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The ICJ’s Historic Advisory Opinion: A New Era for Climate Justice?

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By Sarah Thompson, PhD Candidate, Université libre de Bruxelles and Senior Research Programme Manager, European Chair for Sustainable Development and Climate Transition, Sciences Po

On 23 July 2025, the International Court of Justice (ICJ), key judicial body of the United Nations, delivered a landmark advisory opinion[1] on states’ obligations to address climate change under international law.

How did we get here, what does this advisory opinion mean, and what are the potential implications of this historic decision?

Background

The unlikely champions that led to this groundbreaking moment? A group of students from the Pacific Islands, a region particularly vulnerable to the impacts of climate change. A university assignment at the University of the South Pacific in Fiji sparked an idea: what if vulnerable states could bring the case of climate change to the highest court in the world in order to advance climate justice? This idea ultimately spurred the creation of the Pacific Island Students Fighting Climate Change (PISFCC) in 2019, which began advocating for legal clarity on states’ duties amid existential climate threats.[2]

The efforts of these students then inspired Vanuatu to lead a coalition of states (with over 130 co‑sponsors)[3], prompting the UN General Assembly to adopt a resolution on 29 March 2023,[4] formally requesting the ICJ to issue an advisory opinion. While this type of legal advice provided to the UN is not binding, it “carr[ies] great legal weight and moral authority,” and “contribute[s] to the clarification and development of international law and thereby to the strengthening of peaceful relations between States”.[5]

For this advisory opinion, the ICJ was specifically asked to address the following two key questions:

  1. What are the obligations of states under international law to protect the climate system and environment from anthropogenic greenhouse gas emissions, for present and future generations?
  2. What are the legal consequences for states whose actions or omissions cause significant harm — particularly on vulnerable island states and people, both now and in the future? 

The process, taking place over the last two years, gained unprecedented momentum: during the public hearings in December 2024, the ICJ received the highest level of participation in its history, with 96 states and over 11 international organisations providing oral and written inputs.[6]  

Key points from the ruling

The Court’s advisory opinion[7] leaves little room for doubt: climate action is a legal obligation under international law. The judges unanimously affirmed that states have binding duties — coming from both international treaties (such as the Paris Agreement) and customary international law (less formal and often unwritten expectations of behaviour) — to prevent and mitigate climate change. Importantly, this responsibility is not limited just to parties of specific environmental treaties: it is grounded in long-standing principles such as the “no-harm” principle,[8] which requires states to prevent activities within their territory from causing significant damage to the environment of other states or to the global commons. This is particularly relevant given President Donald Trump’s announcement to pull out of the Paris Agreement again earlier this year.[9]

A striking element of the opinion was the legal validation of the 1.5°C threshold. While mainstream climate discourse often frames 1.5°C as aspirational and 2°C as the more realistic target, the ICJ treated 1.5°C as the binding reference point. This also means that states do not have unlimited discretion in crafting their Nationally Determined Contributions (NDCs), the detailed climate action plans submitted to the UN that outline their commitments to reduce greenhouse gas emissions and adapt to the impacts of climate change. NDCs have often been criticised for their flexibility, allowing countries to include whatever targets or policies they choose, which has led to pledges that are widely seen as insufficiently ambitious to meet the Paris Agreement’s goals.[10] Set against these critiques, the ICJ affirmed that countries must pursue the “highest possible ambition,” and collectively ensure that warming remains within 1.5°C, emphasising that weak or symbolic NDCs fall short of international obligations.

Responding to the second question, the judges also clarified what failure to act looks like in legal terms. Actions such as granting new fossil fuel licenses, providing fossil fuel subsidies, or neglecting to regulate emissions were all explicitly cited as conduct that may amount to an internationally wrongful act. In other words, climate inaction — or worse, regressive actions — may generate legal responsibility.

Perhaps most consequentially, the ICJ spelled out the consequences of countries breaching their obligations to act on climate change. Under the general law of state responsibility, wrongful acts would result in the states having the following duties:

  • cessation (to stop the harmful activity),
  • guarantees of non-repetition, and
  • reparation (including compensation and restitution where harm can be shown).

This is a clear invitation for states, and potentially affected communities, to seek redress for climate harms. The opinion was also notable for its integration of human rights law, recognising that climate change directly undermines rights to life, health, food, water, and culture. The ICJ went further, acknowledging that climate refugees— people who may be forcible displaced due to the effects of climate change— must not be returned to dangerous conditions, and crucially that low-lying island states do not lose statehood because of rising seas and loss of land.

Finally, the Court underlined that obligations are “erga omnes”, meaning they are owed to the international community as a whole, including future generations. All states, regardless of whether they are party to a specific climate treaty, bear these responsibilities. Finally, the opinion rejected the “lex specialis” argument put forward by some major emitters, such as the United States and Australia, affirming that climate treaties like the Paris Agreement exist in harmony with, rather than override, other areas of international law. By rejecting attempts to narrow the scope of states’ duties, the Court underscored that climate change must be addressed through the full body of international law, not only through the Paris Agreement.

