The ICJ’s Advisory Opinion on Obligations of States in Respect of Climate Change

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Legal Voices for the Future
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About the Author: Ellie Stringer is a paralegal at Veale Wasbrough Vizards LLP and a First-Class Law graduate from Newcastle University. Ellie has experience in litigation support, pro bono legal practice, and environmental justice advocacy, ranging from grassroots campaigning with Greenpeace to supporting climate displacement recovery efforts with Think Pacific in Fiji.

(i) Introduction

The International Court of Justice (ICJ) has delivered a unanimous advisory opinion confirming that states’ obligations on climate change are binding, not discretionary.1 These obligations stem both from treaties and customary international law, including the duty to prevent significant environmental harm.2 A breach by a state of any of these obligations constitutes an internationally wrongful act, entailing the responsibility of that state, with consequences such as restitution, compensation, or satisfaction owed to injured states.3 This landmark opinion, initiated by Pacific Island law students and shaped by unprecedented global participation, represents a historic moment in international law.4

At its core, the proceedings revealed competing visions of international law, with Rajamani et al contending that international law itself was in the dock.5 Major emitters argued for narrow, consent-based interpretations of the Paris Agreement and rejected the framework of state responsibility. Vulnerable states, by contrast, advanced a far more robust, purpose-driven reading of international law as a tool to confront global crises.6 The ICJ sided progressively and decisively with the latter, confirming that climate obligations are concrete, enforceable, and capable of entailing state responsibility.

This post provides a summary of the ICJ’s key findings on the sources of states’ climate obligations and the legal consequences of breach, before considering how the Opinion may influence the responsibilities and practice of states, international organisations, private actors, and individuals.

(ii) Background to the request: the Pacific’s plea for clarity

Spearheaded by the Republic of Vanuatu and supported by a coalition of Small Island Developing States (SIDS) and youth-led movements, the initiative brought to the UN General Assembly (GA) a compelling plea: to clarify the legal responsibilities of states in a climate crisis they did little to cause, yet suffer from the most.7

The GA responded by submitting two questions to the Court:

  1. What obligations do states have under international law to protect the climate system and the environment from anthropogenic greenhouse gas (GHG) emissions, for the benefit of both present and future generations?8
  2. What are the legal consequences of these obligations for the states that have caused significant harm to the climate system, especially with respect to injured or particularly vulnerable states, such as SIDS and current and future generations?

(iii) Identifying the Obligations of States from the Applicable Law

The ICJ confirmed that states’ obligations arise under treaties, customary international law, and other branches of international law, each to be interpreted within its broader normative framework.

  1. The Climate Change Treaty Framework: the UNFCCC, Kyoto Protocol, and Paris Agreement:

The ICJ delivered a “muscular” interpretation of the core international treaties governing the global response to climate change:9 the UN Framework Convention on Climate Change (UNFCCC),10 the Kyoto Protocol,11 and the Paris Agreement.12 Far from being superseded, each instrument retains legal significance and contributes to a coherent and evolving treaty regime.13

The court held that all three treaties impose binding obligations: mitigation, adaptation, and international cooperation, including financial and technological assistance for developing states.14 It further clarified that non-compliance with commitments, such as Kyoto’s reduction targets or Paris Agreement Nationally Determined Contributions (NDCs), may constitute internationally wrongful acts.15 Interpreting the Paris Agreement within its broader normative environment, the ICJ stressed that NDCs must align with the 1.5°C goal,16 embody progression and the “highest possible ambition” under Article 4.3,17 and satisfy a stringent due diligence standard.18 

  1. Customary International Law: Universal Obligations of States:

The Court rejected arguments that treaty law displaces custom, confirming instead a symbiotic relationship: treaties and custom inform one another but remain independent regimes.19 This means that fulfilling treaty commitments does not automatically discharge customary obligations, and withdrawal from treaties does not absolve states from compliance with custom.20

The ICJ underscored that all states are bound by fundamental obligations under customary international law in relation to climate change, chief among them the duty to prevent significant environmental harm,21 expressly extended to the climate system.22 Informed by the principle of Common but Differentiated Responsibilities and Respective Capabilities (CBDR-RC) and grounded in due diligence,23 this duty requires states to adopt effective measures to mitigate emissions and regulate private actors within their jurisdiction.24 The Court stressed that the global and complex nature of climate change elevates rather than diminishes the standard of care.25 

It further held that compliance with the Paris Agreement will generally satisfy these customary obligations.26 For non-parties, such as the United States, customary due diligence duties are shaped by the practice of Paris Agreement parties, requiring compliance with equivalent standards even outside the treaty regime.27 This expansive reading underscores the binding nature of climate obligations and entrenches cooperation, consultation, and environmental impact assessments as core elements of due diligence.28

