestic and international climate advocacy and litigation mounted on the basis of the various administrative law style tests articulated by the Court in the Advisory Opinion, as such tests can help ensure accountability in the scrutiny of government action in domestic processes.Footnote ⁷² It has always made sense that there must be a relationship of coherence between domestic measures and their objectives, along the lines seen in other areas of international law, and the Court’s description of the implied nature of the relationship in these terms is to be welcomed,Footnote ⁷³ as is the attention the Court gave to the content of NDCs and the required collective effect.

2.3. Customary international law

Customary international law, as the other major source of international law, played a role in the Advisory Opinion that was equally vital to that of the climate change treaties. Customary international law obligations apply to all States—unlike treaties, to which a State must be party to be bound. For instance, the United States (US) will continue to be bound by customary international law relating to climate change even when its withdrawal from the Paris Agreement takes effect. As the Court put it, ‘customary obligations are the same for all States and exist independently regardless of whether a State is party to the climate change treaty’.Footnote ⁷⁴ This makes the Court’s pronouncements on customary international law particularly important. The Court articulated two customary international law duties on States: first, the duty to prevent significant harm to the environment; and, second, the duty to cooperate for the protection of the environment.

The Court was clear that the duty to prevent significant harm to the environment extends to the climate system, which is an integral and vitally important part of the environment to be protected for present and future generations.Footnote ⁷⁵ The multifaceted and diffuse nature of human conduct contributing to climate change does not preclude the duty, which arises as a result of the general risk of significant harm,Footnote ⁷⁶ noting that some harm has already been caused.Footnote ⁷⁷ Indicators of such harm presently include temperature rise, sea-level rise, negative effects on ecosystems and biodiversity and extreme weather events.Footnote ⁷⁸ The risks posed by current activities for the future, including in the long-term, must be taken into account and the probability/foreseeability and magnitude of these risks must be considered.Footnote ⁷⁹ The higher the probability and seriousness of possible harm, the more demanding the standard of conduct attaching to the duty of prevention.Footnote ⁸⁰ Cumulative effects must also be assessed.Footnote ⁸¹ Even ‘activity that is insignificant in isolation’ can give rise to the duty to prevent, although it is the sum of all activities that contribute to anthropogenic GHG emissions over time which produces the risk to the climate system.Footnote ⁸² States must avert this risk through a coordinated and cooperative response.Footnote ⁸³

Drawing on prior jurisprudence, the Court confirmed that the required standard of conduct for the duty to prevent environmental harm is due diligence: States must ‘employ all means reasonably available to them, so as to prevent [harm] so far as possible’.Footnote ⁸⁴ In respect of climate change mitigation, the standard is ‘stringent’ due diligence.Footnote ⁸⁵ Determining what due diligence requires in a particular situation involves taking into account a number of elements. First, the classic formulation of due diligence in respect of environmental harm calls for appropriate measures. This requires States to:

Second, due diligence requires States to acquire and analyse scientific and technological information, and the Court added that States must ‘actively pursue’ the scientific information necessary for them to assess risks,Footnote ⁸⁷ bearing in mind that new knowledge may raise the standard of due diligence required.Footnote ⁸⁸

The third element determining due diligence is relevant international rules and standards,Footnote ⁸⁹ and the fourth is States’ different capabilities. The Court emphasised that although a State’s capabilities are a factor, even a less well placed State is obliged to take all the means at its disposal to protect the climate system.Footnote ⁹⁰ The fifth element is the precautionary principle or approach,Footnote ⁹¹ and the sixth is the procedural requirement for environmental impact assessment, which is also independently required as a general customary international law rule. Environmental impact assessments are important in relation to individual activities that are particularly significant. In such cases, potential specific climate related effects must be assessed at the level of the proposed individual activities, including to assess their downstream effects.Footnote ⁹² Judges Bhandari and Cleveland note that this means the combustion of fossil fuels has to be taken into account in decisions including production, licensing and subsidisation.Footnote ⁹³ ‘Fossil fuels are produced in order to be burned’, the judges underline.Footnote ⁹⁴ The Court further indicated that it may be reasonable for States to conduct high-level overview risk assessments of the risks caused by GHG emissions also by way of general procedures covering different forms of activity.Footnote ⁹⁵ This is open to interpretation. For instance, a general policy or legal decision to reopen bids for oil and gas exploration nationally might qualify, but equally the Court may have had in mind more strategic environmental impact assessments to precede decision-making relating to an even broader range of diverse activities.

The seventh point in relation to due diligence is the need for notification and consultation, which will be ‘particularly warranted when an activity significantly affects collective efforts to reduce harm to the climate system’ or with respect to information necessary to meaningful cooperation.Footnote ⁹⁶ Examples of situations where due diligence can call for notification and consultation include the implementation of policy changes in relation to the exploitation of resources linked to GHG emissions.Footnote ⁹⁷ These pronouncements also appear to have direct relevance for government decisions to grant or decline permits, and for regulatory decisions in relation to oil and gas exploitation. Like environmental impact assessment, notification and consultation have previously been approached as an independent requirement under customary international law as well as an element of due diligence.

The second customary international law duty discussed by the Court, the duty to cooperate for the protection of the environment,Footnote ⁹⁸ is all the more vital when a shared resource such as the climate system can only be protected through close cooperation.Footnote ⁹⁹ Cooperation must be ‘sustained and continuous’ and must take account of the situations of other States.Footnote ¹⁰⁰ While coordinated implementation of treaties forms a principal expression of cooperation, States are not required to conclude treaties—they are, however, ‘required to make good faith efforts to arrive at appropriate forms of collective action’.Footnote ¹⁰¹ Collective action in relation to climate change requires agreement on forms of bona fide cooperation. Specifically, ‘international law requires States to cooperate to achieve concrete emission reduction targets or a methodology for determining contributions of individual States’, including with respect to the fulfilment of agreed temperature goals.Footnote ¹⁰² This duty applies to all States although it varies according to their common but differentiated responsibilities and respective capabilities.Footnote ¹⁰³ It is founded on the recognition of States’ independence, requiring ‘efforts by States to continuously develop, maintain and implement a collective climate policy that is based on an equitable distribution of burdens’.Footnote ¹⁰⁴

