The ICJ's Climate Change Advisory Opinion: A New Era for States

The ICJ has spoken, and the message is clear: climate inaction carries legal consequences.

On 23 July 2025, the International Court of Justice issued its long-awaited Advisory Opinion on the Obligations of states in respect of Climate Change. Prompted by a UN General Assembly request led by Small Island Developing states, the Opinion marks a pivotal moment in international climate law. While not binding, it offers authoritative guidance on how existing legal obligations apply in the face of the climate crisis.

The world is at a turning point. The science is clear, and the consequences are visible: rising seas, extreme weather, and communities under threat. While international agreements like the UNFCCC, Kyoto Protocol, and Paris Agreement have laid important groundwork, the legal obligations of states under broader international law have often remained open to interpretation.

That is what makes the ICJ’s recent Advisory Opinion so significant. As the UN’s highest judicial authority, the Court has now provided much-needed clarity on the legal duties of states in the face of the climate crisis. Though not binding, the Opinion carries substantial legal and moral weight, and it is likely to become a cornerstone for future climate litigation, policymaking, and international cooperation. In short, it may well redefine the legal landscape of global climate governance. For governments, businesses, and individuals alike, the Opinion signals a shift: climate responsibility is no longer just a moral imperative. It is a legal one.

I. Background to the request: the Pacific's plea for clarity

The recent Advisory Opinion of the International Court of Justice (ICJ) finds its roots in a powerful call for justice from those most exposed to the consequences of climate change. Spearheaded by the Republic of Vanuatu and strongly supported by a coalition of Small Island Developing states (SIDS) and youth-led movements, the initiative brought to the UN General Assembly a compelling plea: to clarify the legal responsibilities of states in the face of a climate crisis they did little to cause, yet suffer from the most.

This global campaign underscored not only the existential threats posed by rising sea levels, extreme weather, and ocean acidification, but also the deep inequities embedded in a crisis largely driven by the historical emissions of industrialized nations.

In response, the UN General Assembly submitted two pivotal questions to the ICJ:

• First, what obligations do states have under international law to protect the climate system and the environment from anthropogenic greenhouse gas emissions, for the benefit of both present and future generations?
• Second, what are the legal consequences for states whose actions - or inactions - have caused significant harm to the climate system, particularly in relation to the rights of individuals and communities affected by climate change?

These questions laid the groundwork for what would become one of the most consequential legal inquiries into the intersection of environmental protection, human rights, and intergenerational equity.

II. Unpacking the Opinion: key legal principles and pronouncements

Before addressing the substantive questions, the Court meticulously affirmed its jurisdiction and the admissibility of the request. It underscored its role in providing authoritative legal guidance on questions put forth by UN organs, reaffirming that the advisory function is a crucial aspect of its mandate, distinct from contentious cases but equally vital for clarifying international law. This preliminary affirmation set a robust foundation for the legal weight of its subsequent findings.

A. The Climate Change Treaty Framework: specifics of the UNFCCC, Kyoto Protocol, and Paris Agreement

In its Advisory Opinion, the ICJ delivered a meticulous and authoritative interpretation of the core international treaties governing the global response to climate change: the UN Framework Convention on Climate Change (UNFCCC), the Kyoto Protocol, and the Paris Agreement. Far from being superseded, each instrument retains legal significance and contributes to a coherent and evolving treaty regime.

The UNFCCC: foundational principles and binding duties

The 1992 UNFCCC sets the stage with its overarching objective: to stabilize greenhouse gas (GHG) concentrations at levels that prevent dangerous anthropogenic interference with the climate system, within a time frame capable of ensuring ecosystems adaption, food production and sustainable development. It enshrines key principles - such as common but differentiated responsibilities and respective capabilities (CBDR-RC), the precautionary approach, sustainable development, equity, and intergenerational equity - which, while not stand-alone obligations, are essential interpretive tools for all treaty commitments.