  1. The Wider Legal Framework: The Law of the Sea, and Human Rights:

The Opinion made clear that states’ obligations in relation to climate change extend across the wider body of public international law. In particular, the Court highlighted the relevance of the UN Convention on the Law of the Sea (UNCLOS),29 and international human rights law.30

With respect to UNCLOS, the Court confirmed that anthropogenic GHG emissions constitute “pollution of the marine environment” within the meaning of Part XII.31 States therefore have stringent due diligence obligations to take “all necessary measures” to prevent, reduce and control such pollution, guided by the best available science and their capabilities.32 These duties include ensuring activities under their jurisdiction do not cause transboundary harm, conducting environmental impact assessments, and cooperating in research and rule-making.33 The Court also clarified that sea level rise does not automatically deprive states of their maritime entitlements or statehood, ensuring continuity of legal personality for vulnerable coastal and island states.34

The Court also underscored the indivisible link between climate change and human rights.35 It affirmed that climate impacts threaten a wide spectrum of rights, including the rights to life, health, food, water, housing, and the rights of vulnerable groups.36 Central to this analysis was the recognition that a clean, healthy and sustainable environment is essential for the enjoyment of all other rights, though the Court stopped short of declaring it customary law.37 States are required to integrate human rights considerations into their mitigation and adaptation policies, regulate private actors, and ensure their actions (and inactions) do not result in violations at home or abroad. This “human rights approach to climate action” reinforces accountability while acknowledging States’ need to balance competing rights, for example in projects affecting indigenous communities.38

(iv) Legal Consequences of State Breaches of Climate Obligations

The ICJ reaffirmed the foundational principle that every internationally wrongful act by a State engages its international responsibility, and this applies squarely in the climate change context.39 Wrongful conduct may arise from breaches of treaty obligations, such as failing to prepare or implement NDCs, or from breaches of customary law duties, such as the due diligence obligation to prevent significant transboundary harm or to conduct environmental impact assessments.40 Importantly, attribution is not limited to State emissions alone: omissions, such as failing to regulate private actors or enact adequate legislation to mitigate emissions, may also constitute wrongful acts attributable to the State.41 

On causation, the Court clarified that liability for breach does not require proof of specific damage, though causation is central in determining reparations.42 It endorsed a flexible standard requiring a “sufficiently direct and certain causal nexus” between wrongful conduct and injury.43 This standard can be met through climate attribution science, which can link climate events to anthropogenic emissions and, crucially, determine the extent to which damage is attributable to a particular State or group of States.44 

Where breaches are established, the Court confirmed that the full range of legal consequences under the law of State responsibility applies.45 These include the cessation of the wrongful act, assurances and guarantees of non-repetition, and the duty to make full reparation. Reparation may take the form of restitution (e.g., restoring ecosystems or rebuilding infrastructure), compensation (where a causal link to harm can be shown, potentially quantified through aggregate sums and equitable considerations), or satisfaction (such as apologies or public acknowledgment for non-material harm).46

(v) From Principle to Practice: Next Steps for States and Beyond

The Opinion, while formally non-binding, carries immense persuasive authority across a wide range of legal, political, and institutional arenas. Its clarifications strengthen the hand of climate-vulnerable states, providing them with a more robust legal foundation to demand ambitious mitigation and adaptation measures, as well as financial and technological support. For developed states, the Opinion signals heightened responsibility and accountability, particularly in relation to fossil fuel activities, climate finance, and the regulation of private actors.47

In terms of litigation, the path to contentious proceedings before the ICJ remains limited, given that many high-emitting states have not accepted its compulsory jurisdiction.48 However, the International Tribunal for the Law of the Sea (ITLOS), with compulsory jurisdiction under UNCLOS, could emerge as a key forum for climate-related disputes.49 At the same time, the Opinion is likely to catalyse regional and domestic litigation, providing courts with persuasive reasoning to interpret national law consistently with international obligations.50

Beyond litigation, much of the Opinion’s practical impact will unfold within the ongoing UN climate negotiation processes.51 The Court’s interpretation of the Paris Agreement, including its emphasis on progression, ambition, and fair share in relation to NDCs, will put pressure on states during COP negotiations. However, these findings may also create friction, especially for major emitters concerned that the Opinion constrains the autonomy of NDCs.52 The recognition that cooperation is not a political aspiration, but a legal obligation could also reshape debates around climate finance, adaptation support, and loss and damage.

