Foley Hoag represented the Republic of The Gambia, the Principality of Liechtenstein, the Republic of Malawi, the Republic of Namibia, and the Republic of Sierra Leone in securing a landmark advisory opinion from the International Court of Justice (ICJ) on States’ obligations regarding climate change under international law.

In its 23 July 2025 Opinion, the ICJ determined that all States are required to take effective measures to address climate change, not only pursuant to the climate agreements, but also under customary international law, the law of the sea, and international human rights law. The Court clarified that the applicable standard is one of due diligence, a rigorous standard given the grave and far-reaching risks posed by climate change, as established by the best available scientific evidence. This requires States to adopt and implement effective laws and policies to mitigate climate change and to regulate the conduct of both public and private actors under their jurisdiction. Failure to exercise due diligence in preventing significant harm to the environment, including the climate system, constitutes an internationally wrongful act, giving rise to State responsibility.

The Court further determined that States’ obligations to protect the climate system and other parts of the environment from anthropogenic greenhouse gas emissions are obligations erga omnes—owed to the international community as a whole—meaning that any State may invoke responsibility for breaches by another State.

Christina Hioureas, Partner and Global Co-Chair of Foley Hoag’s International Litigation and Arbitration Department, argued for The Gambia. “The Advisory Opinion confirms that States have obligations under international human rights law, including the right to a clean, healthy and sustainable environment, to take actions to address climate change. In doing so, States are required to regulate activities of both public and private actors. Indeed, the Opinion signals that State measures regulating the conduct of private actors in connection with climate change are not only permissible but required under international law, thereby recalibrating the balance between investment protection and States’ duty to protect human rights and the environment,” she said.

Partner Andrew Loewenstein, who also argued for The Gambia, said that this authoritative Opinion, from the UN’s principal judicial organ, “provides critical benchmarks for assessing States’ actions in tackling climate change on the basis of clear legal obligations and the best available scientific evidence.” He also stressed that the Opinion is unequivocal that “a State may incur international responsibility by failing to take appropriate action to protect the climate system from GHG emissions, including through fossil fuel production and consumption, the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies.” 

Partner and Chair of the Africa Practice Tafadzwa Pasipanodya, who argued on behalf of Malawi, underscored the Opinion’s groundbreaking determinations on remedies for wrongful conduct concerning climate change: “Resisting calls by a few States to exempt harm caused by greenhouse gas emissions from the normal application of international law, the Court clarified that States that fail to act with due diligence and to cooperate internationally to prevent significant harm to the climate system may be required to cease the wrongful conduct, guarantee they will not repeat it, and provide full reparation, which may include financial compensation or restoration of damaged ecosystems and infrastructure.”

In addition to Hioureas, Loewenstein, and Pasipanodya, the Foley Hoag team included Diem Huong Ho, Juan Pablo Hugues Arthur, Sun Young Hwang, Peter Tzeng, Iulia Padeanu Mellon, Juan Francisco Fernández Garcés, and Nour Nicolas, with assistance from Alexander Small, Katie Weiner, Jennifer Schoppmann, and Muhammad Syed. The Foley Hoag team acted with co-counsel Professor Charles Jalloh, member of the UN International Law Commission (Sierra Leone); Professor Phoebe Okowa, member of the UN International Law Commission (Kenya); and Manuel Casas, Barrister (Twenty Essex), and Ndjodi Ndeunyema, supported by the Open Society Justice Initiative and scientists from the Union of Concerned Scientists.