By Catherine Amirfar and Duncan Pickard

Sea level rise is one of the clearest and most devastating expressions of the climate crisis. For low-lying nations, it is not an abstract projection but an encroaching physical reality that inundates homes, salinates aquifers and agricultural land, and, in some cases, is projected to submerge entire countries by mid-century.

In its 2025 advisory opinion on climate change, the International Court of Justice (ICJ) confirmed that international law requires States not only to prevent such harm through mitigation of greenhouse gas (GHG) emissions, but also to assist developing countries in adapting to it. Read together with the 2024 advisory opinion of the International Tribunal for the Law of the Sea (ITLOS), the ICJ’s opinion establishes that, as a matter of international law, the obligation of nations to mitigate and adapt to the adverse effects of climate change – including sea level rise – is legally binding and not a policy choice.

The legal framework governing sea level rise

Both the ICJ and ITLOS grounded their reasoning in the scientific consensus reflected in the findings of the Intergovernmental Panel on Climate Change (IPCC): anthropogenic GHG emissions are driving sea level rise through thermal expansion and melting ice sheets. The 2024 ITLOS opinion concluded that GHG emissions meet the definition of marine pollution under Part XII of the UN Convention on the Law of the Sea (UNCLOS), obliging countries that are parties to the Convention to prevent, reduce, and control such emissions from every source, as well as to cooperate to lessen their significant negative impacts on developing States.

In 2025, the ICJ built on this foundation. The Court identified the duty to prevent significant environmental harm and the duty to cooperate as rules of customary international law that apply to activities affecting the climate system and, by extension, to sea level rise. It held that countries must act with stringent due diligence to cut emissions in line with the 1.5°C goal and to provide financial and technological assistance to those least able to cope with the effects of climate change. The ICJ’s opinion makes clear that the commitments under the UNFCCC and Paris Agreement on climate change constitute binding obligations of conduct and result: mitigation is an obligation of prevention; adaptation support is an obligation of cooperation. Failure to meet either, the ICJ held, entails international responsibility, including a duty to cease the wrongful act and to make full reparation for its consequences.

These obligations give shape to a collective project: slowing sea level rise through mitigation while enabling the most vulnerable countries to remain liveable despite it. The International Law Commission’s (ILC) 2025 Final Report on Sea-Level Rise in Relation to International Law reaches the same conclusion, stating that “international cooperation is required to protect persons and communities affected by sea-level rise, including to protect their culture, cultural heritage, identity and dignity and to meet their essential needs.” The ICJ took that reasoning further, holding that even if sea level rise were to submerge a country’s entire territory, “the disappearance of one of its constituent elements would not necessarily entail the loss of its statehood.” This affirmation of continuity – highlighted in companion commentaries on statehood and self-determination and on equity and intergenerational justice – means that States’ legal personality endures even if their land does not. What is now required is practical cooperation to make that continuity meaningful for small islanders.

Tuvalu: Surviving the rising tide

Tuvalu embodies the human and legal stakes of sea level rise. Built on porous limestone and with an average elevation of just 1.55 meters, the country’s land is already regularly inundated. Without substantial emission reductions, most of Tuvalu’s territory will be below the high-tide line within decades and possibly uninhabitable long before that. Saltwater intrusion has contaminated wells, pulaka pits, and cropland, storm surges have swept away ancestral graves, and coral degradation has undermined coastal protection.

Yet Tuvalu is striving to adapt. Through the Tuvalu Coastal Adaptation Project (TCAP) – supported by the Green Climate Fund (GCF) and bilateral donors – the government has constructed nature-based sea walls, strengthened atoll shorelines, and undertaken large-scale land reclamation on Funafuti. Tuvalu presented evidence to the ICJ showing how these adaptation efforts have already provided critical protection to its largest atoll, meaningfully reducing the impacts of once-in-a-century inundation events.

But far more is needed. The most ambitious of Tuvalu’s adaptation plans is to raise the elevation of its most vulnerable coasts by up to 5.75 meters, placing them above the worst case projections for sea level rise by 2100. Yet the funds received to date fall far short of what would be required to complete the project, with severely insufficient contributions from the States most responsible for the climate crisis. Both the ICJ and ITLOS confirmed that States with greater capacity have a duty to cooperate through financial assistance, technology transfer, and capacity building to help others adapt. Supporting Tuvalu’s adaptation efforts, therefore, is not benevolence – it is compliance with international law.

Tuvalu’s efforts illustrate the shared challenges confronting low-lying developing countries worldwide. Individually and collectively, they face a spectrum of slow- and fast-onset sea level rise impacts that threaten to erase decades of development progress. Effective adaptation demands innovative and integrated responses – reinforced coastal defences, resilient infrastructure, storm-resistant housing, sustainable drainage and water systems, and beyond. These measures require stable, long-term finance and enduring technology partnerships. International cooperation consistent with the legal framework articulated by the ICJ and ITLOS is therefore essential not only to avert physical loss, but also to safeguard the cultural identity, livelihoods, and sovereignty of affected communities.

The legal consequences of inaction

By clarifying that the obligations to mitigate and to assist are binding norms, the ICJ has linked scientific urgency to legal accountability. Countries that fail to take adequate measures to curb emissions or that neglect their duty to assist vulnerable nations risk breaching international law. The opinion anchors sea level rise within the established international law of State responsibility, making it possible to evaluate national policies not just politically but also legally.

This jurisprudence reshapes the global response to rising seas. It requires the world’s major emitters to reduce emissions consistent with the 1.5°C limit and to ensure effective, equitable assistance for adaptation projects addressing sea level rise. It also affirms that preserving the habitability and sovereignty of small island countries is a shared legal responsibility. As the ocean rises, the measure of compliance with international law will be seen not in pledges but in the concrete actions – mitigation, finance, and technology transfer – that keep the world’s lowest-lying countries above the seas.

* * *

Catherine Amirfar is Co-Chair of the International Dispute Resolution and Public International Law Groups at Debevoise & Plimpton LLP. Duncan Pickard is an associate in the same Groups. They were part of the counsel teams of Tuvalu and of the Commission of Small Island States on Climate Change (COSIS) at the ICJ and ITLOS.

This Guest Article is part of a project that seeks to raise awareness and build momentum and knowledge around the ICJ advisory opinion on obligations of States in respect of climate change and to promote a better understanding of the implications of the advisory opinion among sustainable development decision makers.