Importantly, States may find they have not discharged the duty to cooperate in relation to climate change only by concluding and implementing the climate treaties presently in existence.Footnote ¹⁰⁵ There is a bigger picture, including to the point where, as the Court indicated, there may be a time when further treaty-based obligations are needed, with the increased urgency of the climate crisis. Here, the customary international legal obligations discussed earlier in this section constituted, in the Court’s view, a legal standard for assessing whether climate treaties continue to serve their purpose and whether further collective action has to be undertaken, including adopting further treaty obligations.Footnote ¹⁰⁶

Readers will appreciate that customary international law and treaty obligations have a separate existence, though the Court interpreted them harmoniously. Indeed, the Court went so far as to suggest, in a carefully caveated manner, that:

How should this statement be understood? Importantly, in saying this, the Court both leant on and lent weight to the existing treaty regime. Further, this statement enabled the Court to give clear guidance to non-party States. For non-parties, the Court:

Interestingly, the Court suggested that a non-party State not cooperating in such a way will have the full burden of demonstrating that its policies and practices are in compliance with its customary international law obligations.Footnote ¹⁰⁹ One wonders whether the Court was signalling a potential reversal of the usual adjudicatory burden of proof?Footnote ¹¹⁰ At the same time, the Court was clear that its comments do not mean that customary obligations will be fulfilled simply by compliance with climate change treaties.Footnote ¹¹¹

As far as due diligence is concerned, one point worth noting, for those who are interested in the regularity with which international courts and tribunals now distinguish between obligations of conduct and obligations of result, is that in its Paris Agreement analysis the Court implied directly that the standard of performance for any international law obligation of conduct is one of due diligence.Footnote ¹¹² This is of course without prejudice to the possibility that a heightened due diligence standard may apply, including requirements for ‘stringent’ due diligence as seen both in this Advisory Opinion and in the ITLOS Advisory Opinion.Footnote ¹¹³ However, the point itself is new. Until now, the jurisprudence had been open to the possibility that there could be a variety of standards applying to the range of international law obligations that could be described as obligations of conduct. Inter alia, the Court’s statement would seem to consolidate due diligence’s emerging role as a global regulatory standard, being read widely into international law, including both treaty law and customary international law.Footnote ¹¹⁴ At the same time, there may be a hint of differing views within the Court on the utility of the ongoing distinction between obligations of conduct and obligations of result. While employing these terms in a predictable fashion throughout the Advisory Opinion, the Court also stated that the distinction was not necessarily a strict one, that breach of either type of obligation may incur State responsibility,Footnote ¹¹⁵ and that such obligations may be interconnected and mutually supportive.Footnote ¹¹⁶

2.4. Human rights

The Court’s strong engagement with the right to life, the right to health, the right to an adequate standard of living and the right to privacy, family and home conveyed an important and clear message that the conduct leading to climate change is affecting these human rights. Indeed, the Court stated expressly that ‘the adverse effects of climate change may impair the effective enjoyment of human rights.’Footnote ¹¹⁷ Helpfully for future climate advocacy and litigation, the Court also specified that States must give due account to the protection of human rights in their climate standards and legislation and in the regulation of private activities.Footnote ¹¹⁸

As directed in the UNGA’s Resolution requesting an advisory opinion,Footnote ¹¹⁹ the Court applied a range of UN human rights instruments. Decisions of the Human Rights Committee also feature in support of the Court’s findings, including Teitiota v New Zealand and Daniel Billy and others v Australia. Footnote ¹²⁰ The Court further observed, specifically, that the rights of women and children and the rights of indigenous peoples may be impaired, including in the context of severe weather events and heightened water scarcity,Footnote ¹²¹ and also encouraged States to ensure their consideration in the design of mitigation and adaptation measures so as to help to ensure non-discrimination and not undermine relevant rights.Footnote ¹²²

Observers have discussed whether or not the Advisory Opinion recognised a human right to a clean, healthy and sustainable environment.Footnote ¹²³ This particular topic is not a straightforward one. By the time that the ICJ reached the end of its analysis, it had, indeed, recognised ‘the human right to a clean, healthy and sustainable environment’ as ‘inherent in the enjoyment of other human rights’.Footnote ¹²⁴ Significantly, though, this does not feature in the summary of the Court’s views at the end of the human rights section, and neither is it selected to feature in the dispositif at the end of the Advisory Opinion.

There are important nuances here. Regional human rights courts, applying their respective human rights instruments, have taken different views about reading in such a right.Footnote ¹²⁵ The Court was clear, to begin with, that the protection of the environment is a precondition for the enjoyment of human rights.Footnote ¹²⁶ The Court also clearly stated that, specifically, degradation of the climate system and other parts of the environment may impair the enjoyment of a range of established human rights protected under international law.Footnote ¹²⁷ The Court then reasoned that ‘[th]e right to a clean, healthy and sustainable environment results from the interdependence between human rights and the protection of the environment’, describing the new right in the first place only as a ‘right’ and not a ‘human right’.Footnote ¹²⁸ The Court added that it is difficult to see how the human rights obligations already written into the law can be fulfilled without ensuring the protection of the human right to healthy environment ‘as a human right’.Footnote ¹²⁹ The Court finally appeared then to deduce that a ‘human right’ to a healthy environment is inherent in the enjoyment of other human rights.Footnote ¹³⁰ Indeed, Judge Aurescu’s remarks confirm that the Court did not explicitly find the right to be a norm of customary international law, although he himself believed that to be the case.Footnote ¹³¹

Perhaps the Court’s conclusion was thought inevitable based on the premises. At the same time, the Court’s view was bolstered by its reference to regional human rights instruments recognising a human right to a healthy environment,Footnote ¹³² its reported recognition in the constitutions or domestic legislation of over 100 StatesFootnote ¹³³ and UNGA Resolution 76/300, adopted in 2022 with 161 votes in favour, no votes against and eight abstentions.Footnote ¹³⁴ However, the Court did not seem to say that there is a basis for recognising the right as a matter of customary international law or treaty interpretation.Footnote ¹³⁵

Significantly, the Court referred to the science when discussing States’ obligations under international human rights law,Footnote ¹³⁶ as well as under other multilateral treaties including the UNCCD, where various intergovernmental scientific bodies play their part.Footnote ¹³⁷ In relation to sea-level rise the science is crucial, with the Court noting that the Intergovernmental Panel on Climate Change (IPCC) had concluded with a high level of confidence that risks for coastal ecosystems, people and infrastructure will continue to increase.Footnote ¹³⁸ This leads potentially, inter alia, to forced internal and external displacement of populations,Footnote ¹³⁹ even provoking application of the principle of non-refoulement if there are substantial grounds for believing that there is a real risk of irreparable harm to the right to life.Footnote ¹⁴⁰

2.5. The conventions on biological diversity, desertification and the ozone layer

Directly applicable law further included not only the climate change treaties and customary international law, the law of the sea and international human rights law, but also the CBD, the UNCCD and the Vienna Convention together with the Montreal Protocol (the ozone treaties).Footnote ¹⁴¹ The Court’s attention to these treaties is refreshing and forms a key part of the Advisory Opinion. The state of global conservation, as well as climate change’s impacts upon it, receives too little attention in major international legal fora and inter-State politics beyond specialised treaty regimes.

