Seven highlights from the ICJ’s Climate Change Advisory Opinion

August 31, 2025

What began as a campaign by law students at The University of South Pacific led to the Pacific Island State of Vanuatu seeking an Advisory Opinion on climate change from the principal judicial organ of the United Nations, the International Court of Justice (ICJ, the Court). The request was supported by over 130 States.

The Court was asked to clarify the obligations of States under international law to ensure the protection of the climate system, and what are the legal consequences, where by their acts or omissions States have caused significant harm to the environment with respect to:

(i) States, including, in particular, small island developing States, which due to their geographical circumstances and level of development, are injured or specially affected by or are particularly vulnerable to the adverse effects of climate change?

(ii) Peoples and individuals of the present and future generations affected by the adverse effects of climate change?

On 23d July 2025, the ICJ delivered its Advisory Opinion on Obligations of States in respect of Climate Change, and the results went beyond expectations of even the most hopeful climate campaigners.

Christina Figueres described it as: “Without a doubt, the most far reaching, the most comprehensive and the most consequential legal opinion we’ve ever had.” [1]

UN Secretary-General António Guterres said: “This is a victory for our planet, for climate justice and for the power of young people to make a difference.”

The Opinion, which was rendered unanimously by all members of the Court, is highly detailed, containing over 140 pages of analysis, including scientific literature and the applicable law that governs the questions.

This blog highlights seven of the most important parts of the Opinion.

Unequivocal recognition of climate science

The court recognised that the reports by the Intergovernmental Panel on Climate Change (IPCC) constitute the best available science on the causes, nature and consequences of climate change (para. 74).

The court noted that:

Climate change is an existential problem of planetary proportions that imperils all forms of life and the very health of our planet. (para. 456)
The well-being of present and future generations of humankind depends on our immediate and urgent response to it. (para. 1)
The problem of climate change requires the contribution of all fields of human knowledge, whether law, science, economics, or any other to develop a complete solution. (para. 1)
There is a rapidly closing window of opportunity to secure a liveable and sustainable future for all. (para. 87)

The court also observed that the most vulnerable people and systems are disproportionately affected by more frequent and intense extreme events. (para. 1)

2. A healthy environment is a precondition for the enjoyment of many human rights

The ICJ provided a clear explanation of the indivisibility of climate justice and human rights.[2]

The ICJ referenced General Assembly Resolution 76/300 evidencing a right to a clean, healthy and sustainable environment (paras 392-393)

Para. 373 states: “The environment is the foundation for human life, upon which the health and well-being of both present and future generations depend.” [3]

“The Court thus considers that the protection of the environment is a precondition for the enjoyment of human rights, whose promotion is one of the purposes of the United Nations as set out in Article 1, paragraph 3, of the Charter.”

Since Member States are parties to numerous human rights treaties, including the Universal Declaration of Human Rights, the Court concluded that they are required to guarantee the enjoyment of such rights by addressing climate change (para. 393).

3. States have an obligation to protect the environment from greenhouse gas (GHG) emissions

“The ICJ’s message is clear: climate obligations are not aspirational – they are legal, substantive, and enforceable.” [4]

The Court found that States are obligated to protect the climate system and other parts of the environment from anthropogenic GHG emissions, based on multiple legal sources, including:

the UN Charter,
climate treaties (UNFCCC, Kyoto Protocol, Paris Agreement),
customary international law,
environmental treaties,
and international human rights law. [5]

The most discussed obligations in the Advisory Opinion are the obligation under the Paris Agreement to limit global warming to 1.5°C above pre-industrial levels, [6] and a duty under customary international law to prevent significant harm to the environment. The Court has recognised that “[a] State is . . . obliged to use all the means at its disposal in order to avoid activities which take place in its territory, or in any area under its jurisdiction, causing significant damage to the environment of another State” (citing Pulp Mills on the River Uruguay (Argentina v. Uruguay), Judgment, I.C.J. Reports 2010 (I), p. 56, para. 101).

Climate obligations discussed by the Court include:

mitigation,
adaptation, and
cooperation duties, guided by principles such as sustainable development, precaution, and intergenerational equity.[7]

Finally, the climate change regime is not lex specialis (a specialised regime to displace general obligations), and should be read with other general international law obligations as being mutually reinforcing, “mutually supportive”,[8] and “inform each other” (para. 404).

4. Stringent Due Diligence Standards

The Paris Agreement encourages all parties to submit:

“Ambitious, economy-wide emission reduction targets, covering all greenhouse gases, sectors and categories and aligned with limiting global warming to 1.5°C, as informed by the latest science, in the light of different national circumstances” (para. 224).

Nationally Determined Contributions (NDCs) outline each country’s actions and should be submitted every five years. However, previously, some States and many commentators had argued that NDCs are entirely discretionary. This has been the main criticism of The Paris Agreement.