The Court emphasized the Convention’s differentiated obligations for developed and developing states. All states shall prepare and submit GHG inventories, formulate national programmes and inform the COP, and endeavour in cooperation. Developed states (Annex I and II parties) bear enhanced responsibilities, including binding commitments under Article 4 to adopt national policies and take corresponding mitigating measures, further enhancing GHG sinks and reservoirs, report emissions, and support technology transfer. These obligations are both of result (e.g., submitting GHG inventories, formulating national programmes and taking corresponding mitigating measures) and conduct (e.g., best efforts to cooperate), and their breach may trigger state responsibility.

On adaptation, all parties must implement national adaptation programmes, while Annex II states are specifically required to assist vulnerable developing states - through funding, insurance, and technology transfer - underscoring the indispensable role of international cooperation, grounded in customary law and subject to a due diligence standard.

The Kyoto Protocol: specificity and accountability

The Kyoto Protocol operationalizes the UNFCCC’s principles by imposing quantified emission reduction targets on developed states for defined commitment periods. The ICJ clarified that the Protocol remains legally relevant, even in the absence of post-2020 targets, serving as a critical interpretive tool and a benchmark for assessing past compliance. Importantly, the Court affirmed that failure to meet these commitments may constitute an internationally wrongful act.

The Paris Agreement: a dynamic and binding framework

As the most recent and comprehensive instrument, the Paris Agreement builds upon and refines the existing legal framework. Despite the Agreement’s central goal is to limit global temperature rise to “well below 2°C” while pursuing efforts toward 1.5°C, the Court noted that 1.5°C has, through decisions of the Conference of the parties, become the agreed scientific benchmark. The Court recognized these decisions as “subsequent agreements” under the Vienna Convention on the Law of Treaties, thus carrying authoritative interpretive weight.

The Agreement retains the CBDR-RC principle, but introduces flexibility by referencing “different national circumstances.” This, the Court explained, does not dilute the principle but adapts it to evolving development contexts.

The Paris Agreement imposes both obligations of conduct (e.g., due diligence in mitigation efforts) and obligations of result (e.g., preparing and maintaining Nationally Determined Contributions, or NDCs).

A key clarification by the Court concerns NDCs. States are not merely required to submit them; the content must reflect a “progression” and the “highest possible ambition.” This is not a discretionary standard. Interpreting Article 4(3) of the Paris Agreement in light of its object and purpose - and the customary duty to prevent significant harm - the Court held that NDCs must be capable of making an adequate contribution to the temperature goal. The due diligence standard here is stringent, though context-sensitive, taking into account historical emissions and national capacities.

Moreover, the obligation to “pursue domestic mitigation measures” (Article 4(2)) is a substantive duty of conduct, extending to the regulation of private actors. The standard of due diligence is stringent.

On adaptation, Article 7(9) requires states to engage in planning and implementation of adequate adaptation measures with due diligence. The Agreement also reinforces obligations of international cooperation - financial, technological, and capacity-building - as essential tools for addressing the climate crisis.

Through this comprehensive review, the ICJ has unequivocally affirmed that the UNFCCC, Kyoto Protocol, and Paris Agreement impose binding and enforceable obligations on state parties to protect the climate system from anthropogenic GHG emissions.

B. Customary international law: universal states’ obligations on climate change

In addition to interpreting treaty law, the ICJ delivered a significant contribution by clarifying the role of customary international law in the climate context. It confirmed that all states, regardless of treaty participation, are bound by core obligations rooted in long-standing international practice.

First, the Court reaffirmed the customary duty to prevent significant environmental harm, explicitly extending it to the climate system. This obligation, grounded in due diligence, requires states to take all reasonable measures to avoid causing climate-related damage - whether through their own actions or those of private actors. The Court stressed that the complexity and global nature of climate change do not diminish this duty. On the contrary, the standard of care is particularly high, requiring states to act with vigilance, adopt effective mitigation policies, and consult with others when their actions may have broader impacts. The principle of common but differentiated responsibilities continues to apply, adjusting expectations based on each state’s capacity.