The Court found that the CBD, UNCCD and the ozone treaties all mutually inform the climate change treaties and customary international law, and that parties must take these environmental treaties into account when implementing the climate change treaties and customary international law, as well as vice versa.Footnote ¹⁴² The CBD merits particular attention, given the current extraordinary level of threat to biodiversity globally and the high level of species extinction accompanying human activities, to which attention has been drawn elsewhere.Footnote ¹⁴³ The Court found that the biosphere is one of the components of the climate system,Footnote ¹⁴⁴ and noted that the Kunming-Montreal Global Diversity Framework adopted by the Conference of the Parties (COP) to the CBD in 2022 specifically encourages parties to the CBD to minimise the impacts of climate change and ocean acidification on biodiversity.Footnote ¹⁴⁵

The Court then highlighted a number of obligations within the CBD which relate closely back to those it had already identified in respect of climate change, potentially significantly heightening international legal awareness of relevant hard law obligations.Footnote ¹⁴⁶ Article 3 CBD reflects the customary international law duty to prevent significant harm to the environment by providing that States parties have the responsibility to ensure that activities within their jurisdiction or control do not cause damage to the environment of other States or areas beyond national jurisdiction. Article 5 CBD establishes the obligation of States Parties to cooperate for the conservation and sustainable use of biodiversity in areas beyond national jurisdiction and on other matters of mutual interest. Again, this parallels customary international law and, in addition, duties to cooperate were also identified by the Court as principles within the climate treaty framework.Footnote ¹⁴⁷ Article 7(c) CBD requires parties to identify and monitor activities that are likely to have significant adverse impacts on biodiversity. This flows into Article 8(l), which requires parties, where significant adverse effects have been identified under Article 7, to regulate or manage as far as possible relevant processes and categories of activities, including those which contribute to anthropogenic GHG emissions.Footnote ¹⁴⁸

States also contribute to the protection of the climate system and other parts of the environment by complying with the UNCCD.Footnote ¹⁴⁹ Intergovernmental scientific bodies have highlighted the links between desertification, land degradation, drought and climate change.Footnote ¹⁵⁰ The physical connection is clear. Accordingly, the UNCCD requires States Parties to promote cooperation on environmental protection, land and water resource conservation as relating to desertification and drought.Footnote ¹⁵¹ The Court emphasised that States Parties must take account of climate change.Footnote ¹⁵² Further, the UNCCD requires that developed country parties must support affected developing country parties in combating desertification and mitigating drought, including by providing financial resources.Footnote ¹⁵³ Additionally, the UNCCD specifically requires parties to encourage coordination of activities taken under it, the UNFCCC and the CBD.Footnote ¹⁵⁴ As to the ozone treaties, the Court also found that the ozone layer is part of the atmosphere and that these obligations contribute to ensuring the protection of the climate system as a whole.Footnote ¹⁵⁵

2.6. Principles of international environmental law

Importantly, the Court determined early in the Advisory Opinion that a number of interrelated principles familiar to international environmental lawyers form part of the applicable law for the purposes of the Advisory Opinion. These principles guide both the interpretation and application of treaties and the determination and application of customary international law obligations.Footnote ¹⁵⁶ All of the applicable principles are also found specifically within the climate treaty regime.Footnote ¹⁵⁷

The overarching principle of sustainable development is first on the list, noting that the Court adopted the articulation of this principle from the Gabčíkovo-Nagymaros Project case as concerning the ‘need to reconcile economic development with protection of the environment’.Footnote ¹⁵⁸ This juxtaposes with the often-cited formulation of sustainable development from the 1987 Brundtland Report as a process that ‘meets the needs of the present without compromising the ability of future generations to meet their own needs’.Footnote ¹⁵⁹ While the two formulations are reconcilable and complementary, the Court’s framing of the principle may ultimately be the less anthropocentric of the two phrasings.

Second on the list, and of real instrumentality in the climate change field, is the principle of common but differentiated responsibilities and respective capabilities (CBDR-RC). Differentiation among States based on the CBDR-RC principle has long been a feature of international environmental law, including climate change law, and it also features centrally in the climate change treaties.Footnote ¹⁶⁰ The meaning of the CBDR-RC principle is a matter that is critical to the international climate change law edifice, both for the equity considerations involved and because both developed and developing country action on climate change will determine the success or failure of the regime. Indeed, the Court’s ruling on customary international law placed every State under the stringent due diligence obligation to use all the means at its disposal to protect the climate system, albeit in accordance with its CBDR-RC.Footnote ¹⁶¹

Here, matters became interesting. Key among the tensions during the advisory proceedings was the extent to which, in actuality, legal obligations in the climate sphere depend on a State’s blend of contributions to atmospheric GHG concentrations (historically and currently), and on States’ current capabilities and national circumstances as referred to in the Paris Agreement. National circumstances include a country’s level of economic and social development. The Court found that the CBDR-RC principle embraces all these dimensions. Further, the Court importantly observed that the status of individual countries is not static; rather, their national circumstances continue to change.Footnote ¹⁶² This means that countries are on a spectrum, rather than in two groups of developing and developed States. It also means that there is clear scope for an increasing level of obligation on a particular State. This has concrete, real-world implications. Most obviously, at present, although the Court did not mention it, China is poised to outpace the US as the major GHG contributor and also the European Union which has occupied second place until now, while China’s capacity to contribute to mitigation is also growing. It was of interest that Judge Xue disagreed with the Court’s view that States sit on a spectrum, considering that this description distorts the foundational structure of the climate change treaty regime.Footnote ¹⁶³

The principle of CBDR-RC is a manifestation of the third principle, the principle of equity.Footnote ¹⁶⁴ The Court remarked that equity is an emanation of the idea of justice, and the Court cited its own jurisprudence for the idea that when applying positive law it may choose the interpretation that appears in the circumstances of the case closest to the requirements of justice.Footnote ¹⁶⁵ This raises certain issues. Interpretations of the law have effects going beyond individual cases. It is important to consider the extent to which a particular interpretation of the law can be considered to be confined to the case-specific circumstances before a judicial body. International lawyers will need to be mindful of the ongoing interplay between the importance of equity in the law, as revealed in the circumstances of particular cases, and the question of a legal rule’s overall objective substantive content, which will be binding on all relevant States.