The Opinion is clear in that NDCs are not entirely discretionary and must satisfy certain standards – the States are under an obligation to exercise due diligence when preparing their NDCs, ensuring that, when taken together, are capable of achieving the 1.5°C temperature goal (para. 245).”[9]

The Court clarified that due diligence requires States parties “to use all the means at [their] disposal” with a view to fulfilling their international obligations (citing Pulp Mills on the River Uruguay (Argentina v. Uruguay), Judgment, I.C.J. Reports 2010 (I).

Due diligence requires the “highest possible ambition” and may include several key elements, such as:

Setting and updating robust national climate plans under the Paris Agreement;
Regulating the activities of private actors – interestingly, in para. 403 this is linked to international human rights law;
Providing support to more vulnerable nations;
Adoption of appropriate legal and regulatory measures, such as effective policies aimed at achieving deep, rapid, and sustained reductions in greenhouse gas emissions;
Actively assessing relevant scientific and technological information (para. 136);
Building the resilience of socio-economic and ecological systems, including through economic diversification and the sustainable management of natural resources (para. 257).

The Court acknowledged the principle of common but differentiated responsibilities and respective capabilities (para. 179), and acknowledged varying historical contributions, noting that the Least-Developed Countries, for example, contributed only 0.4% of cumulative CO2 emissions (para. 80).

5. Legal consequences

Inaction, or failure to act decisively, may constitute an internationally wrongful act – triggering consequences under the law of state responsibility. [10]

The ICJ confirmed that these remedies can apply for wrongful acts caused by cumulative greenhouse gas emissions (para. 429) or failures to regulate private actors (para. 438), provided they constitute a breach of an international obligation.

These obligations are erga omnes – owed to the international community as a whole (paras. 439ff). This means that any state may invoke responsibility, not just injured states.[11]

“Full reparation” goes beyond the Paris finance channels, and may include:

ecosystem restoration,
rebuilding defences,
and compensation for affected individuals.

The Court has discussed attribution and causation, stating that cases where reparation is claimed, it must be shown that the damage for which reparation is claimed has been factually and legally caused by a State (para. 422). Any exercise of attribution would need to be undertaken on the facts of the particular case – but even the mention of attribution is undoubtedly historic for an international court.

6. Not regulating private actors, especially fossil fuel companies, may constitute a wrongful act

Para. 428 states that: “A State may be responsible where, for example, it has failed to exercise due diligence by not taking the necessary regulatory and legislative measures to limit the quantity of emissions caused by private actors under its jurisdiction.”

Examples include the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies. Both may constitute an internationally wrongful act which is attributable to that State (para. 427).

The Court also emphasised that the internationally wrongful act in question is not the emission of GHGs per se, but the breach of conventional and customary obligations identified pertaining to the protection of the climate system from significant harm resulting from anthropogenic emissions (para. 427).

The Declaration of Judge Cleverland cited the IPCC’s concerns that the “international investment agreements may lead to ‘regulatory chill’, which may lead to countries refraining from or delaying the adoption of mitigation policies, such as phasing out fossil fuels” [12]

This Declaration clarifies that “.. 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.”

Investors of every kind might take note of this Declaration in their decision-making regarding fossil fuel investments.

7. What’s next?

Although the Advisory Opinion is not legally binding for nations, the hope is that the States will indeed act on “the highest possible ambition” and implement robust policies and actions to address climate change. If they do not, they may face legal consequences that in turn may compel them to act.

Jennifer Robinson (a human rights lawyer who represented the Republic of Vanuatu) said:

“This decision provides a roadmap to climate justice and will be game changing for negotiators. It will be game changing for litigators around the world. This decision will be used for strategic litigation in domestic and regional courts around the world.” [13]

We expect to see more climate litigation between states and against private actors. We know that even cases that are not successful in court have potential for profound impact, as parties realise that it’s cheaper and easier to act on climate change than face lengthy and expensive court proceedings. High profile litigation can also attract negative media attention, regardless of the outcome, and that in turn can influence risk assessments of investors and others.

We will soon see how influential this Opinion is on diplomacy, as COP 30 starts in Brazil this November. We will be able to assess whether it has given the desired diplomatic leverage for Small Island States and what difference it may make.

It’s impossible to predict exactly what will happen, but Julian Aguon (the leading Attorney in this case) summarised it best. He said: “[This Advisory Opinion] .. didn’t just open the door to a new era and that era being one of climate accountability, but it swung it wide open.” [14]

We will also undoubtedly be coming back to this Advisory Opinion over the coming months and years, as the depth and breadth of analysis included are also a very welcome gift to everyone who teaches climate change and climate justice.

You can read the full Opinion here.