From ‘Nuclear Weapons’ to ‘Climate Change’: The ICJ’s Approach to the lex specialis Maxim

[Amir Abbas Kiani is a collaborating researcher in International Law at Shiraz University, Iran]

On July 23, 2025, the International Court of Justice (ICJ) issued its ‘historic’ Advisory Opinion on Obligations of States in respect of Climate Change. As part of its reasoning, the Court examined the issue of lex specialis derogat legi generali (lex specialis) to determine “…the relationship between the climate change treaties and other rules of international law” (para. 162). In particular, “[s]everal states had argued that the rules set out in the climate change treaties should generally take precedence over customary international law and/or other rules of international law on these grounds”.

The very first case to take lex specialis into account was Nuclear Weapons (1996). Likewise, the cases Construction of Wall (2004) and Armed Activities (2005) took lex specialis into consideration. But Climate Change is possibly the case in which the ICJ discussed lex specialis most comprehensively. Moreover, the abovementioned cases concerned only the relationship between international human rights law (IHRL) and international humanitarian law (IHL) during the conduct of hostilities, whereas the Court in Climate Change explored the issue in another context, namely climate change.

Generally, the lex specialis maxim is recognized to be a general principle of reasoning aimed at determining whether “special norms are normally better tailored for the regulation of an issue, and special institutions are normally better equipped to apply them” (p. 682). This maxim, alongside other interpretive principles such as lex posterior and lex superior, as reflected in Article 30 of the Vienna Convention on the Law of Treaties (VCLT), seeks to harmonize the fragmented system of international law.

Below, this blog post examines how the ICJ has conceptualized and applied the lex specialis maxim across its jurisprudence, tracing its evolution from Nuclear Weapons to Climate Change, to assess whether the Court has developed a coherent approach to managing the relationship between specialized and general norms of international law.

Lex specialis in Doubt: The ICJ Took an Implicit Approach

Paragraph 25 of Nuclear Weapons states that the protections of the International Covenant on Civil and Political Rights (ICCPR), including the right to life, continue to apply during war. However, in wartime, whether a killing is considered an ‘arbitrary deprivation of life’ is determined by the laws of armed conflict (lex specialis), not by the ICCPR alone. This paragraph has occasionally been perceived that IHL overrides IHRL during armed conflict. Particularly, this perception could find support as the “maxim, in its strict sense, and as is clear from the word ‘derogat’, implies the (partial or total) disapplication or displacement of the general law in favor of the special law” (p. 8).

Nevertheless, the function of lex specialis is not confined to displacing the general rule. As revealed in the International Law Commission (ILC)’s commentary on the fragmentation of international law, “[t]he application of special law does not normally extinguish the relevant general law. That general law will remain valid and applicable and will…continue to give direction for the interpretation and application of the relevant special law…” (p. 178) (emphasis added).

Further, in Construction of Wall, the ICJ shed light on its Opinion, stating that some rights “…may be matters of both these branches of international law…namely [IHRL] and, as lex specialis, [IHL]” (para. 106). It has been argued that, in this case, the ICJ has acknowledged that “IHL and [IHRL] can sometimes be integrated and applied concurrently” (p. 231, footnote 62).

The ICJ, however, did not mention lex specialis in Armed Activities and merely found Uganda responsible for numerous violations, including killings, torture, destruction of civilian property, use of child soldiers, and failure to protect civilians, which breached both IHL and IHRL (para. 345). Although the Court did not specify which acts violated which branch, it cited its Construction of Wall, confirming that both IHL and IHRL must be applied together (para. 216).

A cumulative reading of these three cases in relation to lex specialis leads us to a unified understanding: The ‘harmonious’ or ‘integrated’ approach concerning international law, which the Court articulated in its Climate Change advisory opinion.

Harmonious Approach: The ICJ’s Explicit Approach on Lex Specialis

The ICJ’s findings on lex specialis in Climate Change can be seen as a commendable effort to align the various branches and sources of international law. In this regard, some States argued that “…climate change treaties…constitute lex specialis in respect of other rules and principles…In their view…the climate change treaties have priority over any other rule of a conventional or customary nature and cannot be superseded or altered by other sources of law” (para. 163). For example, China argued that the “…UNFCCC, together with its Kyoto Protocol and Paris Agreement, constitute the basic legal régime of global climate governance. It is a lex specialis, tailor-made to address climate change” (pp. 29-30). Likewise, Saudi Arabia reasoned that “the specialized treaty régime on climate change provides a complete answer to the questions on obligations and consequences…A judicial response that departs from the lex lata and alters the specialized treaty régime on climate change would undermine the credibility and effectiveness of the existing legal régime, and the framework for negotiation, co-operation and consent that the treaties establish” (pp. 25 and 27). In brief, the Nordic countries, the United States, the United Kingdom, Russia, and Germany are of the latter view.