Intergenerational equity is a related, fourth principle, as referred to in the preamble of the Paris Agreement and discussed in Section 2.7 with respect to the interests of future generations. Fifth, the precautionary approach or principle is also key, as referred to in Article 3(3) UNFCCC and addressed in Section 4.4 in relation to the role of science in the Advisory Opinion.Footnote ¹⁶⁶ One of the interesting questions arising from the Advisory Opinion might be how, in due course, the precautionary principle could play into the determination of the temporal scope of international climate obligations.Footnote ¹⁶⁷ For instance, should the time from which customary international law duties of prevention might run be decided by reference to when prudent States would have taken action, taking into account the precautionary principle’s tenet of acting rather than delaying action when faced with scientific uncertainty?

The sixth principle considered by the Court was the polluter pays principle. Uniquely among the general principles considered, it was determined not to be part of the applicable law, having not been specified in the treaty context.Footnote ¹⁶⁸ This is an interesting conclusion because, if States are viewed as ‘the polluters’, the principle of CBDR-RC could be connected with ideas of the polluter paying because of how the CBDR-RC principle takes account of States’ historical emissions. However, the Court viewed the polluter pays principle as relating primarily to the conduct of private actors and to concepts of strict liability. The Court added that this does not preclude the possibility of international law developing to generate strict liability for acts that may not be internationally wrongful.Footnote ¹⁶⁹ Ultimately, the Court’s view may assist in the development of the CBDR-RC principle on its own terms, keeping this distinct from the polluter pays principle.

Finally the customary international law duty to cooperate was also considered by the Court to be a guiding principle in the climate change treaty framework.Footnote ¹⁷⁰ This duty was further elaborated elsewhere in the Advisory Opinion, as described in Section 2.3 of this article in relation to customary international law.Footnote ¹⁷¹ Several provisions of the UNFCCC, the Kyoto Protocol and the Paris Agreement implement this principle in different ways.Footnote ¹⁷² Indeed, the duty to cooperate runs strongly throughout the Advisory Opinion and is addressed in Section 4.1 in connection with climate finance.

2.7. Intergenerational equity

A key contribution of the Advisory Opinion is the Court’s specific statement that the principle of intergenerational equity requires ‘due regard for the interests of future generations and the long-term implications of conduct’.Footnote ¹⁷³ With ‘due regard’ emerging as a test for legal conduct in an increasingly wide variety of international legal fields, greater elaboration of what due regard comprises may begin to emerge in the near future.Footnote ¹⁷⁴ For instance, there may be an important procedural dimension requiring the analysis of the interests of those to whom due regard is owed.Footnote ¹⁷⁵ As the Court stated, the considerations in question ‘need to be taken into account where States contemplate, decide on and implement policies and measures in fulfilment of their obligations under the relevant treaties and customary international law’.Footnote ¹⁷⁶ This is a valuable contribution to the general understandings in international law of the meaning of due regard, and provides a useful and concrete connection point between domestic and international law for invocation in the context of domestic and international policy development processes, negotiations and climate litigation.

The Court grounded the requirement of due regard for future generations in equity infra legem, with intergenerational equity being a manifestation of equity in a general sense, taking into account the added dimension that intergenerational equity is an expression of the idea that ‘present generations are trustees of humanity tasked with preserving dignified living conditions and transmitting them to future generations’.Footnote ¹⁷⁷ As the IPCC has observed, many of the changes that continued emissions cause to major climate system components will be irreversible on time scales from the centennial to the millennial, such that ‘[w]ithout urgent, effective, and equitable mitigation and adaptation actions, climate change increasingly threatens ecosystems, biodiversity, and the livelihoods, health and well-being of current and future generations (high confidence)’.Footnote ¹⁷⁸

Even with this hint of an international legal ‘turn to the future’,Footnote ¹⁷⁹ there is room to suggest that the Court underplayed the matter of future generations’ interests. Additional arguments have been put forward in the literature. One argument is that future generations are ‘others’ whose interests it would be consistent to consider in accordance with the maxim sic utere tuo ut alienum non laedas (use what is yours in such a way that you do not injure that of another).Footnote ¹⁸⁰ This can be extended into an argument that the customary international law obligation of prevention of harm to the environment might be understood as protecting the interests of future generations. There might possibly also be associated substantive constraints on State conduct requiring avoidance of ‘manifestly excessive adverse impacts’ on the interests of future generations.Footnote ¹⁸¹ The Court’s approach to intergenerational equity does not preclude such findings in future. Judge Sebutinde considered that the Court should have affirmed that States owe customary international law obligations not only to present generations but also to future generations.Footnote ¹⁸²

Elsewhere, the Court blended the matter of future generations’ interests subtly but foundationally into its consideration of human rights, observing at the outset of the human rights section of the Advisory Opinion that ‘[t]he environment is the foundation for human life, upon which the health and well-being of both present and future generations depend’.Footnote ¹⁸³ However, the Court made the point when parsing the UNGA’s request that, because international law is a law among States, individuals by and large lack standing to bring claims against States in respect of fulfilment of their climate obligations, except as might be recognised in specific treaties, such as human rights treaties.Footnote ¹⁸⁴ Certainly, there is blue sky between the Court’s view and that of the Inter-American Court of Human Rights, which has recognised that human rights have collective dimensions in which they are owed to both present and future generations.Footnote ¹⁸⁵

3.1. The applicability of the law on State responsibility

The Court decided to address the second question posed by the UNGA by setting out the general legal framework governing States’ responsibility for failure to comply with their obligations to protect the climate system. Importantly, the Court determined that the general international legal regime on State responsibility applies, and that the climate change treaties do not provide a lex specialis on responsibility, as some participants had argued.Footnote ¹⁸⁶

3.2. Attribution and causation

The issues of attribution and causation were distinct and specific matters arising under the law on State responsibility. On attribution, failure of a State to fulfil a relevant obligation could constitute an attributable and internationally wrongful act. For greater clarity, the Court explained that, in the context of the obligations which apply in respect of mitigating climate change, the wrongful act was likely to take the form, not of the emission of GHGs per se, but of the breach of either treaty obligations or customary international law obligations on the protection of the climate system from significant harm resulting from anthropogenic emissions of such gases.Footnote ¹⁸⁷ Thus, a State could be internationally legally responsible for its failure to exercise due diligence and take the necessary regulatory and legislative measures to limit private actors’ emitting activities.Footnote ¹⁸⁸