Second, the Court underscored the customary duty to cooperate in protecting the environment. Climate change, as a shared global challenge, demands coordinated international action. While states have some flexibility in how they cooperate, this cannot be used to justify inaction or shield efforts from scrutiny. Cooperation must be genuine, transparent, and effective.

Finally, the ICJ clarified the relationship between customary and treaty law. While distinct, the two bodies of law are complementary. Compliance with climate treaties, when interpreted in good faith, may indicate alignment with customary obligations - but it is not a substitute. Customary duties apply universally, and even non-party states must demonstrate that their conduct meets these fundamental legal standards.

C. Obligations under other environmental treaties and the Law of the Sea

The Court further broadened its legal analysis by considering obligations under other significant international environmental treaties and the crucial framework of the law of the sea, recognizing their synergistic relationship with climate change obligations.

The ICJ examined the relevance of agreements such as the Vienna Convention for the Protection of the Ozone Layer and its Montreal Protocol, the Convention on Biological Diversity, and the United Nations Convention to Combat Desertification. The Court concluded that obligations stemming from these instruments significantly “contribute to the protection of the climate system and other parts of the environment”. This highlights an integrated approach to environmental governance: states are expected to consider their obligations under these environmental treaties when implementing their climate change commitments (under treaties and customary international law), and vice-versa. This perspective reinforces the understanding that environmental protection is a holistic endeavour, where actions in one domain can have profound impacts on others.

A particularly significant segment of the Opinion delved into United Nations Convention on the Law of the Sea (UNCLOS), the overarching treaty governing maritime activities. The Court unequivocally found that anthropogenic greenhouse gas (GHG) emissions can be characterized as “pollution of the marine environment” within the meaning of UNCLOS. Consequently, Part XII of UNCLOS, dedicated to the protection and preservation of the marine environment, is directly applicable in the context of climate change and anthropogenic GHG emissions. The ICJ identified several key obligations under UNCLOS relevant to climate action. States have a positive obligation to take measures to protect and preserve the marine environment, and a negative obligation not to degrade it. Furthermore, states are obliged to take “all necessary measures” to prevent, reduce, and control pollution from all sources, with the ultimate aim of preventing its occurrence. While not requiring immediate cessation of GHG-induced marine pollution, the Court clarified that the standard of due diligence in implementing these measures is stringent. What constitutes “necessary” must be assessed objectively, taking into account the best available science (including IPCC findings), international climate change rules and standards, and the state's capabilities. Further obligation highlighted are the following: a requirement for states to ensure that activities under their jurisdiction do not cause significant harm to other states or areas beyond national jurisdiction; a continuing obligation to cooperate through research, data sharing, and the development of international rules; and the duty to conduct environmental impact assessments for activities likely to cause substantial marine pollution, including those affecting areas beyond national jurisdiction.

This ICJ stance on UNCLOS is highly significant, complementing the ITLOS Advisory Opinion issued on May 21, 2024, at the request of the Commission of Small Island states on Climate Change and International Law. Last year, the ITLOS Opinion similarly concluded that anthropogenic GHG emissions constitute marine pollution under UNCLOS and affirmed a “stringent” due diligence obligation for states to take “all necessary measures” to prevent, reduce, and control this pollution, guided by the best available science. Both opinions, though non-binding in contentious cases, strongly reinforce the applicability of the law of the sea to climate change, creating significant legal pressure on states to regulate their emissions to protect the oceans.

The ICJ emphasized that environmental protection must be approached holistically - states cannot address climate change in isolation from their broader legal duties under the law of the sea and other environmental treaties.

D. Obligations under International Human Rights Law: the indivisible link

In a profound articulation that solidifies a growing trend in international jurisprudence, the ICJ firmly established the indivisible link between climate change and human rights. The Court's Opinion makes it unequivocally clear that states have obligations under international human rights law to protect individuals and communities from the adverse impacts of climate change.

Specifically, the ICJ identified threats to the right to life, the right to health, the right to an adequate standard of living (encompassing access to food, water, and housing), the right to privacy, family and home, and critically, the rights of vulnerable groups such as women, children, and indigenous peoples. This broad recognition means that climate inaction or insufficient action by states can directly translate into human rights violations, opening new avenues for accountability and legal claims.

