Introduction
The International Court of Justice’s (ICJ) 2025 Advisory Opinion on the Obligations of States in Respect of Climate Change[1] marked a milestone moment in international law. It underscored the growing urgency of addressing the global climate crisis and reaffirmed the central role of States in fostering cooperation to protect the environment. At the same time, it acknowledged the increasing importance of non-State actors in the climate discourse. It is this very context that displays the enduring character and true value of Professor Koen De Feyter’s work on the duty to cooperate and non-State actors.
Professor De Feyter drafted the thematic study on the duty to cooperate and non-State actors, published in July 2023 by the Expert Mechanism on the Right to Development.[2] The study offers an original exploration of these issues. It provides an essential framework for understanding the interplay between the right to development and the responsibilities of non-State actors. In light of the ICJ’s Advisory Opinion, Professor De Feyter’s work takes on even greater significance as it invites further reflection on the modalities of international cooperation and the gamut of actors involved in the face of climate change.
This contribution seeks to assess Professor De Feyter’s study on the duty to cooperate, exploring its relevance and impact within the context of the recent ICJ’s Advisory Opinion. By examining both the study and the Advisory Opinion, this piece reflects on how Professor De Feyter’s conceptualization of the duty to cooperate in the context of the right to development can enrich our understanding of global legal frameworks for climate action.
The Duty to Cooperate in International Law
The duty to cooperate is a foundational principle in international law,[3] and it represents ‘a central obligation under the climate change treaties and other environmental treaties.’[4] It draws its source on the natural need for States to work collectively to address issues that transcend national borders. Historically, it has found expression in various legal regimes, from environmental law to human rights and development. In the realm of climate change, the duty to cooperate assumes critical importance, as no single State can combat climate change independently. It is a phenomenon that transcends any single territorial or non-territorial unit and, for that matter, any boundary one might conceive. Therefore, it necessitates a multilateral approach.
The duty to cooperate also finds resonance in the substance of sustainable development and the right to development. The 1972 Stockholm Declaration[5] and the 1992 Rio Declaration[6] both emphasize the importance of international cooperation in ensuring sustainable development.
This duty, however, is not solely a legal obligation of States. It extends to other actors in international relations, including international organizations, private corporations, and civil society. In recent years, the role of non-State actors has gained increasing prominence, particularly in the context of global environmental governance.
Professor Koen De Feyter’s Thematic Study on the Duty to Cooperate and Non-State Actors
In this thematic study, Professor De Feyter takes a comprehensive approach to the duty to cooperate, analyzing its implications for both state and non-state actors. Departing from the traditional view of the duty to cooperate as a State-centric obligation, this study explores the role of non-State actors, such as multinational corporations, non-governmental organizations, and other civil society groups, in promoting international cooperation for the implementation of the right to development.[7]
One of the study’s key contributions is its exploration and clarification of the interrelationship between the right to development and the duty to cooperate. It highlights the centrality of human rights to sustainable development.
Professor De Feyter also provides an insightful analysis of the practical mechanisms through which non-state actors can contribute to international cooperation, such as through advocacy, litigation, and the promotion of norms and standards.[8] His study offers a new perspective on the traditional State-centric narrative of international law, advocating for a more inclusive and multi-actor approach to global challenges, including climate change.
The ICJ’s Advisory Opinion on Climate Change
The ICJ’s 2025 Advisory Opinion on the Obligations of States in Respect of Climate Change was requested by the UN General Assembly to clarify the legal obligations of States in relation to climate change. The Court affirmed that States have an obligation to prevent harm to the climate system, noting that climate change poses a threat to the enjoyment of human rights, particularly the right to life, health, food and housing.[9]
While the ICJ was requested to render an opinion on the obligations of States, it did however also acknowledge those of ‘non-State actors within their jurisdiction or effective control,’[10] and the role of international organizations in climate governance.[11] Significantly, the Court also notes that ‘certain treaties enable actors other than States, such as individuals or other private actors, to bring claims against States on the international plane.’[12]
Given the questions put to it, the Court obviously refrained from offering a detailed analysis of the role of non-State actors in its legal framework, but it clearly acknowledged the potential of private actors to cause ‘significant harm to the environment,’[13] and ‘the regulation of the activities of private actors’[14] as a mitigating measure or as a matter of States’ due diligence obligation. In this connection, Professor De Feyter’s study had also articulated and, indeed, offered an even more detailed account of the right of the States to regulate, ‘within the confines of their obligations under international law, including human rights law,’[15] as well as ‘the right to legislate on the conduct of foreign non-State actors,’[16] and ‘the duty to supervise the conduct of non-State actors and to act when necessary, including through legislation or other forms of standard setting.’[17]
Assessing Professor De Feyter’s Study and the ICJ’s Advisory Opinion
Professor De Feyter’s study on the duty to cooperate provides a valuable lens through which to view the ICJ’s Advisory Opinion on climate change. Both share a commitment to multilateral cooperation as essential to addressing the global climate crisis. Professor De Feyter’s emphasis on the role of non-State actors aligns with the ICJ’s recognition that climate change cannot be tackled by States alone. While the Court highlights State obligations, Professor De Feyter’s work calls for a broader conception of the duty to cooperate, one that includes the responsibility of corporations, NGOs, and other actors in the global climate regime.
