“The Advisory Opinion Could Reshape Global Climate Governance.”

This week, the International Court of Justice (ICJ) began hearings in the globally anticipated landmark case on Obligations of States in Respect of Climate Change. The advisory proceedings address fundamental questions regarding state responsibilities in relation to the climate crisis, reparations, the rights of future generations, and other critical issues. What is the focus of these proceedings, and how might an Advisory Opinion from the ICJ impact global climate governance? We spoke with Tejas Rao, Marie-Claire Cordonier Segger, and Markus Gehring, three globally leading experts in climate law from the University of Cambridge.

1. Since Monday, the International Court of Justice has been holding hearings in what has been described as “the world’s biggest legal case”. In the Advisory Opinion requested by the United Nations General Assembly on the obligations of States in respect of climate change”, nearly 100 countries and several organizations are presenting their submissions. What is the case about?

This case originated from a law student-led initiative in Vanuatu that culminated in the United Nations General Assembly adopting Resolution 77/276 on March 29, 2023. The resolution, adopted by consensus among all 193 UN Member States, requested an Advisory Opinion from the ICJ on states’ obligations regarding climate change.  

The Court is being asked to address two fundamental questions: First, what obligations do states have under international law to protect the climate system and environment from greenhouse gas emissions, both for current and future generations? Second, what are the legal consequences for states whose actions or inactions have caused significant climate harm, particularly concerning vulnerable nations like small island developing states and the rights of present and future generations?

The Advisory Opinion is expected to have far-reaching implications. It could provide authoritative guidance on states’ climate obligations under international law, establish a framework for climate reparations, and potentially reshape global climate governance. The Court must consider various sources of international law, including the UN Charter, human rights covenants, environmental treaties, and customary international law.

What makes this case particularly significant is its potential to establish clear legal standards for climate action and accountability, while also breaking new ground in recognising the rights of future generations in international law. As noted by Vanuatu’s Minister of Climate Change, the true measure of success will be whether this process amplifies the voices of the most vulnerable and translates legal clarity into tangible action for climate justice. The student initiative and the Small Island States who led the campaign to “bring the world’s largest problem to the world’s highest court” deserve all our gratitude for their many years of hard work – we have been delighted to host them in the UNFCCC COPs during our Climate Law and Governance Days, and last year, they won the CLG Global Leadership Awards for their efforts.¹⁾

2. In addition to the student-led initiative you mentioned, small island states played a central role in the request for an Advisory Opinion. What are the main arguments of small island states regarding state responsibility, and how do they differ from the legal arguments of major polluters?

The small island states, led by Vanuatu and supported by other nations like Antigua and Barbuda, Fiji, and the Marshall Islands, present several core arguments regarding state responsibility:

First, they assert that international law imposes robust individual obligations on states to prevent climate harm, stemming from both customary international law and treaty obligations. They argue that the “no harm” principle requires states to prevent significant environmental damage to other states, and this principle applies to greenhouse gas emissions just as it would to other forms of transboundary pollution. Many interventions also made positive reference to the ITLOS Advisory Opinion highlighting that GHGs can count as pollution of the ocean.

Second, they contend that legal causation between specific states’ emissions and climate harm can be scientifically established. Barbados specifically countered claims about causation being too complex, arguing this is a matter of scope rather than causation, since the causes of climate change are direct, foreseeable, and proximate.

Third, they argue that human rights, especially those enjoyed by their current and future citizens, are directly impacted by climate change and they provided factual evidence as to the extent and cost of the impact already experienced in many small island states. They argue that human rights must inform the interpretation of international climate law and have a direct link to the climate crisis.

Fourth, they emphasise that historical responsibility matters legally. Several island nations provided evidence that major emitters were aware of potential climate harm for decades but continued with harmful activities regardless, breaching obligations of due diligence.

Fifth, they argue for comprehensive legal remedies, including:

• Cessation of harmful conduct
• Guarantees of non-repetition
• Reparations proportional to historical contributions
• Debt relief as compensation
• Compensation for both economic and non-economic losses
• Ecosystem restoration
• Guarantees for continued statehood stemming from international responsibility
• Several other legal consequences

In contrast, major polluting nations like the United States, Russia, Iran and Saudi Arabia present significantly different legal interpretations: 

First, they argue that climate obligations stem exclusively or primarily from the specific climate treaties (UNFCCC and Paris Agreement and the Kyoto Protocol), rather than broader international law. The US specifically argued that the Paris Agreement does not set standards for judging the adequacy of national commitments or allow for apportioning fair shares of the global carbon budget. They claim that the UNFCCC is lex specialis for all legal obligations related to climate change.    

Second, they reject the application of traditional “no harm” principles to climate change, arguing that these principles were developed for bilateral, clearly traceable harm rather than complex global phenomena like climate change. They also claim that the causality is too complex to assign international responsibility.

Third, they contest historical responsibility, with several nations arguing that legal obligations can only arise from the point when climate treaties entered into force or when scientific consensus about harm became clear (which they generally date to the 1990s).

Fourth, they emphasise that the Paris Agreement’s mechanisms for addressing loss and damage are political rather than legal obligations, and argue against applying traditional state responsibility principles to climate change. They also argue that any view to the contrary would undermine the possibility of a negotiated resolution for the problem.

Fifth, while many accept that an Advisory Opinion request unanimously by the General Assembly is an exemption, they argue to evaluate the admissibility of the request carefully and if found to be admissible for a very narrow interpretation of the question and not to be led by the formulation of the question for example by its reference to human rights.