A cornerstone of this part of the Opinion is the Court's explicit recognition that a clean, healthy and sustainable environment is a precondition for the enjoyment of many other human rights. This pronouncement significantly elevates the legal standing of this emerging human right, reinforcing its foundational role in human well-being and obliging states to guarantee its protection.

Court also addressed the territorial scope of universal human rights treaties, reiterating its prior recognition that such treaties can apply when a state exercises jurisdiction outside its territory. This point holds particular relevance in the climate context, suggesting that a state's human rights obligations might extend to harms caused by its climate-related activities (e.g., GHG emissions) that impact individuals and communities beyond its geographical borders.

In its overarching conclusion for this section, the Court underscored that the full enjoyment of human rights cannot be ensured without the protection of the climate system and other parts of the environment. To guarantee these effective human rights, states are required to take necessary measures to protect the climate system. These measures are comprehensive and include, but are not limited to, taking robust mitigation and adaptation actions (with due regard for human rights protection), adopting appropriate standards and legislation, and crucially, regulating the activities of private actors that contribute to climate change and its human rights impacts. States must integrate their human rights obligations when implementing climate and environmental commitments, and vice-versa. This necessitates a comprehensive legal lens for policymakers, ensuring that climate action is not only effective in reducing emissions and building resilience but also fully compliant with, and indeed protective of, fundamental human rights.

III. Determination of state responsibility in the climate change context

The ICJ acknowledged that climate change presents a particularly complex challenge for the traditional rules of state responsibility. The global and cumulative nature of greenhouse gas emissions - spread across time, borders, and both natural and human sources - raises difficult questions around attribution and causation. Still, the Court was clear: these complexities do not prevent the application of existing international legal rules.

On attribution, the Court reaffirmed that the conduct of any state organ is attributable to the state under established principles of international law. In the climate context, the wrongful act is not the emission of GHGs itself, but the failure to meet specific treaty or customary obligations to protect the climate system. This could include inaction or inadequate action in areas such as fossil fuel licensing, subsidies, or emissions regulation.

Importantly, the Court addressed the role of private actors. It clarified that while private conduct is not directly attributable to the state, states are nonetheless responsible for regulating such conduct under the due diligence standard. In other words, a state may be held internationally responsible if it fails to adopt or enforce adequate laws and policies to control emissions from private entities operating within its jurisdiction. This includes oversight of industries, corporations, and other non-state actors whose activities contribute significantly to climate change. The Court emphasized that the duty to regulate private actors is not optional - it is a core component of a state's climate obligations.

The Court also noted that, despite the diffuse nature of emissions, science now allows for the quantification of each state’s contribution - past and present - to global emissions. This makes it possible, in principle, for any injured state to invoke the responsibility of another state that has breached its obligations and caused harm to the climate system.

On causation, the Court distinguished between establishing a breach of obligation and proving causation for the purpose of reparation. It rejected the idea that causation is either impossible or should be presumed in climate cases. Instead, it reaffirmed its existing standard: a “sufficiently direct and certain causal nexus” must be shown between the wrongful act and the harm suffered. In climate litigation, this involves two steps - first, establishing that a climatic event or trend is linked to human-induced climate change (which can often be done through scientific evidence), and second, showing that the damage can be traced back to a particular state’s wrongful conduct. While this may be more complex than in traditional pollution cases, the Court confirmed it is legally feasible and must be assessed case by case.

Finally, the Court made a significant statement on the nature of climate obligations: many of them are erga omnes - owed to the international community as a whole. This includes customary duties like preventing transboundary harm, as well as treaty-based obligations under the UNFCCC and Paris Agreement. As a result, any state - or any party to the relevant treaties - has legal standing to hold another state accountable for failing to meet its climate commitments.