Further, as the Court discussed, previously where there have been multiple causes of injury the Court itself has held that ‘responsibility for part of such injury should [be] allocated among [the] actors’.Footnote ¹⁸⁹ This statement has been described as significant at the level of principle, though leaving much to be determined.Footnote ¹⁹⁰ The Court also observed that States have contributed to the atmospheric concentration of GHGs in significantly different degrees.Footnote ¹⁹¹ Further, although it is the collective and aggregate effects of the GHGs emitted by all States as well as from natural sources that cause damage to the climate system,Footnote ¹⁹² the responsibility of a single State could be invoked without invoking the responsibility of all States that might be responsible.Footnote ¹⁹³ This does appear to give rise to the point that, if contentious proceedings were to ensue, the Court might have to address objections to admissibility on the basis that proceedings could involve the determination of obligations of third parties not party to the proceedings. The extent to which the Monetary Gold principle—that the Court cannot decide a dispute which would involve the determination of a third party’s legal interestsFootnote ¹⁹⁴—could pose an impediment might depend on the legal issues before the adjudicators.

That leaves the problem of causation. Reparation can only follow in relation to damage that has been caused by a State or States’ wrongful conduct. Causation has often been a thorny issue in relation to climate change. What, then, does the Advisory Opinion have to say about causation? The Court reiterated the established international law test developed in its own jurisprudence which requires ‘a sufficiently direct and certain causal nexus’ between the wrongful act and the injury.Footnote ¹⁹⁵ In the context of climate change, this has two elements: first, scientific evidence establishing that harm to the environment has been caused by anthropogenic GHG emissions and, second, the attribution to a particular State or group of States of the relevant conduct. The Advisory Opinion can be read as saying that scientific evidence before the Court considered that the first element of causation can be established by clearly linking observed increases in heat waves, flooding and drought to anthropogenic GHGs (although only at a general level and only in respect of the increases in these phenomena).Footnote ¹⁹⁶ Additionally, in respect of the second element, the Court highlighted that it is possible to determine scientifically the amount of emissions that each State has contributed to the atmosphere (although not all of these emissions will be unlawful).Footnote ¹⁹⁷ Thus, there is scope for a contentious case arguing that certain States’ conduct has caused increases in destructive weather. On the Court’s reasoning, it seems that causation in a general sense could be established. The physical consequences are, at least in part, a result of the conduct. There is thus a ‘core’ causative connection. As the Court stated:

Finally the Court’s remarks on compensation must be considered. The Court commented, citing past jurisprudence, that ‘the absence of adequate evidence as to the extent of material damage will not, in all situations, preclude an award of compensation for that damage’.Footnote ¹⁹⁹ Combined with what the Court had to say about causation, it appears that even if it is unclear how much of the material damage a State may have suffered is due to another State or States’ GHG emissions, and what the extent of that damage is, compensation may be possible, provided that there is a core causative link. It seems significant that, subsequently, when discussing reparation, the Court also highlighted that compensation in the form of what it has referred to as a ‘global sum’ may be possible, on an exceptional basis, where the exact extent of damage caused is uncertain, which the Court expected it would be in a climate change context.Footnote ²⁰⁰ This occurred in Armed Activities on the Territory of the Congo (Democratic Republic of the Congo v Uganda), where the Democratic Republic of the Congo had sought some USD 13 billion in reparations with the Court ordering reparations of USD 325 million, i.e. about 2.5 per cent of the amount sought.Footnote ²⁰¹ If only ‘core’ causality in the sense described can be established, then perhaps only the award of a global sum can potentially be contemplated.

3.3. Legal consequences

The Court was clear that State responsibility may ensue in the case of wrongful conduct.Footnote ²⁰² State conduct precipitating State responsibility includes all wrongful acts. Concrete examples of wrongful acts under international law could include: the failure of a State in various ways to take appropriate action to protect the climate from GHG emissions, including through failure to regulate GHG emissions in accordance with the customary international law obligation of due diligence in the prevention of environmental harm; fossil fuel production; fossil fuel consumption; the granting of exploration licences; or the provision of fossil fuel subsidies.Footnote ²⁰³ Wrongful acts might also, for instance, take the form of parties’ failures to communicate and maintain NDCs under the Paris Agreement (around 170 parties to the Paris Agreement did not meet the February 2025 deadline for successive NDCs);Footnote ²⁰⁴ or failure to conduct environmental impact assessments.Footnote ²⁰⁵

The Court also made clear that the full panoply of legal consequences found in the law of State responsibility would be applicable,Footnote ²⁰⁶ in addition to the continued duty to perform relevant obligations.Footnote ²⁰⁷ These consequences would include the duties of cessation and non-repetition, with the additional duty to make reparation, assuming that causation of the damage suffered by the injured State, or suffered by individuals if there were a breach of international human rights law, was established.Footnote ²⁰⁸

A State has continuing primary duties to perform its obligations, despite having breached these obligations. Given the range of obligations concerned, many examples could be provided, but this might include, for instance, the continuing duty to preserve and improve reservoirs’ and sinks’ absorption capacity. Equally, a State that has set an NDC which is inadequate under the Paris Agreement would have a continuing duty to set an adequate NDC.Footnote ²⁰⁹ The duty of cessation will also apply. The Court specified that in the climate context, cessation might require a State that is not already doing so to employ all means at its disposal to reduce its GHG emissions.Footnote ²¹⁰ It also might require a State ‘to revoke administrative, legislative and other measures that constitute an internationally wrongful act’.Footnote ²¹¹ For instance, a State could potentially be legally obliged to cancel oil and gas licences. Judge Bhandari suggested the Court could have been more specific that cessation in general requires discontinuing practices directly contributing to GHG emissions and adopting policies to facilitate deep and immediate emissions cuts.Footnote ²¹² Fulfilling such duties of cessation would naturally equally involve fulfilling a State’s continuing primary duties to perform its obligations.

