4. Climate Change and Other Areas of International Law

International law has expanded substantially in the range of topics it addresses. Obviously, it is the climate regime that is of primary significance to the climate crisis, however other aspects of international law are also relevant, particularly those dealing with environmental issues. All areas of international law are implicated in some way and can assist in small or larger ways in addressing the problem.^[1]

There is also the potential for some areas of international law to operate as handbrakes on action. For instance, international trade law, which seeks to liberalise global trade, may prevent states from adopting progressive climate policies if it involves unjustifiable trade restrictions. Furthermore, international investment law, which is designed to protect foreign investments in host countries, can come into tension with the rights of states to address climate change in the public interest.^[2] Investor–state dispute settlement under investment treaties can, for instance, enable foreign fossil fuel companies to challenge climate policies.

4.1. Other Relevant International Environmental Regimes

4.1.1. Ozone Depletion Regime

An important precursor to the climate regime were several treaties dealing with another atmospheric pollution problem — the depletion of the ozone layer. As ozone-depleting substances are greenhouse gases the ozone regime has also played an important role in mitigating climate change.

Following the discovery in the 1980s of a substantial hole in ozone layer, a region of the stratosphere that protects the Earth from harmful ultraviolet radiation, the international community responded promptly through the adoption of the 1985 Vienna Convention for the Protection of the Ozone Layer and the 1987 Montreal Protocol to phase out the production, trade and use of ozone depleting substances such as chlorofluorocarbons. This response, which has become more stringent over time through amendments to the Montreal Protocol, has had significant success, with global emissions of ozone-depleting substances falling by more than 99 per cent.^[3] The ozone hole is now closing.

The ozone regime has substantial co-benefits for mitigating climate change given that ozone-depleting substances are powerful greenhouse gases. The Kigali Amendments to the Montreal Protocol to limit the use of hydrofluorocarbons are expected to prevent the emissions of up to 105 billion tonnes of carbon dioxide equivalent of greenhouse gases.^[4]

4.1.2. Convention on Biological Diversity

Another example of a regime with clear relevance to the climate crisis is the CBD. The CBD seeks to conserve biological diversity, and promote the sustainable use of its components, and recognises the close connection between safeguarding nature and addressing climate change. This is seen in the CBD’s Kunming–Montreal Global Biodiversity Framework, adopted in 2022.^[5] The framework succeeds and replaces the Aichi Targets. It includes four overarching goals and 23 accompanying targets to be achieved by 2030. These include Target 8, which is to ‘[m]inimize the impact of climate change and ocean acidification on biodiversity and increase its resilience through mitigation, adaptation, and disaster risk reduction actions’.

4.2. The Law of the Sea

An area of international law that has had increasing engagement with climate issues is the law of the sea, the main content of which is found in the 1982 United Nations Convention on the Law of the Sea (‘UNCLOS’).^[6]

UNCLOS includes detailed provisions relating to the protection of the marine environment from all sources of pollution, including the atmosphere. In an advisory opinion issued in 2024, the International Tribunal for the Law of the Sea (‘ITLOS’) found that parties to UNCLOS have the specific obligation under Article 194 of UNCLOS to take all necessary measures to prevent, reduce and control marine pollution from anthropogenic GHG emissions.^[7] Importantly, ITLOS found that this obligation will not be satisfied simply by complying with the obligations and commitments under the Paris Agreement.^[8]

ITLOS has elaborated in detail what parties to UNCLOS, which include most states, must do to protect the ocean, which is the most important component of the climate system. The ocean has absorbed around 30 per cent of the carbon dioxide emitted by human activities, and 90 per cent of the extra warmth added to the climate system by heat-trapping gases.^[9] ITLOS found that governments have a due diligence obligation to take all necessary measures to prevent, reduce and control marine pollution from GHG emissions. These measures are to be determined objectively, taking into account the best available science and the relevant international rules set out in the UNFCCC and the Paris Agreement, including the 1.5 C temperature goal. ITLOS also observed that this standard of due diligence is stringent, given the high risks of serious and irreversible harm posed to the marine environment from GHG emissions, and that states must adopt and implement national laws to give effect to the due diligence obligation.

4.3. General Principles of International Law

Climate change raises a number of questions for general principles of international law. These include whether states can be held responsible for climate change induced damage to other states, and whether states can continue to exist if their territories are submerged or become uninhabitable.

The state responsibility question has been challenging for several reasons, including the difficulty in establishing causation given that climate change is a global problem and different from the ‘classic’ case of transboundary harm (such as the celebrated Trail Smelter Case^[10] involving air pollution from a metals smelter in Canada damaging farmland across the border in the United States).

In relation to statehood, there are four key criteria under international law. These are territory, a permanent population, effective government and the capacity to enter into international relations. The first two of these, territory and population, are considered the most vital requirements. The loss of these, certainly if permanent, would appear to deprive an existing state of its status as such.^[11] The International Law Commission, a United Nations body of international legal experts tasked with codifying and progressively developing the law, examined the issue in a Study Group on Sea Level Rise in International Law.^[12] The co-chairs of the study group concluded that there is a strong presumption of the continuity of statehood, and that this position has been endorsed by a range of states including the United States (which in 2023 declared that human-induced sea level rise should not cause any country to lose its statehood or membership of the United Nations).^[13]

Pacific Island states have been at the forefront of state practice on this issue. In November 2023, the Pacific Islands Forum (‘PIF’) adopted a Declaration on the Continuity of Statehood and the Protection of Persons in the Face of Climate Change-Related Sea-Level Rise.^[14] PIF members recognised that under international law there is a general presumption that a state, once established, will continue to exist and endure, and maintain its status and effectiveness, and that international law does not contemplate the demise of statehood in the context of climate change related sea level rise.

This position was endorsed by the ICJ in its 2025 advisory opinion on the Obligations of States in Relation to Climate Change.^[15] The Court concluded that once a state is established, the disappearance of one of its constituent elements would not necessarily entail the loss of its statehood.^[16] The Court also endorsed the PIF’s position^[17] that baselines and maritime zones do not need to be changed as a result of climate change induced sea level rise.^[18] This largely puts to rest two fundamental questions that had generated extensive academic commentary.

To address the consequences of climate change for a vulnerable state, two members of the PIF, Australia and Tuvalu, adopted the Falepili Union Treaty, which entered into force in 2024.^[19] Under the treaty, Australia recognises Tuvalu’s continuing statehood and sovereignty, notwithstanding the impact of sea-level rise, has committed to assist Tuvalu in response to a major natural disaster, health pandemic or military aggression, and will create a special visa pathway to support mobility with dignity, which will enable citizens of Tuvalu to come to Australia to live, work and study.