Legal Consequences Arising from Wrongful Acts

The Court unequivocally reaffirmed the fundamental principle that every internationally wrongful act of a State entails its international responsibility. In the climate change context, such wrongful acts could range from breaches of specific treaty obligations, like a State's procedural duty to prepare, communicate, or implement its Nationally Determined Contributions (NDCs) under the Paris Agreement, to breaches of customary international law obligations, such as failing to regulate GHG emissions under its due diligence duty to prevent significant harm, or neglecting to conduct environmental impact assessments (EIAs). While acknowledging it could not specify precise consequences in the abstract given the advisory nature of the proceedings and the case-specific dependence on the breach and harm, the Court broadly stated that breaches of climate obligations may trigger the “entire panoply of legal consequences” available under the law of State responsibility. Importantly, the Court stressed that a breach does not extinguish the responsible State's underlying duty to perform the obligation.

These consequences include the duty to cease the breach of climate obligations and to offer assurances and guarantees of non-repetition to prevent future breaches, but extends to the duty to make reparation. Rooted in the principle that full reparation must “wipe out all the consequences of the illegal act,” responsible states are obligated to make full reparation for the damage caused. This can take various forms:

• Restitution, where possible, aims to restore the situation to what it was before the breach. In the climate context, this might involve rebuilding infrastructure or restoring ecosystems - though such remedies are often complex and context-dependent.
• Compensation is required when restitution is not feasible. The Court acknowledged the challenges in quantifying climate-related damage but confirmed that compensation may be owed if a clear causal link can be established. In some cases, this could take the form of a global sum, based on available evidence and equitable considerations.
• Satisfaction addresses non-material harm - such as injury to dignity or legal standing - that cannot be remedied through restitution or compensation. This might include formal apologies, public acknowledgments, or educational initiatives, depending on the nature of the breach.

This framework also reinforces the accountability of states for failing to regulate private actors. The Court emphasized that states have a duty to exercise due diligence in overseeing emissions from private entities. If a state fails to enact or enforce adequate laws to control such emissions, it may be held responsible - not for the private conduct itself, but for its own regulatory inaction.

IV. Implications for states and Non-State Actors

The ICJ's Advisory Opinion, while non-binding in the sense of a contentious judgment, carries immense persuasive authority. It clarifies and strengthens existing international law, providing a potent tool for various actors, influencing legal and political landscapes far beyond the Court's chambers.

A. For Developed Nations: heightened responsibility and accountability

For developed nations, with their historical emissions and greater financial and technological capacities, the Opinion signals a heightened level of responsibility and accountability. The Court's emphasis on the stringent due diligence standard and the principle of common but differentiated responsibilities means these states will face increased legal and political pressure to enhance their emission reduction targets, provide more substantial climate finance, and ensure their domestic policies align with international environmental and human rights obligations.

Their conduct, particularly concerning fossil fuel activities and their regulation of private actors, will face intensified scrutiny. In fact, the Court underscored that states have a duty to regulate private actors under their jurisdiction. This means that governments cannot turn a blind eye to emissions from corporations or other non-state entities. If a state fails to enact or enforce adequate laws to control such emissions, it may be held responsible for breaching its due diligence obligations.

Failure to demonstrate robust efforts could lead to increased climate litigation both domestically and, as the Court clarified, potentially internationally through State responsibility claims.

The Opinion will likely empower civil society, human rights advocates, and climate litigators to exert greater pressure on states and corporations to comply with these clarified obligations. It provides a robust legal foundation for national courts to interpret domestic laws in line with international obligations and for civil society to demand higher standards of climate governance.

B. For Developing Nations: support, capacity building, and climate justice

For developing nations, especially Small Island Developing states and Least Developed states who are on the front lines of climate impacts, the Opinion offers significant legal leverage. It reinforces their long-standing calls for climate justice, emphasizing the legal imperative for developed nations to provide adequate support for mitigation, adaptation, and to address loss and damage. The clarity on the right to a clean, healthy, and sustainable environment, and the linkages to other human rights, strengthens their position in international negotiations and provides a firmer legal basis for advocating for their rights and seeking redress for climate harms already endured or projected.