In response, the Court draws its argument on the ILC’s commentary to Article 55 of Articles on State Responsibility (ASR) and provides that “[f]or the lex specialis principle to apply it is not enough that the same subject matter is dealt with by two provisions; there must be some actual inconsistency between them, or else a discernible intention that one provision is to exclude the other…”(para. 167). Nonetheless, the ICJ clarifies 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).

With respect to ‘actual inconsistency’, it can be argued that, for example, States’ obligations to reduce emissions cannot be reconciled with certain investment treaties. For instance, the issue was raised in Eco Oro Minerals Corp. v. Republic of Colombia, ICSID Case No. ARB/16/41. Actually, Article 805 of the Free Trade Agreement (FTA) between Canada and Colombia ensures fair and equitable treatment for foreign investors based on the customary minimum standard of treatment for aliens. Although Article 2201(3) of the FTA provides an environmental exception that encourages positive discrimination and restriction on investments in order to protect human, animal or plant life and health and to conserve living or non-living exhaustible natural resources, the tribunal found Colombia in breach of Article 805 and held: “…The text of Article 2201(3) is explicit and specific: the ordinary meaning is that it does not prevent the payment of compensation… Indeed, the payment of compensation, as a result of an internationally wrongful act such as a breach of the FTA, is a default rule under international law…” (para. 367). As has been observed by a commentator, “such reasoning indicates that measures taken for the protection of the environment can be challenged and deemed a violation of [international investment agreements].” In a broader sense, the tribunal’s interpretation could produce a potential ‘actual inconsistency’ between the FTA and Article 4 of the UNFCCC (to which Canada and Colombia are parties), which requires all Parties to develop and implement national measures to mitigate and adapt to climate change (See also RWE v. Netherlands).

However, the ICJ and many states seemingly did not consider such a conflict as an ‘actual inconsistency’. For instance, Antigua and Barbuda contended that the “climate change treaties, and other sources of law, mutually reinforce and shed light on one another. Compliance with the Paris Agreement is necessary, but may not be sufficient, for compliance with States’ obligations under custom, UNCLOS and human rights instruments” (p. 16). Additionally, Mexico considered that the “Paris Agreement encourages co-operation but does not derogate from general obligations…In this vein, it is Mexico’s position that climate treaties must coexist with broader principles to create a unified and holistic legal framework that ensures comprehensive accountability” (p. 10) (See also the approaches of Kenya and Samoa).   

At the outset, the Court confirms that the two main functions of lex specialis are either to set aside a general rule in favor of a specific one or to elaborate more precisely on the same subject matter, without excluding the relevant general rule that concurrently remains applicable behind the scenes. Nevertheless, in their joint declaration, Judges Charlesworth, Brant, Cleveland and Aurescu claimed: “The Court ⎯ in our view, correctly ⎯ unequivocally rejects the argument that the climate change treaties constitute lex specialis in their relation to other rules of international law” (para. 2). One may view this discussion as overlooking the ‘interpretive’ role of lex specialis. In other words, it may open the door to viewing lex specialis as a principle that is used solely to exclude the general rule of applicability. This, by contrast, runs counter to the ICJ’s consistent approach to the lex specialis maxim, under which it has generally refrained from excluding the applicability of various rules of international law that deal with the same subject matters. Such a debate may have been raised given the Court’s wording that for lex specialis to apply, “there must be some actual inconsistency between them, or else a discernible intention that one provision is to exclude the other…” (citing the ILC’s commentary on ASR, para. 167). It seems that the ICJ’s view is that while tensions can exist in practice, these do not amount to actual legal inconsistencies. All these regimes are mutually compatible; where overlap occurs, interpretation resolves it. That’s why the ICJ proclaimed “…that argument according to which the climate change treaties constitute the only relevant applicable law cannot be upheld and finds that the principle of lex specialis does not lead to a general exclusion by the climate change treaties of other rules of international law” (para. 171).