Beyond the duty of cessation, the general law on State responsibility holds that States can be required to make reparation in three combined forms: satisfaction; restitution; and compensation. Satisfaction could take the form of expressions of regret, formal apologies, public acknowledgement or statements, education of society about climate change or a formal judicial declaration.Footnote ²¹³ Judge Bhandari’s Separate Opinion suggested also recognition of States and communities as victims of climate change, commemorations and tributes to the affected as well as a trust fund to manage funds for beneficiaries and symbolic monetary awards for non-pecuniary damage.Footnote ²¹⁴

Restitution might not be feasible for environmental harm, although the restoration of ecosystems and reconstruction of damaged infrastructure could be called for.Footnote ²¹⁵ Judge Bhandari would have liked the Court to refer more specifically to reparation measures aimed at protecting, preserving and enhancing the absorption capacity of GHG reservoirs and sinks as well as, where feasible, returning lost territory or property.Footnote ²¹⁶ Judge Sebutinde promoted reference also to reforestation, prevention of coastal erosion, infrastructural rebuilding and disaster or debt relief, as plead by certain countries including from the African continent.Footnote ²¹⁷ Most importantly for certain States, Judge Aurescu took the view that restitution could embrace the continued recognition of the entitlement of States affected by sea-level rise to their prior maritime territories as well as of their continued statehood.Footnote ²¹⁸

Compensation would correspond as per the law on State responsibility to financially assessable damage not made good by restitution, and the Court has previously confirmed that compensation can fall due for environmental damage.Footnote ²¹⁹ This might include damage to the environment in and of itself, which might involve indemnification for impairment or loss of environmental goods and services, and also damage in the form of expenses incurred by injured States as a consequence of such damage.Footnote ²²⁰ Given the high number of potential claimants, Judge Bhandari made suggestions about institutional frameworks for delivery in the form of international claims commissions or a UN-administered compensation fund.Footnote ²²¹ There is no reference in the Advisory Opinion to the matter of whether full reparation is affordable.

3.4. Conclusion on State responsibility

Hot on the heels of the Advisory Opinion came the BBC News headline ‘Top UN Court Says Countries Can Sue Each Other over Climate Change’.Footnote ²²² Indeed, it is thought by many that the Advisory Opinion is likely to form part of a wider litigation strategy giving rise to contentious proceedings. However, even if there may be a State or States interested in undertaking contentious litigation internationally, a close reading of the Advisory Opinion implies that expectations should perhaps be calibrated down.Footnote ²²³ A particularly prudent reading might perhaps expect a declaratory remedy as satisfaction in respect of damage caused by the breach of obligations and/or potentially an inexact global sum award as reparation.

4.1. Cooperation and climate finance

As the ICJ has accurately perceived, international cooperation is essential in relation to climate change. The Advisory Opinion has made a major contribution in highlighting this, with the theme of the duty to cooperate running right through the decision. Momentum has been building toward greater international recognition of international obligations of cooperation, including in the ILC, and the recognition of the customary international law duty on States to cooperate with each other in good faith to prevent significant harm to the climate system and other parts of the environment is a breakthrough moment.Footnote ²²⁴ Ultimately, as the Court said: ‘Climate change is a common concern. Co-operation is not a matter of choice for States but a pressing need and a legal obligation.’Footnote ²²⁵

The Court also made further important findings under treaty law in relation to international cooperation.Footnote ²²⁶ The climate change treaties address a global challenge, and States’ cooperation is a ‘paramount principle’ in solving global problems:Footnote ²²⁷ ‘it is by cooperating that the States concerned can jointly manage the risks of damage to the environment’, just as in a bilateral setting.Footnote ²²⁸ This includes cooperation in relation to reducing GHG emissions and mitigating the risk of climate change, adaptation, and also through financial assistance, technology transfer and capacity building. In addition to those in the Paris Agreement, there are a wide range of obligations relating to cooperation under the UNFCCC,Footnote ²²⁹ and the climate change treaties and customary international law are mutually informing when it comes to the duty to cooperate.Footnote ²³⁰ Meaningful cooperation may require the sharing of information, notification and consultation.Footnote ²³¹ Additional cooperative duties feature in the CBD,Footnote ²³² and the UNCCDFootnote ²³³ as well as in UNCLOS.Footnote ²³⁴ Further, when it comes to sea-level rise, the Advisory Opinion places a central emphasis on cooperation with a view to achieving equitable solutions.Footnote ²³⁵

The Court addressed climate finance, too, under the rubric of cooperation. Indeed, often the Advisory Opinion talks about ‘support’ and ‘cooperation’ rather than finance flows. By doing this, the Court was placing the accent on the aim and the obligation rather than monetary matters, perhaps making the discussion about climate finance more palatable. The topic was addressed first as part of the Court’s analysis of the requirements of the UNFCCC. Here, the Court observed that: ‘a case-by-case determination of the adequacy of current financial aid and technology transfer commitments is to be made by the application of the principle of good faith, which governs the duty of cooperation’.Footnote ²³⁶ This does imply that a State could be judged for its efforts in respect of finance, although the yardstick of good faith would perhaps only reveal conduct that was manifestly insufficient.

In general, the Court handled the question of climate finance adroitly although unemphatically. This is perhaps because the sums called for are vast, although the efforts the world must go to will be sorely needed. In international climate negotiations there is a distinction between provision of finance to assist developing countries with mitigation and adaptation, and finance for the loss and damage caused by climate change. Both have been challenging subjects of negotiation in the climate change treaty context. Slow progress on the latter especially was among the motivations for the campaign to request the Advisory Opinion. Progress has since been achieved at the COP to the UNFCCC, although there is a long way to go. This progress includes the agreement at COP27 in Sharm el-Sheik, Egypt, in 2022 to establish a fund on finance for loss and damage as part of a mosaic of funding arrangements and its formal operationalisation the following year.Footnote ²³⁷ As to finance to assist developing countries with mitigation and adaptation, the New Collective Quantified Goal on Climate Finance adopted at COP29 in Baku, Azerbaijan, in 2024 is to comprise at least USD 300 billion per annum by 2035 from public and private sources, with up to USD 1.3 trillion coming primarily from private financing.Footnote ²³⁸

Finance features centrally in the analysis of mitigation obligations under the Paris Agreement, where the Court placed it squarely within the rubric of cooperation, alongside capacity building and technology transfer.Footnote ²³⁹ For developed countries, there is not only a binding obligation of financial assistance,Footnote ²⁴⁰ but also binding obligations to communicate in advance the projected finance, under Article 9.Footnote ²⁴¹ The Court made the key point that the Paris Agreement does not specify the amount or level of financial support to be provided, but the relevant provisions are to be interpreted in light of other provisions, including the Article 2 temperature goal: ‘parties are to implement their obligations under Article 9 in a manner and at a level that allows for the achievement of the objectives listed in Article 2’.Footnote ²⁴² This level of contribution can be evaluated on the basis of factors including developed States’ respective capacity and developing States’ respective needs. There are also important obligations to regularly communicate action taken, under Article 11(4), and to enhance capacity building activities through appropriate institutional arrangements, under Article 11(5).

