A key takeaway of the International Court of Justice’s (ICJ or Court) 23 July 2025 advisory opinion on the Obligations of States in Respect of Climate Change is that the climate treaties do not displace other rules and principles of international law in responding to climate change (para 170), and that the ‘entire corpus of international law’ is potentially relevant (para 98). In that interpretive light, the Court came to the view that a State’s failure to take appropriate measures to protect the climate system from greenhouse gases (GHGs) applies in respect of production of fossil fuels, not just consumption (para 427). Without the full corpus of international law being potentially relevant, a much narrower answer may have been given.
Issues of applicable law and related interpretative issues of whether the Paris Agreement was lex specialis or to be interpreted in accordance with the principle of systemic integration were front and central both in the deliberate design of the Question asked of the ICJ by the UN General Assembly and in States’ oral submissions (see e.g. the US and Saudi Arabia).
The reason an answer on integration is useful is that it unlocks a raft of applicable law in respect of climate change. In the following, I will focus on the Court’s affirmation of systemic integration as an interpretative technique and its use of cross-citation as a form of institutional integration. In taking an integrative approach, the Court has made international law more coherent.
The Question
The Question asked about: (A) obligations of States under international law to ensure protection of the climate system from anthropogenic emissions of GHGs; and (B) legal consequences under those obligations where States have caused significant harm to the climate system.
The chapeau to the Question referred the Court to have ‘particular regard’ to a specific list of sources: the UN Charter, the ICCPR and ICESCR human rights treaties, the UNFCCC, the Paris Agreement, and UNCLOS as well as a number of customary principles including due diligence, prevention, and the duty to protect and preserve the marine environment.
The Court’s decision on applicable law
Interpretation of the question
The framing of the chapeau did not bind the Court as to applicable law. Nevertheless, the Question’s chapeau was intended to guide the Court, and did in fact have that function (para 99). The Court interpreted the Question as broadly as was intended by the group of co-sponsoring States. In the Court’s words, the ‘unqualified reference to obligations “under international law” indicates the intention of the General Assembly to seek the Court’s opinion … [on] the entire corpus of international law” and, critically, ‘not to limit the Court’s reply to any particular source or area of international law’ (para 98). The Court interpreted the phrase ‘particular regard to’ as a request to identify ‘the most directly relevant applicable law’ (para 114).
Applicable law: Directly relevant and guiding principles
The Court determined there to be two categories of applicable law. This categorisation exercise illustrates the range of areas of law that were applied by the Court.
The Court classified some rules as ‘directly relevant’ (paras 115-140) and others as ‘guiding principles for the interpretation and application of the most directly relevant rules’ (paras 141-161). By and large, the rules that the Court considered to be directly relevant are the sources set out in the Question’s chapeau—climate/environmental, human rights and law of the sea. In addition, the Court added other environmental treaties (paras 125-129) and the customary duty to cooperate to protect the environment (paras 140-142) to its category of directly relevant laws. The Court also built on other normative notions in the Paris Agreement, such as the Article 4.1 requirement that mitigation be based on the ‘best available science’ (paras 74, 224). In its category of guiding principles, the Court included sustainable development, common but differentiated responsibilities, equity (including intergenerational equity), and the precautionary approach.
This goes beyond what the minimum that the Court could have found. Last year, Rajamani characterised some of these guiding principles as the ‘normative environment’ of the Paris Agreement but potentially falling short of bindingness. The ICJ’s opinion affirms that all of those listed are binding, and affirmed the substantive content of the phrase ‘best available science’. In doing so, it did exactly what Rajamani had argued: the Court ‘constrain[ed] unbridled subjective [NDCs]’ (for commentary on the Court’s clarification of mitigation obligations, see Voigt).
Determining there to be two categories of applicable law begs the question: how do these laws interact? I offer an answer to the meta-question: what rules govern how these rules interact?
Rejecting the lex specialis argument
Whether the climate treaties constitute lex specialis was contested by States; the majority suggested they do not (para 163). The lex specialis principle requires there to be an actual inconsistency between two provisions, but the Court was unable to find any actual inconsistency (para 167-168). Moreover, States were aware of the ‘normative context’ of the climate treaties and did not choose to displace other rules when making negotiating the climate rules (para 170). In rejecting the lex specialis argument, the Court paved the way for accepting one that would radically shift the scope of applicable law: systemic integration.
Systemic integration in climate litigation
Systemic integration is a principle of interpretation under Article 31(3)(c) of the Vienna Convention on the Law of Treaties: treaty interpreters (such as the ICJ) must take account of ‘any relevant rules of international law applicable in the relations between the parties’. Often heralded as a solution to fragmentation, the principle of systemic integration requires judges to ‘mak[e] sure that the outcome is linked to the legal environment’. The foundational idea is that assorted rules of international law ‘take their meaning from their connections to each other as a set of interlocking norms that are part of a larger system’ (McLachlan).
The Court affirmed the ‘generally recognised 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’ (165, citing the ILC).
Applied, the Court took the view that ‘international human rights law, the climate change treaties and other relevant environmental treaties, as well as relevant obligations under customary international law, inform each other’ (para 404, emphasis added). In other words, States must take their obligations under each area of law into account when implementing their obligations in other areas of law.
Implications
In doing so, the ICJ went far further in terms of systemic integration than the ITLOS advisory opinion on climate change (see e.g. ITLOS’ cursory reference to human rights law, para 66). Some criticised ITLOS’s opinion as a ‘missed opportunity’ to fully integrate these areas of applicable law. We might have expected ITLOS to be more reticent, given its core mandate is to adjudicate disputes arising from UNCLOS (but see Article 293 UNCLOS, which empowers ITLOS to apply ‘this Convention and other rules of international law not incompatible’). Nevertheless, the ICJ has truly picked up the baton, and taken the opportunity to synthesise all three areas: climate/environmental law, law of the sea, and human rights, as well as international humanitarian law and international investment law (Declaration Cleveland).