The ambitious goals and stringent obligations articulated by the ICJ require substantial global resources. A key challenge will be ensuring that the principles of common but differentiated responsibilities and respective capabilities translate into tangible and sufficient financial investment, technological transfer, and capacity building, particularly for developing nations. Equitable climate action is predicated on robust international support, and the Opinion implicitly calls for greater solidarity and resource mobilization to enable all nations to meet their legal duties and ensure sustainable development.

C. For International Organizations: strengthening global governance

The ICJ's advisory opinion significantly bolsters the mandate and urgency of international bodies involved in climate governance. By providing a clearer, more authoritative legal framework, the Opinion enables these organizations to exert greater influence in facilitating state compliance, promoting more ambitious climate action, and strengthening existing global governance mechanisms. It underscores the importance of science-based policymaking, aligning with the work of bodies like the Intergovernmental Panel on Climate Change (IPCC), and encourages a more robust, legally informed approach to multilateral climate efforts.

D. For businesses and investors: navigating evolving legal landscapes and enhanced ESG scrutiny

The reverberations of the ICJ's Opinion will undoubtedly be felt deeply within the corporate and financial sectors. While directly addressed to states, the Opinion's clarification of state obligations translates into new and enhanced expectations for businesses operating within national jurisdictions and globally. The Court's explicit mention of states' obligation to regulate the activities of private actors as a matter of due diligence is a particularly salient point for corporate responsibility.

What businesses can expect: increased due diligence, disclosure demands, and litigation risks

Businesses should anticipate a significant tightening of regulatory frameworks as states move to align their domestic laws with their clarified international obligations. This will lead to increased demands for climate-related due diligence across supply chains, requiring companies to rigorously assess, mitigate, and report on their direct and indirect GHG emissions and broader climate impacts. Transparency will become paramount, with expanded mandatory and voluntary climate disclosure requirements (e.g., in line with IFRS S2, CSRD, or various national climate reporting mandates) becoming the norm, necessitating robust internal systems for data collection, verification, and reporting.

Furthermore, the Opinion provides a powerful new foundation for climate litigation. Companies perceived as contributing significantly to climate change, or those engaged in “greenwashing” face heightened risks of lawsuits from governments, NGOs, affected communities, and even shareholders.

The emphasis on the link between climate change and human rights, as well as the duty of states to regulate private actors to prevent harm, means that corporate activities can increasingly be scrutinized through a human rights lens. Sectors heavily reliant on fossil fuels, or those with large carbon footprints, should re-evaluate their transition plans, potentially facing legal challenges related to stranded assets or inadequate decarbonization strategies.

V. Conclusion: charting a course towards climate justice and accountability

The International Court of Justice's Advisory Opinion on the Obligations of states in respect of Climate Change is far from a mere academic exercise; it is a profound and timely declaration that reinforces the urgency of climate action and firmly entrenches it within the realm of international law. The Court has spoken with a unanimous voice, leaving no room for doubt: climate action is not merely a policy option, but a legal obligation that demands stringent due diligence from all states.

By unequivocally linking climate change to fundamental human rights, codifying stringent due diligence obligations, and clarifying the applicability of State responsibility, the Opinion underscores that delayed action is no longer merely imprudent, but potentially legally culpable. It serves as a powerful reminder of the global community's collective duty to confront this crisis with immediate and decisive measures.

This Opinion is a critical catalyst, pushing states towards more ambition and stronger international commitments. It provides a potent legal tool for climate advocates, civil society, and vulnerable nations to demand greater accountability from high-emitting states and to accelerate the global transition to a sustainable future. It empowers dialogue, negotiation, and potentially, litigation, driving enhanced ambition across all levels of governance.
The future of our planet hinges on shared responsibility and decisive action. While the ICJ has illuminated the legal path, true progress requires unwavering collective action from states, international organizations, non-state actors, and individuals alike.

For our firm, and indeed for our clients navigating the complexities of ESG, this Opinion reinforces the critical importance of integrating climate risk and opportunity into core strategies. It mandates a proactive approach to compliance, robust due diligence, and genuine commitment to sustainability, collectively forging a just and sustainable future for all generations.