These important observations from the Court about finance flows must be distinguished from later sections of the Advisory Opinion addressing the subject of the legal consequences where States’ conduct has caused significant harm to the climate system, including reparation. Cooperation’s overall importance is reflected in the general structure of the Advisory Opinion, addressing, in turn, mitigation, adaptation and ‘cooperation’.

4.2. Sea-level rise and self-determination

The Court dealt with sea-level rise in an attentive way, although the care it took not to overcommit itself may lead to complaints of reticence. Sea-level rise is an especially sensitive issue for small island States, particularly those who would lose maritime territory if the baselines of maritime zones recognised in the law of the sea were to shift landward and to undergo alteration in other respects. This is likely, given the normal baseline is the low tide mark and taking into account also the special archipelagic baselines regime. For certain States, the further possibility of losing the entirety of their land territory to the rising oceans by the end of the century is the spectre. Questions then arise about ongoing Statehood.Footnote ²⁴³ This is the most obvious of the potentially existential threats dealt with in the Advisory Opinion.

The Court validated such concerns, observing that the IPCC has described sea-level rise as unavoidable and has concluded with a high level of confidence that associated risks will continue to increase.Footnote ²⁴⁴ This may potentially lead to forced population displacement within or across borders and affect States’ territorial integrity and permanent sovereignty over their natural resources.Footnote ²⁴⁵ In respect of baselines, the Court made the specific finding that the provisions of UNCLOS ‘do not require States parties, in the context of physical changes resulting from climate-induced related sea level rise’, to update charts and coordinates showing either their maritime zone baselines or the outer limits of the maritime zones once established in accordance with UNCLOS.Footnote ²⁴⁶

This is significant as a win for small island States, but it is a somewhat minimalist finding. The Court did not state, for instance, that baselines are fixed as a matter of general international law, whereas Judge Aurescu calls for the fixed baseline solution to be recognised as established in customary international law.Footnote ²⁴⁷ The Court’s interpretive reasoning appeared to be based on the text of UNCLOS. The Court identified as relevant the views that many States and the ILC expressed in support of this interpretation, and customary international law arguments, but did not appear, interpretively, to rely greatly on either.Footnote ²⁴⁸ Time may be required here for customary international law to develop.

In respect of statehood, the Court was again minimalist, although importantly expressing the view that once a State is established, ‘the disappearance of one of its constituent elements would not necessarily entail the loss of its Statehood’.Footnote ²⁴⁹ This was a significant statement because it confirmed speculation that there may be room to accommodate the possibility that fulfilling the traditional criteria for statehood in the Montevideo Convention on the Rights and Duties of States on a continuing basis may not be essential to statehood in all cases (a defined territory, permanent population, government and the capacity to enter into relations with other States). Judge Tomka considered this remark deeply troubling, due to the possibility that it might give rise to readings of the Advisory Opinion as endorsing the deconstruction of statehood.Footnote ²⁵⁰

The Court also remarked that the potential effects of sea-level rise on the international legal principles of territorial integrity and permanent sovereignty over natural resources are closely connected with the right to self-determination and held that ‘sea level rise is not without consequences for the exercise of this right’.Footnote ²⁵¹ This is a remark that could herald later legal arguments and developments. The Court was making the point that States’ GHG emissions might undermine the right to self-determination of the peoples of other States, a fundamental principle of international law.Footnote ²⁵² The right to self-determination is both jus cogens and an obligation erga omnes, and this potential but preventable result would appear to be a serious breach of the right.

It seems that the scheme of responsibility in ARSIWA indicates that States would be obliged not to recognise the change in legal situation that resulted, for instance, from the disappearance of a State and its land territory.Footnote ²⁵³ This would be in addition to any secondary legal obligations to cooperate to bring such a serious breach to an end. It may be argued that these secondary legal obligations already apply today, if it can be scientifically predicted with relative certainty, that present-time GHG emitting conduct constitutes a breach that will undermine the right of self-determination in the future. Little consideration appears to have been given to this point.Footnote ²⁵⁴ These cooperative obligations would supplement those already recognised in primary law. In any event, the Court emphasised that States are obliged to cooperate in dealing with climate change-induced sea-level rise, a duty lying at the core of the UN Charter.Footnote ²⁵⁵

4.3. What the Court left for later

Throughout the Advisory Opinion, the Court discerned matters on which it might and might not be appropriate to give views. This applies both in relation to the primary law and also the Court’s response to the second part of the UNGA’s question relating to legal consequences. In many cases, the Court has approached the drawing of this line by noting that certain matters would call for assessment in concreto. This is a positive way of phrasing the matter; an alternative would have been to convey that the Court was unable to address certain matters in abstracto. The Court opted for a forward-looking tone, which assists in maintaining the Advisory Opinion’s affirmative energy. Another example of this approach is seen in relation to the setting of NDCs under the Paris Agreement, where the Court explained that the question of how the principle of CBDR-RC in light of national circumstances plays into the due diligence required of each State would need to be assessed in concreto.Footnote ²⁵⁶

The Court’s line-drawing was particularly apparent in relation to the requirements of customary international law. Matters that would call for assessment in concreto here include, centrally, the requirements of due diligence in a given case,Footnote ²⁵⁷ as well as whether any specific risk of harm or actual harm would constitute a relevant adverse effect of climate change,Footnote ²⁵⁸ and also the relevance of decisions of the COP as current standards for determining risk when assessing the due diligence required in a given case.Footnote ²⁵⁹ There are additionally other areas of the primary law where the Court took the view that the matter would need to be argued and determined based on specifics. This is the case, for instance, for the continuity of Statehood, which the Court left to be argued in caso.Footnote ²⁶⁰ In relation to the territorial scope of human rights treaties, the Court took the view that it need not determine the specific circumstances under which a State can be regarded as exercising its jurisdiction outside its own territory, since this will depend on the provisions of the particular treaty in question.Footnote ²⁶¹

