International Court of Justice Advisory Opinion on States’ obligations in respect of climate change – Interview with Ana Kantzelis

Since 2 December, the International Court of Justice (ICJ) has been hearing oral submissions in the long-awaited advisory opinion proceedings on the obligations of states concerning climate change. On 13 December, the International Union for Conservation of Nature (IUCN) will address the ICJ during the final day of hearings. I spoke with 6 Pump Court Associate Member, Anastasia Kantzelis, who was part of the IUCN counsel team, about these historic proceedings and her involvement.

What is the background to this ICJ advisory opinion?

This ICJ advisory opinion request arose from a diplomatic effort led by Vanuatu, following advocacy by youth activists who put the issue on the agenda and encouraged states to support it. In March 2023, the UN General Assembly adopted a resolution requesting the opinion, co-sponsored by 132 countries, including the United Kingdom. The request asks the Court to clarify states’ obligations under international law to protect the climate system from anthropogenic greenhouse gas emissions and the consequences of failing to meet these obligations (paraphrased; the full text of the question is very broad and detailed).

This comes as progress to reduce emissions under the Paris Agreement has been slow, while the global carbon budget available to meet its goals—holding warming well below 2°C and pursuing efforts to limit it to 1.5°C—is rapidly depleting. Other international courts and tribunals, such as the ITLOS and the IACtHR, are also addressing related advisory opinion requests, reflecting growing frustration with political inaction and a shift toward international courts for accountability.

What is the procedure for requesting an ICJ advisory opinion?

A UN organ or international organisation authorised by the ICJ Statute submits a legal question. In this case, the UN General Assembly requested the opinion. States and authorised organisations provided written submissions, followed by comments on other participants’ submissions. Oral hearings are now underway, where participants present their positions before the Court. The Court is uploading the written materials submitted by each participant as they appear in oral hearings, allowing the process to be followed online. The judges will then deliberate and are expected to issue an advisory opinion early next year.

What is your involvement?

I was part of the counsel team for the International Union for Conservation of Nature (IUCN), an organisation with both state and non-state members dedicated to environmental protection. The IUCN, through its World Commission on Environmental Law (WCEL), was authorised by the Court under Article 66 of its Statute to participate in the proceedings. The team was led by Professor Christina Voigt.

What do you hope an advisory opinion achieves? 

I hope for clear legal guidance on states’ obligations under international law, both within the climate treaty regime and beyond. An advisory opinion that confirms the relevance of customary international law principles to climate harm would be significant. It could also define the standard of conduct required of states and clarify how climate treaties inform obligations under other regimes. This would strengthen accountability, provide coherence on legal standards, and help distinguish legal duties from areas requiring political solutions.

What issues will the Court be considering?

Not all submissions are public yet, but those uploaded online and statements made by participants during oral hearings already highlight some key issues.

A central question is whether the customary law obligation to prevent significant transboundary harm applies in the context of climate change, and if so, what standard of conduct it requires of states. If recognised, this obligation would bind all states, including those outside the Paris Agreement and the broader climate treaty regime. Failure to comply could carry legal, diplomatic, and geopolitical consequences, even for states that do not accept the Court’s contentious jurisdiction.

The Court may also examine how obligations under climate treaties interact with and inform obligations under other treaty regimes, and vice versa. It may also consider the legal status and application of broader principles of international environmental law, such as intergenerational equity and the right to a healthy environment.

ICJ advisory opinions are not formally binding on States, so why is this so significant?

An advisory opinion carries significant authority as a definitive statement of law from the same Court that would hear any subsequent contentious case on these issues. In the climate context, clarifying legal obligations and distinguishing them from matters of political negotiation could reduce uncertainty in the UNFCCC multilateral process. The opinion may also contribute to the recognition of customary international law, clarify how existing legal principles apply, and provide a foundation for state action. Its reasoning could also influence domestic courts, offering authoritative insights that reinforce accountability and guide climate-related obligations.

Will an advisory opinion change the behaviour of states?

It will certainly have an impact, though the extent depends on its content and how it is received. States are most likely to adjust their behaviour if the opinion challenges their international reputation or exposes them to legal risks. Civil society and other actors may also use the opinion to advocate for stronger climate action, including through national courts. However, it won’t be a cure-all. Some states may resist its conclusions, especially if they conflict with national interests. Still, even indirect effects—such as influencing international negotiations, reinforcing legal norms, and supporting domestic litigation—could play a crucial role in advancing climate action.

What are the risks of requesting an advisory opinion?

One risk is that the breadth of the question put to the Court could lead to a vague or overly cautious opinion if submissions do not clearly define the issues. This might limit its usefulness in guiding states or strengthening accountability. Resistance from powerful states could undermine the opinion’s authority, potentially challenging international institutions and the rule of law. It may also complicate diplomatic efforts, shifting focus from negotiations that could deliver immediate results.

However, even with these risks, the process of seeking an advisory opinion highlights critical legal questions and reinforces the importance of international cooperation on climate issues. The opinion, regardless of its immediate impact, has the potential to shape long-term legal standards, influence state behaviour, and support a more accountable global response to climate change.