Climate Advisory Opinions and the Emergence of General Principles

On 23 July 2025, the International Court of Justice delivered its Advisory Opinion on the Obligations of States in Respect of Climate Change, outlining the States’ obligations under treaty and customary international law. The Inter-American Court of Human Rights published an Advisory Opinion AO-32/25 just prior, describing climate change as a “human rights emergency” that leads to obligations “erga omnes and of an intergenerational nature” (para. 287). This post argues that, combined, these courts’ decisions recast climate obligations into general principles. Prevention, due diligence, intergenerational equity, solidarity, precaution and equity gradually take over the role of general principles of law within the meaning of Article 38(1)(c), even though neither of the two courts expressly refers to them as such.

General Principles in the ICJ’s Climate Framework

The ICJ identifies the “applicable law” (paras. 113 ff.) in the UNFCCC, the Paris Agreement, other environmental treaties, and customary international law, which provides for obligations to prevent significant transboundary environmental harm. The Court confirms that States are required to ensure that activities within their jurisdiction “respect the environment of other States or of areas beyond national control.” This presents climate change as a problem which should be solved by due diligence of the States, mandating the latter to prevent, reduce, and control greenhouse gas emissions (paras. 271-315).

Still, the opinion does not merely characterize the notions of prevention and due diligence as minimal, customarily binding rules. The Court takes the liberty of connecting them with a whole range of principles. It interprets common but differentiated responsibilities and respective capabilities (CBDR-RC) as indicating “the need to distribute the burdens” of climate obligations equitably, while at the same time considering States’ “historical and current contributions to cumulative GHG emissions” and their different capabilities (paras. 148-151). The Court points out that development status is not something fixed but rather depends on an evaluation of the current situation of the State in question (para. 148).

Moreover, the Court also incorporates the precautionary principle (para 158) and the principle of intergenerational equity (para. 155-157). It associates due diligence with “the best available science” and the precautionary approach (para. 254), especially where there is risk of “serious or irreversible harm” (para 180). Additionally, the Court expresses that States must fulfill their climate obligations with “[d]ue regard for the interests of future generations” (para. 157)  and depicts present generations as trustees of the environment. A recent study has shown that these ideas are not mere policy foundations but serve to determine the extent of due diligence and specify the content of prevention obligations.

Erga Omnes and Intergenerational Dimensions in the IACtHR Opinion

The IACtHR Opinion explicitly extends the discussion into the field of community interests and general principles. It identifies climate change as part of a “triple planetary crisis” (para. 42 and footnote 38) and a “human rights emergency” that threatens the rights to life, health, housing, food, water, and a healthy environment (para. 239). The Court argues that the obligations of prevention and protection against climate-related harm are “erga omnes and of an intergenerational nature” (paras. 157–161), thus binding all States towards the international community as a whole and future generations (paras. 103–105).

Once more, prevention and due diligence are emphasized in the IACtHR Opinion through the principles of solidarity, equity, and precaution. The Court points out that all States are under the obligation to prevent human rights violations resulting from climate change (para. 230). However, the ones that have historically contributed more to greenhouse gas emissions and have greater financial, scientific and technological capacities are the ones that must shoulder the heavier burdens (paras. 56, 63, 255, 372). The Court links the precautionary principle with the protection of human rights; thus, States are required to take actions “in the presence of scientific uncertainty” where a credible risk of serious climate-induced harm exists (paras. 166–167). Some academic commentators argue that this reformulation, which combines erga omnes and intergenerational elements, effectively draws on general principles—solidarity, prevention, equity—to create a composite duty that cannot be broken down into any single treaty regime.

Principles as Connective Tissue Between Treaties and Custom

Neither court refers to Article 38(1)(c) of the Statute directly. However, the manner in which the ICJ and IACtHR utilize prevention, due diligence, equity, intergenerational equity, solidarity, and precaution is very much in line with Article 38(1)(c) for three reasons.

