The Die Is Cast: Some Key Implications of the ICJ Advisory Opinion on Climate Change

Introduction

On 23 July 2025, the International Court of Justice (ICJ or Court) delivered its long-awaited Advisory Opinion (AO or Opinion) on Obligations of States in Respect of Climate Change, sparking widespread praise. Widely described as "historic" (see e.g. here and here), the Opinion affirms the growing role of international law in addressing the climate crisis. Spanning over 140 pages, it examines the interplay between climate treaties and general international law. The Court emphasised due diligence, international cooperation, the right to a healthy environment and the prevention of harm, among other key principles. Accompanied by five separate opinions and six declarations, the ruling reflects significant legal complexity. This post offers a first look at three key takeaways: (1) the rejection of lex specialis; (2) the role of fossil fuels in international law; and (3) reparations.

1. Lex Specialis Defeated? The Triumph of Systemic Integration in the ICJ's Climate Opinion

One of the first and most contested issues raised during the oral hearings, the lex specialis argument—advanced by major polluting States such as the US, Russia, Saudi Arabia, and Germany—claimed that the Paris Agreement should be interpreted narrowly, without recourse to general principles of international law, emphasising the autonomy of the specialised climate change regime. This position was firmly rejected by the Court (paras. 162–171), which instead endorsed a systemic integration approach, affirming that climate treaties must be interpreted in harmony with other rules and principles of international law (para. 171, in response to Question (a)). The Court found no contradiction between UN climate agreements and broader international legal norms, stating that they are complementary and mutually reinforcing.

In doing so, the ICJ aligned itself with the approach taken by ITLOS (see e.g. here, here and here). The Court explicitly endorsed the principle of systemic integration (see generally McLachlan, 2024), noting that preambular references to general principles of international law allow treaties to operate within the broader international legal system (para. 168). This is reinforced by the Court's explicit reference to systemic integration and the usage of "guiding principles" in treaty interpretation (para. 164).

Turning to Question (b), the Court considered whether the Paris Agreement constitutes a lex specialis regime overriding general rules of international law. It held that Articles 8 and 15 of the Paris Agreement do not displace the application of general international law, affirming that ARSIWA rules on State responsibility continue to apply (paras. 410–420). With this, the Court made clear that climate treaties operate within—not outside—the broader framework of international law, placing it at the heart of interpreting and enforcing climate obligations.

2. A Reckoning for the Fossil Fuel Industry

The ICJ's AO could reshape the legal landscape for fossil fuel-producing States. In response to the direct question posed by Judge Cleveland during the December 2024 hearings, the Court addressed the specific climate obligations of States engaged in fossil fuel production. Given that fossil fuels account for approximately 75% of global greenhouse gas (GHGs) emissions, the Court's findings in this area may prove transformational.

The Court reaffirmed that a State's failure to take appropriate measures to protect the climate system from GHGs—including via fossil fuel production, consumption, licensing, or subsidies—may amount to an internationally wrongful act (para. 427). Importantly, the wrongful act lies not in the emissions themselves, but in breaching the obligation to prevent significant climate harm.

States also have a legal duty to regulate private actors under the principle of due diligence. Here, attribution concerns the State's failure to act, not the conduct of private entities. Where a State fails to adopt adequate laws or regulations to limit emissions from private actors within its jurisdiction, it may incur international responsibility. The Court cited Pulp Mills to confirm that omissions can breach international obligations (para. 428).

The Court acknowledged that climate change stems from the cumulative emissions of "a plurality of States that cause injury to a plurality of injured States" (para. 429). However, it affirmed that scientific tools now allow for quantifying each State's individual contribution, both historic and current. Despite the complexity of linking specific harm to specific breaches, the Court stressed that customary international law on State responsibility can address such cases. Even when multiple States contribute to a single wrongful act, each can be held individually responsible, and injured States may bring separate claims (paras. 429–432).

Despite the fossil fuel industry being put on notice, there is more to be said. As noted in the Joint Declaration of Judges Bhandari and Cleveland, the Court could have taken a more assertive stance on fossil fuel and related activities (para. 4 of the Joint Declaration).

First, the Court should have afforded greater weight to the "best available science" in the IPCC reports, which highlight that fossil fuel production—especially subsidies—is a major driver of climate change (para. 10 of the Joint Declaration). The need to phase out fossil fuel subsidies and transition away from fossil fuels in general is firmly embedded in the First Global Stocktake and COP decisions, which qualify as subsequent agreements between parties (para. 224).

The principle of common but differentiated responsibilities and respective capabilities (CBDR-RC) must guide this transition, aligning with the third pillar of the proposed Fossil Fuel Treaty: a fair and equitable phase-out based on States' technological and resource availability (Fossil Fuel Treaty Report, May 2024, p.5). This also requires wealthier States to support developing countries in phasing out fossil fuel dependence. Notably, the elimination of subsidies must be balanced against the imperative of eradicating energy poverty—a legitimate exception recognised in the Joint Declaration (para. 26 of the Joint Declaration). The Court could have explored this issue further.

Second, the Judges underscored the extraterritorial dimensions of State obligations. Extracting or burning fossil fuels abroad does not absolve a State of its duty to prevent irreversible climate harm. This aligns with the "stringent" due diligence obligation (para. 15 of the Joint Declaration) articulated in the ITLOS Advisory Opinion on Climate Change (see e.g. Desierto).