The Opinion also gives momentum to emerging legal initiatives aimed at strengthening the international framework for climate and environmental protection. One such development is the proposal for a Fossil Fuel Non-Proliferation Treaty, currently supported by seventeen states including Tuvalu, which seeks to establish binding obligations on a coordinated fossil fuel phase-out and a just transition to renewable energy.53 In parallel, Vanuatu, Fiji, and Samoa are spearheading efforts to amend the Rome Statute of the International Criminal Court to include “ecocide” as a fifth core international crime, a move that has gained notable backing from states such as the Democratic Republic of the Congo.54 

The broader implications extend beyond states to international organisations, civil society, and the private sector. The Opinion strengthens the mandates of international bodies engaged in climate governance, promotes science-based policymaking, and provides new tools for accountability. For businesses and investors, it signals stricter regulation, enhanced due diligence and disclosure obligations, and greater litigation risk. Unresolved questions of causation, attribution, and overlapping regimes remain, but the true significance of the Opinion will be measured by how states, courts, and institutions implement its principles to drive meaningful climate action.

Footnotes

  1. International Court of Justice, Obligations of States in respect of Climate Change (Advisory Opinion, 23 July 2025) General List No 187 https://www.icj-cij.org/case/187 accessed 7 September 2025 ↩︎
  2. Margaret Young, Phoebe Okowa and Lavanya Rajamani, ‘The ICJ’s Advisory Opinion on Climate Obligations: Remarkable, Radical and Robust’ (EJIL: The Podcast!, 30 July 2025) https://www.ejil.org/podcast.php?guid=e48c88df-1d3f-47ad-81b4-0d298daab189 accessed 7 September 2025. ↩︎
  3. Ibid (2) ↩︎
  4. Ibid (2) ↩︎
  5. Ibid (2) ↩︎
  6. Ibid (2) ↩︎
  7. Simmons & Simmons, ‘The ICJ’s Climate Change Advisory Opinion: A New Era for States’ (Simmons & Simmons, 28 July 2025) https://www.simmons-simmons.com/en/publications/cmdrd1ftn00j0u11oqepyqgop/the-icj-s-climate-change-advisory-opinion-a-new-era-for-states ↩︎
  8. Ibid (1), [88] ↩︎
  9. Lavanya Rajamani, ‘Interpreting the Paris Agreement in its Normative Environment’ (2024) 77 Current Legal Problems 167. ↩︎
  10. United Nations Framework Convention on Climate Change (adopted 9 May 1992, entered into force 21 March 1994) 1771 UNTS 107. ↩︎
  11. Kyoto Protocol to the United Nations Framework Convention on Climate Change (adopted 11 December 1997, entered into force 16 February 2005). ↩︎
  12. Paris Agreement (adopted 12 December 2015, entered into force 4 November 2016). ↩︎
  13.  Ibid (2), [195] ↩︎
  14. Ibid (1), [175]-[270] ↩︎
  15. Ibid (1), [444] ↩︎
  16. Ibid (1), [223] ↩︎
  17. Ibid (1), [179], [242]-[245] ↩︎
  18. Ibid (1), [252]-[254] ↩︎
  19. Ibid (1), [309]-[313] ↩︎
  20. Ibid (2) ↩︎
  21. Pulp Mills on the River Uruguay (Argentina v Uruguay) (Judgment) [2010] ICJ Rep 14, [101], [197]. ↩︎
  22. Ibid (1), [273] ↩︎
  23. Ibid (1), [290]-[293] ↩︎
  24. Ibid (1), [282] ↩︎
  25. Ibid (1), [272], [280]-[300] ↩︎
  26. Ibid (1), [314] ↩︎
  27. Ibid (1), [287], [315] ↩︎
  28. Ibid (2) ↩︎
  29. United Nations Convention on the Law of the Sea (adopted 10 December 1982, entered into force 16 November 1994) 1833 UNTS 3. ↩︎
  30. Ibid (1), [369] ↩︎
  31. Ibid (1), [340] ↩︎
  32. Ibid (1), [346], [349] ↩︎
  33. Ibid (1), [353] ↩︎
  34. Ibid (24); Ibid (1), [363] ↩︎
  35. Ibid (2) ↩︎
  36. bid (1), [371]-393] ↩︎
  37. Ibid (2) ↩︎
  38. Margaret A Young, ‘Climate Change and Law: A Global Challenge for Legal Education’ (2021) 40 University of Queensland Law Journal 351. ↩︎
  39. Ibid (1), [405] ↩︎
  40. Ibid (7) ↩︎
  41. Ibid (24) ↩︎
  42. Ibid (7) ↩︎
  43. Ibid (1), [433]-[438] ↩︎
  44. Ibid (7) ↩︎
  45. Léa Weimann, ‘Breaking News: A Historic Moment at the International Court’ (LinkedIn, 5 July 2025) https://www.linkedin.com/pulse/breaking-news-historic-moment-international-court-l%C3%A9a-weimann-ll-m-vsqcf/ ↩︎
  46. Ibid (7) ↩︎
  47. Ibid (2) ↩︎
  48. Ibid (2) ↩︎
  49. Ibid (24) ↩︎
  50. Alice Foulks, ‘The ICJ Speaks: Climate Justice is Not to be Ignored’ (International Lawyers Project, 25 July 2025) ↩︎
  51. Ibid (2), [184] ↩︎
  52. Ibid (2) ↩︎
  53. Maina Vakafua Talia, ‘States now have a legal duty to prevent climate harm – justice is in reach’ (Nature, 01 September 2025) https://www.nature.com/articles/d41586-025-02752-y ↩︎
  54. Léa Weimann, ‘When the Law Catches Up: How a Youth-Led Call from the Pacific is Redefining Environmental Justice’ (Earth.Org, 21 May 2025) https://earth.org/when-the-law-catches-up-how-a-youth-led-call-from-the-pacific-is-redefining-environmental-justice/ ↩︎