Firstly, the principles are genuinely cross-regime. They are found in environmental agreements (UNFCCC, Paris Agreement, UNCLOS), human rights instruments, as well as in the precedents of the ICJ, ITLOS, and regional human rights courts. The ICJ Climate Opinion, the IACtHR Opinion, the ITLOS Advisory Opinion on Climate Change and International Law, and judgments such as Verein KlimaSeniorinnen Schweiz v Switzerland all call for prevention, due diligence, and intergenerational equity to account for climate obligations. Their presence across different fields is one of the characteristics of general principles.

Secondly, the courts employ these principles as a means to harmonise the different sources. The ICJ explicitly refers to systemic integration under Article 31(3)(c) VCLT and indicates that “when several rules concern a single issue, they should, as far as possible, be interpreted to result in a single set of compatible obligations” (para. 165). To establish a connection between treaty-based mitigation, adaptation, and finance obligations and customary obligations of prevention and cooperation, the Court employs the concepts of CBDR-RC, intergenerational equity, and precaution to create a comprehensive framework (para. 404). Similarly, the IACtHR arrives at solidarity and equity as the binding factors that unite human rights-based duties and climate treaty commitments (paras. 157–158, 176). As one of the commentators puts it, the ICJ Opinion functions as a “sources decision,” which implicitly relies on general principles to establish an integrated climate law of obligations.

Thirdly, the principles fulfill the traditional roles of gap-filling and conflict resolution as envisaged in Article 38(1)(c) of the Statute. Prevention and due diligence are modified in the context of climate change to incorporate intergenerational equity, solidarity, and precaution, thereby extending their reach to areas where there is no clear settlement of law in climate-specific treaties or in traditional transboundary harm jurisprudence. Recent work views the IACtHR Opinion as a reconceptualization of erga omnes responsibility in climate-related human rights law, grounded in these very principles. At the same time, scholarly analysis considers that the ICJ Opinion employs general principles to strengthen climate duties.

Implications for Future Climate Litigation

This principles-driven framework brings about three implications. To begin with, it lowers the evidentiary threshold for future climate litigation. Parties may present their arguments in terms of widely accepted principles, supported by judicial decisions that follow the same lines, rather than establishing climate-specific customary rules for every single obligation. This is particularly important in cases where treaty clauses are drafted in an open-textured manner (“appropriate mitigation measures,” “best efforts”) and where practice is disputed. A disputed case before the ICJ or a regional human rights court could now allege the violation of general principles of prevention or intergenerational equity as a matter of law, citing both advisory opinions as proof of their status.

Moreover, such a move significantly improves systemic integration and judicial dialogue. By using a shared set of principles, various courts and tribunals can harmonize their interpretations of different legal texts in a mutually supportive manner, thereby facilitating what has been termed a fledgling “climate law of the courts.” The judgments of the ICJ, IACtHR, ITLOS, and ECtHR are being interpreted together in this new fashion, with the ICJ itself referring to ITLOS (paras. 337–338) and regional human rights jurisprudence (paras. 377–378, 381, 385).

Lastly, the question of legitimacy arises. Opponents could argue that by heavily relying on general principles, erga omnes language, and references close to jus cogens (IACtHR paras. 287-294), the courts run the risk of unilaterally extending obligations beyond those explicitly consented to by the States in the treaties. Regardless of whether the argument is ultimately convincing, it clearly indicates the shift that has taken place: climate obligations are no longer comprehensively spelled out only in treaty clauses and customary rules, but are increasingly being delineated and limited by overarching principles that apply to different regimes.

This is why the ICJ and IACtHR climate advisory opinions represent support for the idea that general principles are progressively becoming the core and the framework of international climate law. They form the connective tissue between the treaties and obligations under customary international law. This gives the courts the freedom to build an organized, principles-based law of climate obligations that would otherwise be scattered in different pieces.