What comes next

Although advisory opinions are not legally binding, this one is expected to reverberate across legal, political, and economic arenas, with many calling it a turning point or roadmap for climate justice.[11]

Jennifer Robinson, lead counsel for Vanuatu, stressed in an interview the extraordinary significance of the ICJ’s opinion. Not only was every argument advanced by Vanuatu was unanimously supported by the Court, but the process itself was unprecedented: the request had first been adopted by consensus in the UN General Assembly — the first time in history that such a referral was made without opposition.[12] In addition, several judges issued separate declarations that went even further than the unanimous opinion. Robinson described this as one of the most remarkable decisions in the Court’s history, underscoring how strongly states wanted clarity — and how decisively the Court provided it.

One expected impact this advisory opinion will soon bring will be on climate litigation.[13] Courts around the world have sometimes hesitated to enforce climate obligations, citing legal ambiguity. The ICJ has now removed much of that vagueness. We can expect to see a wave of rights-based claims against governments and even private actors, with litigants pointing to the ICJ’s recognition that emissions cause significant transboundary harm and may constitute wrongful acts. The threat of litigation and reparations is likely to change how states, and companies financing fossil fuels, approach climate risk.[14]

In the policy sphere, governments will also face pressure to revisit and strengthen their next NDCs ahead of upcoming climate summits. The Court’s message was clear: national pledges must be both ambitious and realistic enough to hold warming within 1.5°C. This will narrow the space for “creative ambiguity” in negotiations, where states often rely on vague wording to avoid hard commitments.[15] Negotiators now have legal clarity that certain arguments — such as unlimited national discretion — are off the table.

The opinion also further strengthens the role of Pacific Island nations and youth movements in shaping international law. What began as a bold proposal advocated by a group of 27 motivated students from a region disproportionately affected by climate change yet too often marginalised in global decision-making has culminated in one of the most far-reaching climate decisions ever issued. This reinforces the idea that small states and grassroots actors can drive global legal change, even against the resistance of larger powers.

For the private sector, the opinion signals that states must regulate corporate conduct consistent with climate obligations. That may accelerate existing trends in investor scrutiny, divestment, and risk assessment, making fossil fuel projects less attractive and more legally precarious.

Finally, the opinion’s symbolic weight cannot be overstated. By unanimously declaring climate change an existential threat and a legal duty, the ICJ has potentially equipped vulnerable states, negotiators, and campaigners with a powerful new lever. Yet, it is important to keep in mind that international law alone is insufficient: implementation ultimately depends on political will.[16] What remains to be seen is whether governments will uphold international law and translate the ICJ’s clarity into meaningful climate action — the decisive factor in whether this moment will usher in a new era for climate justice.

[1] https://www.icj-cij.org/sites/default/files/case-related/187/187-20250723-adv-01-00-en.pdf

[2] https://www.bbc.com/future/article/20250721-how-a-bold-student-plan-swept-to-un-world-court

[3] https://www.unep.org/news-and-stories/story/un-resolution-billed-turning-point-climate-justice

[4] https://digitallibrary.un.org/record/4008332

[5] https://www.icj-cij.org/advisory-jurisdiction  

[6] https://www.climateinthecourts.com/court-hearings-in-the-most-consequential-case-in-the-history-of-humanity-have-concluded-heres-what-you-need-to-know/

[7] https://www.icj-cij.org/sites/default/files/case-related/187/187-20250723-adv-01-00-en.pdf

[8] https://shs.hal.science/halshs-03286152/document

[9] https://www.whitehouse.gov/presidential-actions/2025/01/putting-america-first-in-international-environmental-agreements/

[10] https://www.unep.org/resources/emissions-gap-report-2024

[11] https://www.unep.org/news-and-stories/story/un-resolution-billed-turning-point-climate-justice ; https://www.outrageandoptimism.org/episodes/breaking-planetary-news-the-icj-climate-opinion-explained?hsLang=en; https://www.boell.de/en/2025/08/05/lotte-leicht-era-of-voluntary-climate-targets-over

[12] https://www.outrageandoptimism.org/episodes/breaking-planetary-news-the-icj-climate-opinion-explained?hsLang=en ; https://www.abc.net.au/news/2025-07-24/australian-barrister-representing-vanuatu-in/105566110

[13] https://www.carbonbrief.org/icj-what-the-world-courts-landmark-opinion-means-for-climate-change/

[14] https://www.outrageandoptimism.org/episodes/breaking-planetary-news-the-icj-climate-opinion-explained?hsLang=en

[15] https://www.outrageandoptimism.org/episodes/breaking-planetary-news-the-icj-climate-opinion-explained?hsLang=en

[16] https://www.outrageandoptimism.org/episodes/breaking-planetary-news-the-icj-climate-opinion-explained?hsLang=en

Further resources

This post draws on reporting and analysis from a range of sources, including UN documents, media coverage, and commentary from Think tanks and international law scholars. Key references are provided in the footnotes above with a list of further resources below.