Bibliography

Treaties and Conventions:

Paris Agreement (2015)

Kyoto Protocol to the United Nations Framework Convention on Climate Change (1997)

United Nations Convention on the Law of the Sea (adopted 10 December 1982, entered into force 16 November 1994) 1833 UNTS 3

United Nations Framework Convention on Climate Change (UNFCCC, adopted 9 May 1992, entered into force 21 March 1994)

Cases:

International Court of Justice, Obligations of States in respect of Climate Change (Advisory Opinion, 23 July 2025) General List No 187 

International Court of Justice, Pulp Mills on the River Uruguay (Argentina v Uruguay) (Judgment) [2010] ICJ Rep 14.

Academic Journals:

Vakafua Talia M, ‘States now have a legal duty to prevent climate harm – justice is in reach’ (Nature, 24 July 2025) https://www.nature.com/articles/d41586-025-02752-y 

Rajamani L, ‘Interpreting the Paris Agreement in its Normative Environment’ (2024) 77 Current Legal Problems 167 

Young MA, ‘Climate Change and Law: A Global Challenge for Legal Education’ (2021) 40 University of Queensland Law Journal 351

Reports and Working Papers:

Norton Rose Fulbright, ‘The International Court of Justice Confirms the Obligations and Consequences for Nation States to Address Climate Change under International Law’ (Norton Rose Fulbright, September 2025) https://www.nortonrosefulbright.com/en/knowledge/publications/de4b6726/the-international-court-of-justice-confirms-the-obligations-and-consequences-for-nation-states 

Simmons & Simmons, ‘The ICJ’s Climate Change Advisory Opinion: A New Era for States’ (Simmons & Simmons, 28 July 2025) https://www.simmons-simmons.com/en/publications/cmdrd1ftn00j0u11oqepyqgop/the-icj-s-climate-change-advisory-opinion-a-new-era-for-states 

Podcasts and Multimedia:

EJIL: The Podcast!, Margaret Young, Phoebe Okowa and Lavanya Rajamani, ‘The ICJ’s Advisory Opinion on Climate Obligations: Remarkable, Radical and Robust’ (EJIL: The Podcast!, 30 July 2025) https://www.ejil.org/podcast.php?guid=e48c88df-1d3f-47ad-81b4-0d298daab189 

Online Resources and Websites:

Foulks A, ‘The ICJ Speaks: Climate Justice is Not to be Ignored’ (International Lawyers Project, 25 July 2025) https://www.internationallawyersproject.org/post/the-icj-speaks-climate-justice-is-not-to-be-ignored 

Weimann L, ‘When the Law Catches Up: How a Youth-Led Call from the Pacific is Redefining Environmental Justice’ (Earth.Org, 21 May 2025) https://earth.org/when-the-law-catches-up-how-a-youth-led-call-from-the-pacific-is-redefining-environmental-justice/ 

Weimann L, ‘Breaking News: A Historic Moment at the International Court’ (LinkedIn, 5 July 2025) https://www.linkedin.com/pulse/breaking-news-historic-moment-international-court-l%C3%A9a-weimann-ll-m-vsqcf/ 

Photo Credits

[Photo Credit 1: Unsplash // Mathias Reding]

[Photo Credit 2: Unsplash // Markus Spiske]

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