The Court also circumscribed the scope of what the Advisory Opinion can contribute when it comes to legal consequences, where the Court focused essentially on legal consequences under the law on State responsibility. First, the Court addressed the remaining primary law matter of the temporal scope of States’ obligations. The Court was clear that the assessment of the temporal scope of States’ obligations would need to be considered in concreto.Footnote ²⁶² This issue could be crucial in contentious litigation and this was a matter that was fiercely argued in pleadings.Footnote ²⁶³ Perhaps the most distinct position was taken by Judge Nolte: that it was in the second half of 1980s that general understanding and recognition of risks associated with emissions came about, and so this is when customary international law became applicable and wrongful conduct could have begun to occur.Footnote ²⁶⁴

Based on the tenor of the Advisory Opinion, one might perhaps predict that in the future a point in history may indeed be identified when States had or should have had sufficient knowledge of the harm that atmospheric GHG concentrations cause to bring into play the customary international law obligations of prevention and cooperation. One interesting question is whether both duties would run from the same point in time. Claims in relation to these two distinct duties might have different aims. Findings around the point in time at which each could be considered to have been triggered might be needed more in relation to the preventive duty, where financial remedies of some description may be more likely.

The Court then came to matters of secondary law proper, considering that in concreto assessment would be needed in relation to the responsibility of individual States or groups of States.Footnote ²⁶⁵ As the Court reiterated several times, that is not a task the Court was asked to undertake and the determination of State responsibility was beyond the scope of the Advisory Opinion.Footnote ²⁶⁶ Among other matters, causality would need to be determined.Footnote ²⁶⁷ The Court was also not asked to indicate or quantify specified legal consequences in the sense of remedies.Footnote ²⁶⁸

The Court stated that the rules and principles governing injuries arising out of acts not prohibited by international law fell outside the scope of the questions before it, thus excluding consideration of reparation for harm caused by conduct that is not internationally legally wrongful.Footnote ²⁶⁹ On the other hand, Judge Yusuf noted that, first, the regime of State responsibility for wrongful acts and, second, the international legal principles or rules that may apply in relation to allocation of loss caused by conduct that is not wrongful, are intertwined.Footnote ²⁷⁰ Importantly, in the climate change context, if it is assumed that the temporal scope of obligations on which State responsibility for harm caused by climate change may one day be specified, emissions prior to that could be caught by the regime on allocation of loss. This regime could perhaps be considered still under development, although it has received sustained attention in the past from the ILC, leading most recently to the Principles on the Allocation of Loss.Footnote ²⁷¹ Though valuable, these principles may not prove to be the end of the story. The international experience with climate change in the coming years might be a crucible for further development of the law in this respect.

4.4. The role of science in the Advisory Opinion

The Advisory Opinion is based on a smooth synthesis of the science and the law, as far as the engagement with science goes.Footnote ²⁷² This engagement did not extend, for instance, to ascertaining scientific parameters for possible pathways towards the Paris Agreement’s 1.5ºC temperature goal, including, for instance, pathways without overshoot, with overshoot, without atmospheric carbon dioxide removal and with such removal. That said, the Advisory Opinion brings the science it relies on into clear focus from the outset, drawing primarily on the work of the IPCC as the best available science.Footnote ²⁷³ The Court’s pre-hearing meeting with groups of past and present authors from the IPCC, intended to enhance the Court’s understanding of the IPCC’s key scientific findings, is likely to have bolstered the Court’s confidence with the Advisory Opinion’s scientific content.

The Court observed, based on very high-confidence figures provided by the IPCC, that concentrations of the three main GHGs (carbon dioxide, methane and nitrous oxide) are higher now than they have been in at least 800,000 years.Footnote ²⁷⁴ The Court left no doubt that ‘it is scientifically established that the increase in concentration of GHGs in the atmosphere is primarily due to human activities’, whether as a result of emissions or the weakening of carbon reservoirs and sinks including forests.Footnote ²⁷⁵ Further, the consequences are severe and far-reaching, affecting natural ecosystems as well as human populations.Footnote ²⁷⁶ They include rising temperatures, melting ice sheets, sea-level rise and unprecedented flooding for coastal communities, as well as the heightened frequency and intensity of extreme weather events, such as hurricanes, drought and heatwaves.Footnote ²⁷⁷ Impacts include devastation of agriculture, displacement of populations, exacerbation of water shortages and the disruption of natural habitats pushing certain species toward extinction, leading to irreversible biodiversity loss.Footnote ²⁷⁸ Importantly, too, the Court drew on the key understanding that the best available science is clear that the ‘[r]isks and projected adverse impacts and related losses and damages from climate change escalate with every increment of global warming’, as the IPCC had found, again with very high confidence.Footnote ²⁷⁹

In addition, the Advisory Opinion embraces the precautionary principle with thoroughness and care, providing valuable guidance on how precaution informs both treaty law and customary international law. Both science and the precautionary principle directly infuse the Court’s analysis under the Paris Agreement, customary international law and other applicable treaties. For instance, when reading in the requirement for due diligence in States’ fulfilment of their Paris Agreement obligations on the setting of NDCs and pursuit of domestic mitigation measures, the Court recalled that the due diligence standard varies according to the circumstances, including the level of scientific knowledge, the risk of harm and the urgency involved.Footnote ²⁸⁰

The role of science is also thoroughly interwoven with the Court’s analysis of the customary international law duty to prevent harm to the environment. The ‘best available science’ standard applies here again when determining the risk of significant harm,Footnote ²⁸¹ and the state of scientific and technological information is one of the elements going to the standard of due diligence attaching to the harm prevention duty.Footnote ²⁸² The science on both the probability and the seriousness of possible harm informs the required due diligence standard.Footnote ²⁸³ The Court was unequivocal in the view that, where there is scientific uncertainty, the precautionary principle also applies: ‘States should also not refrain from or delay taking actions of prevention in the face of scientific uncertainty’.Footnote ²⁸⁴

Indeed, States need to pursue knowledge sharing and cooperation initiatives due to the status of climate change as a matter of common concern and its scientific complexity.Footnote ²⁸⁵ Importantly, they must also share information to help minimise the possibility that a measure taken for purposes of adaptation or mitigation might in itself pose a significant risk of transboundary harm.Footnote ²⁸⁶ The Court went on to say that States are expected to act with ‘prudence and caution’ in relation to technologies for preventing or mitigating harm that pose further risks.Footnote ²⁸⁷ The Court was warning here against isolated unilateral action. This could include solar radiation management activities as well as terrestrial or marine sequestration of carbon. Depending on the situation, information sharing will be required where this may affect other States.Footnote ²⁸⁸