Yet neither the Opinion nor the Joint Declaration clarified how a State could be held internationally responsible for concrete acts such as granting licenses for fossil fuel exploration, approving petrochemical facility expansions, providing public financing for fossil infrastructure, or maintaining subsidies for coal or oil. Is responsibility here grounded in strict liability or fault-based liability? Moreover, exemptions may apply—for instance, fossil fuel subsidies aimed at ensuring energy security, as permitted under the Agreement on Climate Change, Trade and Sustainability (Art 4.6(2)(c)). What would be the precise criteria for these exemptions? A future case will need to engage with these questions.

The Opinion nonetheless strengthens legal arguments for ending fossil fuel subsidies and supports the goals of the Fossil Fuel Non-Proliferation Treaty Initiative. It is regrettable, however, that the Opinion did not mention the initiative at all. Still, even the sparse reference to fossil fuels may influence ongoing negotiations on subsidy reform in the WTO and the development of the Global Plastics Treaty. Beyond its impact on treaties, the Opinion may also spur strategic anti-fossil fuel litigation—targeting fossil fuel production, challenging government approvals and licensing decisions in domestic courts, and holding companies liable for harm caused to specific States or sub-national entities (see e.g. Global Atlas for Social Justice).

3. Beyond Prevention: Legal Consequences and a New Era of Climate Redress

One of the most significant conclusions of the ICJ's AO is its affirmation that States responsible for climate-related wrongful acts must not only cease the conduct and provide assurances of non-repetition but also make full reparation, where appropriate, through restitution, compensation, and/or satisfaction (paras. 445–455). For the first time, the ICJ clearly confirmed that international law requires not only the prevention of climate harm but also redress when it occurs.

The Court emphasised that breaches of international obligations do not relieve States of their duty to comply. For example, a State that submits an inadequate NDC remains under a continuing obligation to bring it into compliance with its treaty commitments (para. 446). Cessation may also require revoking laws, cutting emissions, and taking necessary measures to stop the wrongful conduct (para. 448). Judge Bhandari, in his Separate Opinion, rightfully argued for more specificity, suggesting that cessation should include halting fossil fuel extraction, phasing out subsidies, and adopting immediate emissions cuts (paras. 4–5 of the Separate Opinion).

On reparation, the Court acknowledged that full restitution is often impossible in the climate context due to irreversible harm. Still, it may include restoring ecosystems, rebuilding infrastructure, and enhancing carbon sinks (para. 451). Judge Bhandari proposed that restitution could also encompass Indigenous rights, preservation of maritime entitlements, and ecological restoration (paras. 6–7 of the Separate Opinion).

Where restitution is not feasible, compensation must be provided for environmental and financial harm, even if damage is difficult to quantify. The Court suggested that compensation could take the form of a global sum awarded on an exceptional, equitable basis (paras. 452–454). Judge Bhandari went further, proposing international claims commissions and a UN-administered compensation fund, financed by developed States, to ensure equitable relief (para. 8 of the Separate Opinion). This aligns with broader calls for a Global Climate Reparations Fund, governed by the UN Human Rights Council and financed by high-emitting States and fossil fuel corporations. However, as the challenges facing the established Loss and Damage Fund show, operationalising such mechanisms will be politically and financially difficult (see e.g. here).

The Court also recognised satisfaction as a valid form of reparation, including acknowledgments of wrongdoing, apologies, or public education initiatives (para. 455). Judge Bhandari called for more concrete symbolic reparations, such as commemorations and recognition of affected communities, to affirm dignity and responsibility (para. 9 of the Separate Opinion). By grounding its analysis in both treaty and customary law, the Court confirmed that States may be held responsible for climate-related harm caused by action (e.g. emissions or fossil fuel subsidies) or omission (e.g. failure to regulate emissions or conduct environmental assessments).

Though mindful of its advisory role, Judge Bhandari stressed the Opinion's broader significance, invoking impliedly the philosophy of vasudhaiva kutumbakam—"One Earth, One Family, One Future" (para. 10 of the Separate Opinion)—to underline the Court's message of shared responsibility (see e.g. here, here and here).

Ultimately, while the Opinion affirms that climate harm can trigger State responsibility and reparation, it leaves key details to future contentious cases. Even so, it will likely become a foundational text in the evolving discourse on climate justice.

4. Concluding Remarks: One Earth, One Family

The ICJ's AO marks a major step forward for international climate law. For the first time, the Court clarified that existing international law imposes concrete obligations on States to address climate change—dispelling claims that lex lata is unfit for purpose (see e.g. Declaration of Judge Cleveland, para. 2). As Lowe aptly noted, litigation is not the end of the fight, but merely its beginning (Lowe, p. 221).

Yet the Opinion is not without shortcomings. It underplays the need to phase-out fossil fuels, offers little guidance on what such a transition entails, and fails to mention the Fossil Fuel Treaty Initiative. It also leaves several key issues unanswered—such as the legal standing of future generations, and the scope of "enhanced due diligence", recently emphasised by the IACtHR AO.

Importantly, the Court closed with a sobering reminder: law alone is not enough—it takes human will and wisdom to change how we live (para. 456). In the spirit of vasudhaiva kutumbakam—"One Earth, One Family, One Future"—the Opinion should be seen as a catalyst, not a conclusion.

The die is cast. Let the work begin.

Renatus Otto Franz Derler, LLB (Buckingham), LLM, MPhil (Cambridge), is a PhD in law candidate at the University of Cambridge. He serves as Editor-in-Chief of the Cambridge International Law Journal (Volumes 14 and 15), Managing Editor of RECIEL, and assistant to International Law Commission Special Rapporteur Marcelo Vázquez-Bermúdez. All views are personal.

Paulina Rundel, LLM, is a PhD researcher, university assistant (prae doc) and lecturer at the Department of European, International and Comparative Law of the University of Vienna. She is research assistant to Special Rapporteur August Reinisch of the International Law Commission.