The fundamental divide appears to centre around whether climate change should be treated as a unique phenomenon governed primarily by specific treaties, as major polluters argue, or whether it falls within the broader framework of international law regarding transboundary harm and state responsibility, as small island states contend. The resolution of this legal question could have profound implications for future climate action and compensation for climate damages.

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3. The ICJ is not the only international tribunal deciding on an Advisory Opinion request related to the climate crisis. Recently, the International Tribunal for the Law of the Sea delivered its Advisory Opinion on international law and climate change. The Inter-American Court of Human Rights is expected to deliver an Advisory Opinion on the climate emergency and human rights next year. Why do Advisory Opinions play such a central role in international climate law, and what do the requesting parties hope to achieve with them?

The primary motivation behind these requests stems from the frustration of climate-vulnerable states, particularly small island nations, with the slow pace and structural constraints of the UNFCCC process. The consensus-based nature of climate negotiations allows a small number of states to block meaningful progress, even as the window for effective climate action narrows. The 2020s represent a crucial decade for climate action, with scientific evidence indicating that states must cut emissions by nearly 50% by 2030 to maintain a realistic chance of achieving the Paris Agreement’s 1.5°C temperature goal. 

Through these Advisory Opinions, requesting parties aim to achieve several objectives. First, they seek to establish a more robust framework of “adjudicated law” alongside negotiated climate law. This could help clarify states’ legal obligations regarding emissions reductions and climate damages, potentially shifting the discourse from voluntary commitments toward more clearly defined legal responsibilities.

Second, these opinions could strengthen domestic climate litigation efforts. National courts often consider international tribunal interpretations when addressing climate-related cases, making Advisory Opinions potentially influential in domestic legal contexts. The success of cases like Urgenda in the Netherlands demonstrates how judicial decisions can drive more ambitious climate action at the national level. The impact on interpretation at the international and domestic level should not be underestimated. An authoritative opinion by an international court or tribunal carries significant interpretative weight in many legal systems around the world.

Third, requesting parties hope these opinions will provide clear legal principles to frame future climate negotiations. By establishing authoritative interpretations of international law as it relates to climate change, Advisory Opinions could help redistribute argumentative leverage between states in diplomatic discussions.

The distribution of requests across different tribunals – the ICJ, ITLOS, and the Inter-American Court – reflects a strategic approach to leverage each institution’s unique jurisdiction and expertise. The ICJ can address broad questions of international law, while ITLOS focuses specifically on law of the sea implications, and the Inter-American Court examines human rights dimensions. This multi-forum strategy aims to build a comprehensive legal framework addressing various aspects of the climate crisis.

It is possible that a far reaching and from a climate perspective positive Advisory Opinion could form the basis for subsequent contentious cases if a legal basis, for example in customary international law, for a bilateral claim could be found.

4. Let’s delve a bit deeper into the issue you raised about the consensus-based nature of climate negotiations and states being able to block meaningful progress. Unlike negotiated law, as is the case with international climate treaties, judicial decision-making does not rely on consensus among states. Given how fragile the current state of the international climate regime is, does an Advisory Opinion on states’ obligations in the climate crisis also carry risks, particularly regarding the legitimacy of a legally more bold Advisory Opinion?

The tension between judicial boldness and legitimacy presents a critical challenge for any Advisory Opinion on climate change obligations. International courts face a delicate balance: they must offer meaningful guidance while maintaining their institutional legitimacy and avoiding disruption to the existing climate regime.

The fundamental legitimacy challenge stems from the nature of climate change as a deeply political issue. When an international tribunal ventures beyond general principles to specify concrete obligations, it effectively engages in law-making rather than mere interpretation. This raises serious questions about institutional competence and democratic accountability. Why should judges, rather than elected political representatives, determine specific requirements for emissions reductions or establish frameworks for climate reparations?

The risk becomes particularly acute given the purposeful ambiguity built into many climate agreements. Terms like “common but differentiated responsibilities”, “respective capabilities”, “progression”, and even the classification of “developed countries” represent carefully negotiated compromises that enable broad participation in the climate regime. A judicial interpretation that dismantles these constructive ambiguities could undermine the delicate political balance that makes the current system workable, however imperfect it may be.

States that perceive an Advisory Opinion as judicial overreach might react by withdrawing support not only from the opinion itself but from the broader international climate framework. This risk is especially significant for major emitters whose participation is crucial for effective climate action. An expansive Advisory Opinion could thus paradoxically weaken rather than strengthen the international response to climate change. On the other hand, it is not new to advance negotiations through adjudication. In other fields, such as WTO law, this has been the method for many years and while it is new in the climate context, it is not new for international as a whole.²⁾

Moreover, the legitimacy of international courts themselves could suffer. If states widely reject an Advisory Opinion as exceeding judicial authority, it could damage the credibility of international tribunals more broadly. This institutional risk becomes particularly concerning when courts venture into territory that states have explicitly reserved for political negotiation.

Yet excessive judicial restraint carries its own legitimacy risks. If an international court merely restates accepted principles without offering meaningful guidance on their application to the climate crisis, it might be criticised for abdicating its responsibility to develop international law. Climate-vulnerable states, in particular, might view such restraint as undermining the court’s legitimacy as an institution capable of addressing urgent global challenges. Many of the climate vulnerable and small island countries intervened in an international court case before the ICJ for the very first time and their trust in this international institution will depend on an ambitious Advisory Opinion. 

The path forward requires carefully calibrated judicial reasoning that respects both political realities and legal principles. Rather than attempting to resolve all climate-related controversies, an Advisory Opinion might most effectively focus on clarifying foundational legal frameworks while leaving space for political processes to determine specific applications. This approach could help maintain legitimacy while still contributing meaningfully to international climate law’s development.

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