What the ICJ’s advisory opinion on climate change means for military emissions

Pathway to the advisory opinion

The last 36 months have seen numerous outputs from international courts on the extent to which states must act to mitigate and adapt to climate change. Yet the International Court of Justice (ICJ) advisory opinion of 23rd July 2025 — nicknamed the “biggest case in human history”, and “the most significant development in international climate law since the adoption of the Paris Agreement” — is distinguished by its possible significance for military and conflict emissions. This post unpacks these connections.

An advisory opinion is a judicial opinion that offers legal guidance or clarification on a specific legal matter. The ICJ is the UN’s principal judicial organ and provides such authoritative legal opinions following requests from UN bodies and institutions. Following years of advocacy by Vanuatu — initiated by law students — in 2023, the UN General Assembly adopted a resolution requesting the ICJ’s advisory opinion on two questions: what are states’ legal obligations to address climate change, and what are the consequences for failing to meet them?

To answer these questions, the court heard from a variety of states and international organisations, reaching the “highest level of participation in a proceeding”. The uniqueness of the proceedings was also marked by what were termed “phantom experts”: representatives of the Inter-governmental Panel on Climate Change (IPCC) who met with the judges in private. The unanimous opinion that resulted represents the world’s first authoritative, science-based legal roadmap on states’ duties to mitigate and adapt to climate change, and to address its damage.

Determining which law applies

First and foremost, the court had to clarify which law governs states’ actions regarding climate change. While some states had argued that the field is regulated only by climate change treaties, the court concluded that the regulatory space in fact is built from many different sources: the climate change treaty framework, customary international law relating to climate change, other environmental treaties, the law of the sea and related issues, and international human rights law. This means that states have duties on climate change even if they are not parties to treaties like the Paris Agreement.

Therefore, worries that existing climate law is insufficient are unfounded. Instead, states and national and international judges can interpret and apply a wide range of norms when considering the legality of particular actions, or inaction.

Determining the duties of states

Having established the applicable law,  the ICJ then delved  into the obligations that arise from it. 

The court confirmed that states party to climate treaties must adopt the measures required to fulfil the objectives of the treaties. For example, states have a duty to mitigate their emissions through increasingly restrictive emissions targets — as set out in Nationally Determined Contributions (NDCs) — to meet the goal of limiting warming to 1.5 degrees.

States must also manage the emissions of the private sector through regulation and enforcement, and prevent serious environmental damage through exercising due diligence. The court also identified the obligation to cooperate, especially in the provision of support to vulnerable nations. 

Finally, states must uphold fundamental rights. The court noted the essential need to protect the right to a clean and healthy environment as a prerequisite for the enjoyment of other human rights. In doing so it put a halt to the debate over whether this is a separate human right. 

On the question of state responsibility, the court found that failure in these duties, for example by continuing fossil fuel production, consumption, licensing and subsidies, may represent a breach of international law. That would open the door to legal procedures around reparations for states harmed as a result of that inaction. 

What about military and conflict emissions?

Despite the density of the document, the ICJ did not address everything. Some authors have already identified missed or intentionally avoided opportunities, such as guidance on the legal rights of nature, or deeper engagement with the rights of future generations. Also, the judges themselves have criticised the panel for an overly cautious approach. 

The military’s impact on the climate is one such elusive topic. The court did hear about military and conflict emissions in Palestine’s oral statement to the court. That included the emissions caused by the conflict and expected by post-conflict reconstruction, as well as the barriers that occupation can place on climate adaptation and mitigation.

Reference was also made in the independent declaration of one of the presiding judges, Judge Cleveland. Such declarations are published in parallel and are separate from the main advisory opinion, allowing judges to publish their individual reasoning or perspective on specific aspects of the case. The declaration goes into a high degree of detail into military and conflict emissions, arguing that states should pay increased attention to armed conflicts and military emissions as sources of emissions, assessing, reporting and mitigating them (para 18, 19). Judge Cleveland also argues that evaluations of harm to the climate system within a state’s jurisdiction or control must also include the emissions caused by the destruction of carbon sinks and similar activities through conflict (para 18). Failure to pursue this specific duty also might result in state responsibility (para 20). While Judge Cleveland’s declaration was welcome, it is the main advisory opinion that counts and, while it did not directly mention military or conflict emissions, it could still indirectly influence them.

Relevance for military emissions

In defining the duties of states towards their climate obligations, the court indirectly addressed military emissions reporting and decarbonisation measures. While reporting of peacetime military emissions is currently voluntary, keeping at or below 1.5 degrees inevitably demands that military emissions are properly accounted for in national inventories and NDCs, and mitigated. A complete and undistorted picture of the climate crisis requires that states treat their militaries like other high-emitting sectors.  

The court’s criticism of fossil fuels and production subsidies as a potential violation of states’ climate obligations is also notable, especially as it’s also applicable to non-party member states. Militaries remain heavily dependent on fossil fuels; the Pentagon the world’s leading institutional consumer of petroleum. Increased pressure to phase out fossil fuels would have repercussions for militaries, potentially exposing their dependencies and strengthening arguments for decarbonisation. 

Relevance for conflicts

While the court didn’t analyse the relationship between climate change duties and international humanitarian law in detail, it did recognise its relevance (para 173). This is a reminder that norms that prohibit employing warfare methods that cause disproportionate harm to the environment also include harm to the climate system. It follows that emissions estimates should become part of proportionality tests for military actions. Importantly these should not be restricted solely to individual attacks, but should also be determined for campaigns or strategies. 

Broader reach of duties

The court reminded wealthy states of their leadership role in reducing emissions, including the need to mobilise climate finance for fragile and conflict-affected states to support adaptation and mitigation. Ever increasing military spending has pulled funding from these critical areas; in 2022 30 times more was spent on military budgets than climate finance. Fears over litigation might encourage some states to reflect on this imbalance.

What happens next?

ICJ advisory opinions carry significant weight due to the court’s authority and reputation. Moreover, the judges’ declarations and opinions represent the views of the field’s leading legal minds. Thus, the opinion and its amendments will influence regional and national courts and legislators. It will also prove useful for policymakers as a depersonalised and protective justification for stricter measures against polluters, especially in cases where such an agenda does not not comply with the will of their voters. This also applies to the military sphere, an area yet to be touched by strategic climate litigation. 

Yet while improved reporting of military emissions should be feasible, discussion and compromise will be needed from state and other stakeholders on how conflict-linked emissions are determined and where they are reported, particularly if this is to be beyond the territory where they were generated. 

Post-conflict climate justice

The principles of climate justice run through the opinion and it should encourage debate over climate reparations from armed conflict, such as in Palestine or Ukraine. This might encompass the loss of climate adaptation and mitigation capacities. Restitution could include the restoration of destroyed carbon sinks such as forests, and financing and technologies to support green energy transitions as part of recovery processes.

Of course, the law alone is no panacea. The court recognised the critical importance of the “contribution of all fields of human knowledge, whether law, science, economics or any other” and “human will and wisdom — at the individual, social and political levels — to change our habits, comforts and current way of life in order to secure a future for ourselves and those who are yet to come” (para 456). This must surely include concessions by the military.

 

Madara Melnika is a Latvian PhD Researcher at the European University Institute, where she focuses on the environmentally relevant regulations of international security organisations. She obtained an LL.M. from UConn in Human Rights and Social Justice with a special focus on Post-Conflict Justice and Environment, and is a member of CEAN and the Community of Practice on Environment, Climate, Conflict, and Peace (ECCP).