Failure to affirm a systemic integrative interpretation of the applicable law would have had negative consequences. As Voigt points out, such a position could have resulted in the Court establishing alternative obligations in respect of climate change action, further fragmenting the corpus of international law and potentially undermining the relevance of those parts of the international climate treaty regime that are legally binding, as well as potentially derailing political negotiations at the annual Conferences of Parties of the Paris Agreement. Instead, what we saw was a strengthening of the Paris Agreement obligations, buttressed by obligations from other areas of law, such as human rights law and law of the sea.
Arguably, the Court did not go far enough. Although the ICJ cited the Inter-American Court’s (IACtHR) advisory opinion published 3 July on climate change (para 385), the Court as a whole failed to engage with it. The ICJ’s opinion was published just twenty days later, and we can assume it was largely agreed (it was unanimous). Instead, we see only two judges writing separately mentioning it, again treating it relatively lightly (see Sep Op Charlesworth and Sep Op Aurescu). Given the uniqueness of the current situation of the four advisory opinions on climate change in a short period of time (ITLOS, IACtHR, ICJ and the African Court on Human and People’s Rights (still pending)), and that the ICJ is the only of those courts with general jurisdiction, the ICJ could and perhaps should have delayed its opinion by a short period to more fully address the IACtHR opinion in the same way it fulsomely addressed the ITLOS advisory opinion.
Institutional integration
In addition to systemic integration, another form of integration is in action: cross-citation or inter-judicial dialogue as institutional integration. Little attention has been paid to this in the climate literature (but see here), although some scholars have paid attention to it in the human rights law more generally (here) and it has been advocated as a response to fragmentation. See for example, Benvenisti and Downs have posited that cross-citation ‘can function as a kind of global good’, ‘potentially accelerating the evolution of a more coherent international legal system’; more recently, ILC Special Rapporteur Mr Jallo argues that reliance on other judicial decisions under Article 38(1)(d) of the ICJ Statute ‘provides stability’ and ‘may provide a solution to certain … negative consequences of … fragmentation’ (Third report on subsidiary means). Taking the argument a step further, Andenas and Leiss argue that Article 38(1)(d) of the ICJ Statute is a ‘logical corollary’ to the principle of substantive integration, obliging courts to take into account other relevant judicial decisions. Leaving aside whether they are right that this creates an obligation, I adopt their term for this phenomenon: ‘institutional integration’.
I suggest the ICJ’s advisory opinion reflects considerable institutional integration, citing other climate cases and arguably treating them as a ‘subsidiary source’ of law under Article 38(1)(d).
For example, the ICJ relied on ITLOS’ advisory opinion (paras 337-338). The ICJ endorsed ITLOS’ findings that many provisions of UNCLOS constitute obligations of States in respect of climate change. To take just one example, the finding that anthropogenic GHGs pollute the marine environment within the meaning of UNCLOS (ITLOS advisory opinion, paras 161-179, ICJ advisory opinion, para 340; and see others ICJ advisory opinion paras 340-353).
In respect of human rights jurisprudence, the ICJ took an even wider response, citing regional courts as well as the UN human rights treaty bodies (para 144). In particular, the ICJ also endorsed core jurisprudence of the UN human rights treaty bodies, such as the Human Rights Committee’s findings in Teitiota v New Zealand that environmental degradation can lead to a violation of the right to life and can give rise to non-refoulement obligations (ICJ advisory opinion, paras 377-378, citing Teititoa, paras 9.5, 9.11), and the findings in Billy v Australia that States’ ‘failure to implement timely and adequate adaptation measures to address the adverse impacts of climate change may violate the right to privacy, family and home’ (ICJ advisory opinion, paras 381, citing Billy, paras 8.9-8.12). The ICJ also cited, but did not explicitly endorse particular findings of, regional human rights-based climate litigation (para 385).
These findings in other international courts are part of a concerted effort of strategic litigation to use human rights law to fill the gap created by largely non-binding climate treaties. In its advisory opinion, the ICJ used them as a subsidiary source to interpret ‘directly relevant’ sources of law (such as UNCLOS and the human rights treaties).
Implications
Advisory opinions are of course non-binding. That does not mean they are without significance. Much of the anticipated significance reliance on the advisory opinion in other litigation, treating the advisory opinion as a persuasive legal authority that other courts can reference to support them making similar findings. The ICJ has itself taken and therefore given credence to such an approach. In other words, we expect to see not just more climate litigation, but litigation that tends towards institutional integration. This is likely in both other international and regional courts as well as in domestic courts. Such action aids in the evolution of legal norms, principles and standards and builds normative coherence.
Conclusion
As Feria Tinta suggests, there has never been a ‘greater need for coherence’ than in respect of responding to the climate crisis, for these issues are existential. Systemic and institutional integration are not a panacea. But use of these synthesising techniques does mean that the law is a more convincing tool to address the challenges we face. In the words of the ICJ, ‘the point here is to achieve the necessary clarity and the essential consistency of international law, as well as legal security’ (para 338, citing Diallo para 66). Now that we have a clear and consistent authoritative statement of international law about obligations in respect of climate change, international law can be more persuasively used as one of the guiding tools, alongside other ‘fields of human knowledge’ to ‘inform and guide social and political action to address the ongoing climate crisis’ (ICJ advisory opinion, para 456).
Rebecca McMenamin, LLM (Cantab), LLB/BA (Wellington), is a PhD Candidate at the University of Vienna, writing about human rights-based climate litigation and international law.