From Harmonization to Coexistence

Much has already been said about the International Court of Justice’s (ICJ)  Advisory Opinion (AO) on the Obligations of States in Respect of Climate Change which was delivered in July 2025 upon request of the United Nations General Assembly, particularly with regards to systemic integration and harmonious interpretation: harmonious interpretation towards ‘coherence and integrity’ and the Courts turn towards ‘a cohesive legal order through systemic integration’ have been identified, the ‘central role’ of systemic integration in the AO has been highlighted.

According to the ICJ States have to protect the climate and other parts of nature from anthropogenic greenhouse gas emissions. The related obligations do not only arise from the United Nations Framework Convention on Climate Change, the Kyoto Protocol and the Paris Agreement (subsequently: the climate treaties), but equally from, inter alia, the Charter of the United Nations, the United Nations Convention on the Law of the Sea, the Convention on Biological Diversity and the United Nations Convention to Combat Desertification, customary rules and international human rights law (para. 172 and operative part of the AO). The ICJ does not consider the climate framework as the primary source of climate obligations, but as part of a kaleidoscope of international legal frameworks and rules which provide for climate related obligations. The fact that the ICJ derives obligations from multiple instruments and rules indicates possibly wide implications of the AO for future rulings. It also raises more general methodological questions with regards to the application and interpretation of a multiplicity of norms, rules and instruments on the same subject-matter.

The contribution argues, that it is rather the principle of systemic integration, not harmonious interpretation which is applied in the Courts reasoning – a distinction which is rather subtle, as the principles seemingly are invoked interchangeably in the AO. It stresses, that systemic integration is applied as an interpretive means towards parallel application of norms and rules, which do neither create inconsistency nor conflict. Only the latter would require harmonious interpretation.

Two subsections of the AO are particularly relevant to look at in that regard: the subsection in which the ICJ ‘identif[ies] those rules which are most directly relevant’ for question (a) of the request (AO para. 114) and the subsection where it determines if ‘any of the rules identified are excluded by lex specialis’ (AO para. 114).

For that matter, a clarification should be made: The questions addressed by applying the principle of systemic integration, which is a principle of interpretation, are matters of applicability. Applicability is governed by Article 38 (1) ICJ Statute, the principle of systemic integration has found expression in Article 31 (3) (3) VCLT – as a rule of interpretation of the treaties already identified as applicable. Yet, ‘[i]t is only through a process of interpretation that it is possible to determine whether the two rules are incompatible, such that a rule of priority has to be applied’ (see here, para. 3.03; see also Declaration of Judge Nolte, para. 9). As the ILC Fragmentation Report puts it: ‘Interpretation does not intervene only once it has already been ascertained that there is a conflict. Rules appear to be compatible or in conflict as a result of interpretation.’ (Fragmentation Report, para. 412).

No Inconsistency

The Court emphasizes, that it ‘cannot find any actual inconsistency between the provisions of the climate change treaties and other rules and principles of international law’ (para. 168) relevant for the question (a) at hand. It further stresses, that ‘[o]n the contrary’ the preambles of the climate treaties refer to other rules and principles of international law which would indicate that the State parties considered those rules and principles as relevant in the context of climate change (AO para. 168). Neither would it be object and purpose of the climate change treaties to ‘displace’ or ‘contradict’ other rules and principles of international law (AO para. 169), nor would rules on the same subject-matter ‘necessarily constitute a conflict or result in one superseding the other’ (AO para. 189). The Court considers rules from different regimes as the ‘most directly relevant applicable law’ (AO para. 172). The Court states that the lex specialis rule does not apply to the relationship of the climate treaties with other rules (AO para. 171). It thereby dismisses the argument of some participants that the climate treaties form the framework solely applicable with regards to climate obligations (AO paras. 163-164, why this argument seemed attractive to major emitting States, see here). The rules from the climate treaties therefore are not only ‘flanked’ by customary norms and other rules. Rather, they equally apply on the same subject-matter. The Court thus engages in interpretation in order to discern the applicable law.

This further becomes apparent in the ICJ’s considerations on the principle of lex specialis (for an earlier reflection on  the role of the lex specialis principle, depicted as ‘the main bone of contention’, look here): The ICJ points out that ‘lex specialis is a maxim of interpretation that is used for determining which of several potentially applicable rules is to prevail or whether the rules simply coexist’ (para. 166). The principle’s application would not only depend ‘on the circumstances of each case’ (para. 166). It would also require ‘some actual inconsistency’ between the provisions dealing with the same subject matter or ‘a discernible intention that one provision is to exclude the other’ (para. 167, by citing the commentary on the ILC Draft Articles on State Responsibility).

No Normative Conflict

While considering the question of lex specialis (with regards to question (a) of the request), the Court further refers to the principle of harmonization. It states, ‘that 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’ (para. 165, also see para. 310 on the relationship between obligations arising from treaties and customary international law). It does so by citing conclusion 4 of the Conclusions of the work of the ILC Study Group on the Fragmentation of International Law (ILC conclusions). The citation as such might indicate that the Court ‘went far […] in terms of systemic integration’. A glance into the ILC conclusions reveals that conclusion 4 does not address systemic integration, but the principle of harmonization as a means to resolve normative conflict. The principles sometimes are invoked interchangeably (see Fragmentation Report, D.), but in fact are distinct (comp. ILC conclusions, conclusion 43). Conclusion 4 forms part of the general conclusions and has the heading ‘The principle of harmonization’. This should be read in the light of conclusion 43, which addresses the principle of harmonization by reference to conclusion 4. It reads: ‘[…], conflicts between rules of international law should be resolved in accordance with the principle of harmonization, that is, by bearing in mind that, in the event of a conflict, the norms should be interpreted as compatible to the extent possible.’ The ILC conclusions hence consider the principle of harmonization primarily relevant in the case of conflict. Yet, the ICJ does neither see a norm conflict between the three climate treaties, which ‘complement each other’ (AO para. 120), nor between the climate treaties and other relevant rules of international law (AO para. 168). Since there are no norm conflicts to solve by means of interpretation, the principle of harmonization does not need to be applied. Rather, the ICJ is required to apply the rules as ‘a single set’ of coexisting norms relating to an issue, as the Court reiterates – which, in fact, seems to be a matter of systemic integration. The ICJ therefore moves from harmonization to coexistence (compare AO, para. 166) by considering a multiplicity of norms and rules applicable on the same subject-matter. Lex specialis and systemic integration are applied as primary interpretive principles, going hand in hand relating to questions of applicability.

Conclusion

The ICJ’s approach might pave the way towards a more inclusive and cross-sectoral application of international law. However, the Court does neither harmonize, nor fully integrate the respective conflicting rules and instruments, but considers them applicable in parallel on the same subject-matter: they coexist, and inform each other, but do neither create inconsistency, nor overlap. This resonates with what the ILC has called ‘The doctrine of “treaty parallelism”’, which ‘addresses precisely the need to coordinate the reading of particular instruments or to see them in a “mutually supportive” light.’ (ILC Fragmentation Report, para. 417). These methodological aspects might highlight the AO’s relevance for the ‘structural transformation of international law’  towards a more coherent international legal order. Although the AO justifiably has been criticized for its ‘excessively formalistic approach’ (so by Judge Yusuf in his Separate Opinion, para. 2, which has been picked up here) and its lack to address the ‘structural and historical causes of harm’, it might be still of relevance on the very formal and methodological level – yet: parallelism might not yet be integration.