Professor De Feyter’s study also adds depth to the ICJ’s Advisory Opinion by framing the duty to cooperate within a human rights context. The ICJ acknowledged the links between climate change and human rights, noting that ‘the full enjoyment of human rights cannot be ensured without the protection of the climate system and other parts of the environment.’[18]
Additionally, Professor De Feyter’s work provides practical tools for understanding how non-State actors can be held accountable for their role in international cooperation. This focus on accountability is particularly relevant in light of the ICJ’s emphasis on the importance of international legal frameworks for climate action.
Conclusion
Professor Koen De Feyter’s thematic study on the duty to cooperate offers a farsighted and timely perspective on the evolving role of States and non-State actors in global climate governance, as it can be witnessed from the ICJ’s Advisory Opinion. His work calls for a broader, more inclusive understanding of the duty to cooperate that transcends the traditional State-centric paradigm. In this regard, Professor De Feyter’s contribution is critical not only for its scholarly insight but also for its practical application in contemporary global challenges, particularly those related to human rights and environmental protection.
The ICJ’s Advisory Opinion, while a landmark moment in affirming the central role of States in addressing climate change, constructs an important legal edifice that still leaves much to be explored with respect to non-State actors; a result essentially conditioned by the very questions posed by the General Assembly. In this connection, Professor De Feyter’s study complements the ICJ’s view, by framing the duty to cooperate as a multi-actor responsibility, one that acknowledges the indispensable role of international organizations, corporations, and civil society groups in addressing global environmental crises. His focus on the role of non-State actors in ensuring that States meet their international obligations reinforces the importance of shared responsibility in the pursuit of global climate justice.
Professor De Feyter’s insistence on integrating human rights into the duty to cooperate framework offers a significant advancement in understanding the legal obligations of States and non-State actors alike. While the ICJ recognized the human rights implications of climate change, it stopped short of fully addressing how non-State actors could be held accountable for their contributions to climate harm or their failure to engage in meaningful cooperation other than limiting its observation to the States’ due diligence obligation to take measures that ‘limit the quantity of emissions caused by private actors’ under their jurisdiction.[19] By grounding the duty to cooperate in human rights, Professor De Feyter’s work helps bridge this gap, highlighting the potential for non-State actors to be held responsible through international legal and normative frameworks.
Ultimately, Professor Koen De Feyter’s scholarship will continue to resonate long after his passing, shaping how the international legal system evolves to address collective global challenges of the magnitude of climate change. His commitment to ensuring that human rights are at the center of international cooperation, and that all actors, whether State or non-State, are held accountable for their roles in promoting or hindering progress, is a legacy of supreme value. As we reflect on the ICJ’s Advisory Opinion and the present and future of climate change law, Professor De Feyter’s work both clarifies and serves as a reminder that the duty to cooperate is not merely a formal legal obligation but a call to action for all actors (i.e., States, international organizations, corporations, civil society, and individuals alike), to engage in a collective effort to mitigate the impacts of climate change and ensure a sustainable future for all, one rooted in our common quest for human dignity.
[1] Obligations of States in Respect of Climate Change (Advisory Opinion) 2025 <https://www.icj-cij.org/sites/default/files/case-related/187/187-20250723-adv-01-00-en.pdf> accessed 22 September 2025.
[2] UNGA ‘Duty to cooperate and non-state actors: Thematic study by the Expert Mechanism on the Right to Development’ (5 July 2023) UN Doc A/HRC/54/84. Also available at <https://www.ohchr.org/en/documents/studies/ahrc5484-duty-cooperate-and-non-state-actors-thematic-study-expert-mechanism> accessed 22 September 2025.
[3] See eg Charter of the United Nations (24 October 1945) 1 UNTS XVI, art 1; Declaration on Principles of International Law concerning Friendly Relations and Co-operation among States in accordance with the Charter of the United Nations (1970) UN GAOR, 25th sess, Supp no 28, at 121, UN Doc A/RES/2625(XXV).
[4] Obligations of States in Respect of Climate Change, para 140.
[5] UNGA, ‘Declaration of the United Nations Conference on the Human Environment’ (16 June 1972) UN Doc A/RES/2994(XXVII) Principle 24.
[6] Rio Declaration on Environment and Development (1992) UN Doc A/CONF.151/26 (Vol. I) Principle 7.
[7] ‘Duty to cooperate and non-state actors’ para 1.
[8] ibid paras 55-60.
[9] Obligations of States in Respect of Climate Change, in particular paras 291, 377, 379 and 393.
[10] ibid para 95.
[11] ibid paras 317 and 367.
[12] ibid para 111.
[13] ibid para 276.
[14] ibid para 403.
[15] ‘Duty to cooperate and non-state actors’ para 79.
[16] ibid.
[17] ibid.
[18] Obligations of States in Respect of Climate Change para 403.
[19] ibid para 482.