In Conversation: Prof. Philippe Cullet — Professor of International and Environment Law at SOAS University of London

The Q&A is curated by Aditya Kumar for the SPIL Blog.

Introduction

The International Court of Justice’s Advisory Opinion on Climate Change issued on 23^rd July 2025 represents a pivotal moment in the evolution of international environmental law. Beyond clarifying the scope of States’ obligations to mitigate and adapt to climate change, the Opinion foregrounds long-debated principles such as intergenerational equity, sustainable development, and the relationship between climate and human rights.

Yet the questions remain: how far does the Opinion reshape the legal architecture of environmental governance? Does it go beyond a state-centric framework to engage meaningfully with climate justice and the disproportionate burdens borne by vulnerable communities? What implications might it hold for national courts, transboundary disputes, and the future of negotiations under the Paris Agreement?

To examine these questions, SPIL Mumbai is delighted to present a conversation with Prof. Philippe Cullet, Professor of International and Environmental Law at SOAS University of London. Prof. Cullet’s scholarship spans international environmental law, human rights, and equity in resource management, with a particular emphasis on water law and governance in India. His work on environmental justice and equity provides a critical lens through which to assess the Opinion’s contributions and limitations.

1. The ICJ Climate Advisory Opinion has been described as a landmark in clarifying States’ obligations to mitigate and adapt to climate change. From your perspective, what do you see as its most significant doctrinal or normative contribution to international environmental law?

At the most general level, it may be to recentre climate law as part of environmental law. While at the time of the adoption of the UNFCCC, it was understood that climate change was one of many environmental issues arising, over the past couple of decades, climate change has progressively been addressed as if it were a distinct field. The Advisory Opinion (hereinafter “AO”) has reconfirmed not only that international environmental law is a broad field with inter-connected MEAs, and also that it cannot be understood without connected fields, such as in particular human rights.

2. The Opinion emphasizes intergenerational equity as part of its reasoning. How does this understanding of equity relate to broader principles such as common but differentiated responsibilities, and what implications might it have for international environmental governance.

Intergenerational equity is for the Court an emanation of equity. In turn, equity is understood in the narrow sense that the Court has used for a long time, namely as the equity applied at the level of the application of norms.

International environmental law has gone much beyond this understanding of equity with what is known as differential treatment. Under this paradigm,  different states take on different commitments for reasons such as different historical contributions to the climate crisis and/or different present capacity to address the problem. Differential treatment is reflected in the principle of common but differentiated responsibilities and respective capabilities. This principle is acknowledged by the Court as a core guiding principle. At the same time, the Court states that the principle is a manifestation of equity. This is indeed what has often been argued but the equity referred to here is a much narrower concept than the broadly based equity at the root of differential treatment.

3. Much of your work has focused on water law and governance, with particular emphasis on equity and access. In light of the ICJ Opinion, how should States approach disputes over transboundary water resources that are increasingly affected by climate change?

The AO has little to say on water. This is possibly not surprising since water was for a long time subsidiary in climate debates. This is more surprising in 2025 given that the links have become much more readily addressed.

In terms of future transboundary water disputes, the answer may not be very different from other bilateral/regional environmental issues that may arise in the future. The AO may add some legitimacy to certain environmental protection arguments. At the same time, since it is ‘advisory’ and since it does not really give additional content to existing norms, its impact will be as much as what states on both sides of a dispute are happy to agree on in relation to climate arguments.

4. You have consistently argued for the centrality of environmental justice and equity in international law. Do you think the ICJ Opinion adequately addresses the disproportionate burden borne by vulnerable communities, or does it remain too state-centric in its articulation?

The AO remains overwhelmingly state-centric. That is a position, which is in line with a traditional understanding of public international law as only concerning states. This position is outdated in terms of the diminishing influence of states and the growing influence of other actors, in particular private sector actors. Further, it is outdated as it reflects a traditional strict division between the national and the international spheres. The climate crisis (among other environmental crises) cannot be understood by segmenting issues in this way and needs to be addressed simultaneously from the local to the global levels and must directly address the role of non-state actors as well.

More broadly, the AO does not actually engage with climate justice, which in many ways has become the way we understand the connections between the different issues arising in the context of the climate crisis (climate, nature, people, livelihoods etc).

5. The Advisory Opinion has been welcomed by many developing states as reinforcing common but differentiated responsibilities. How do you interpret the Court’s treatment of differentiation, and what implications might this have for South-South cooperation on climate governance?

The AO puts a lot of emphasis on the principle of common but differentiated responsibilities. At the same time, all it does is tell us that it is ‘relevant for the interpretation of treaties’ but ‘does not establish new obligations’ (para 151). Further, while it tries to go beyond the division of the world in the two unwieldy categories of developed and developing countries that have been at the centre of the regime since 1992, it does not actually give any pointers towards finding new ways to reflect equity in climate law. The rather timid treatment of common but differentiated responsibilities has been criticized in several separate opinions. Overall, the way in which the AO approaches the topic is a reflection of the deeply polarized positions around common but differentiated responsibilities and the difficulty to find a consensus position between the global South and global North. It also signals the difficulty to find ways to address the vastly different, and sometimes opposed positions, of countries clubbed within the Global South (such as small-island states and countries whose economies depend mostly on fossil fuel extraction).

6. If we consider the Advisory Opinion alongside your work on environmental justice, what lessons can be drawn for national courts when interpreting constitutional or statutory environmental obligations?

Courts will hopefully be swayed by the legitimacy that comes with certain statements being made by the ICJ. At the same time, since the AO does not really add new understanding but reinforces existing messages, the real impact will depend on the extent to which courts will see the international elements arising being aligned with domestic environmental policy.

One of the points that may be effectively used is the linking of ‘climate’ and ‘environment’, which provides a lens to look at both simultaneously rather than through separate angles.

7. How might this Opinion influence ongoing and future climate negotiations under the UNFCCC and the Paris Agreement?

The AO is one of many other significant developments that reinforce the message that action to address the climate crisis is imperative and urgent. It may not change the direction of negotiations immediately or of its own. At the same time, it will be a significant addition to the different elements that will together eventually put overwhelming pressure on states to take the kind of action they have not taken yet.