In the author’s view, the fact that the Court has not been able to find any actual inconsistency does not mean that the climate change treaties do not constitute lex specialis. To illustrate,  in her declaration, Judge Cleveland clarified: “…the obligations of States in relation to the protection of the climate system impose significant responsibilities on States to adopt and implement appropriate environmental regulations to mitigate and adapt to climate change, including in co-operation with other States, while allowing States some discretion with respect to the particular regulatory paths that may be chosen. Accordingly, the interpretation of investment instruments must be informed by States’ obligations in respect of climate change under international law, including the stringent due diligence standard to which States are bound in implementing such obligations” (para. 22). Moreover, if we accept the aforementioned view, the whole jurisprudence of the ICJ concerning lex specialis would be rendered absurd; it would suggest that IHL could not be seen as a lex specialis in times of war, while the Court has predominantly confirmed it. This view arises from a limited interpretation that sees lex specialis only as overriding the general rule. In contrast, the Court’s precedent and the ILC’s practice clearly differ. When examining the ICJ’s Opinions, it is evident that the Court has consistently given significant weight to the ‘interpretive’ role of the lex specialis maxim, implicitly at first and explicitly later, while not asserting that a rule or branch of law does not constitute lex specialis. In climate change, the ICJ mentioned the ‘interpretive’ role of the lex specialis maxim (para. 162) and provided that in some cases “…the specific rule should be seen as an elaboration of one or more general rules, the latter continuing to play an interpretative role in the background…” (para. 166). The Court did not claim that the climate change treaties do not constitute lex specialis; on the contrary, it emphasized the essence of the interpretive role of lex specialis in ensuring more appropriate compliance with the various obligations regulating a particular subject matter. Therefore, in this context, lex specialis serves as a reinforcing principle that promotes the harmonization of the international legal system as a whole.

This argument can be supported by the declaration of Judge Tladi, in which he expressed: “… the flavour of the Opinion has a definite slant in favour of the climate change treaties, and in particular the Paris Agreement…Since the Paris Agreement, while not the only game in town ⎯ or even the most important game in town, ⎯ has taken centre stage as the response of States to the climate change crisis, the proper interpretation and application of this instrument [have] assumed particular importance…” (paras. 4-5). In addition, if we accept that the ICJ rejected the assertion that the climate change treaties constitute lex specialis, one may ask what logic lies behind the ICJ’s Construction of Wall, which stated that “the Court will have to take into consideration both these branches of international law, namely human rights law and, as lex specialis, international humanitarian law”? It is self-evident that the Court did not intend to reject the specific character of IHL vis-à-vis IHRL, but rather to advocate for their integrated interpretation and application, with special attention to IHL.

In this regard, Koskenniemi states that “…the question of validity has been replaced by a question of priority. The norm that will be set aside will remain as it were ‘in the background’, continuing to influence the interpretation and application of the norm to which priority has been given” (para. 411). Marko Milanovic has characterized the lex specialis maxim as a ‘conflict avoidance’ tool rather than ‘conflict resolution’, arguing that “[i]n international law lex specialis is nothing more than a sub-species of harmonious interpretation, a method of norm conflict avoidance. All it can do is assist in the interpretation of general terms and standards…” (p. 251).

Therefore, the ICJ’s real approach to lex specialis has always been a ‘harmonious’ or ‘systemic’ approach, which seeks to integrate all the rules of international law regarding a subject matter, rather than displace one in favor of another. Such a view is apparent where the Court provides that “…the preambles of the UNFCCC and the Paris Agreement themselves contain references to other rules and principles. The States parties to these treaties have thereby recognized that climate change constitutes a problem for whose solution other rules and principles also play a role” (para. 168. See also paras. 335 and 354, where the Court clarifies that all relevant rules of international law regarding climate change must ‘inform each other’). This perspective is compatible with Article 31(3)(c) of the VCLT of general rules of interpretation that says: “[A]ny relevant rules of international law applicable in the relations between the parties.” It has been put forward that this provision “refers to the international legal system as a whole…and thereby lays the foundation for the systemic approach to treaty interpretation” (p. 603).

Concluding Remarks

The ICJ’s engagement with the lex specialis maxim reveals a gradual but consistent evolution from implicit reliance to explicit articulation. While early decisions, such as Nuclear Weapons, seemed to imply a hierarchical relationship between special and general norms, later cases culminating in the Climate Change advisory opinion confirm a systemic and integrative understanding. This interpretive orientation carries significant implications for international law as a whole. By emphasizing coherence and mutual reinforcement among legal regimes, the ICJ’s approach mitigates fragmentation and strengthens the unity of the international legal order.