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Nations have a duty to address climate change, and their actions must be based on the best available science.

That’s the consensus of the International Tribunal for the Law of the Sea (ITLOS), the court responsible for upholding the United Nations Convention on the Law of the Sea (UNCLOS). The tribunal’s panel of 21 judges unanimously declared in its May 2024 decision that UNCLOS signatory states—a list of 169 members including Canada, the European Union, and the majority of the world’s national governments—cannot simply abide by the 2015 Paris Agreement. Instead, they must take “all necessary measures” to prevent, reduce, and control greenhouse gases.

ITLOS’s advisory opinion—the first such document issued by an international court—states that UNCLOS signatories are legally obligated to address climate change, which represents an existential threat to the oceans and humankind.

The advisory opinion is the result of a case brought to the court by the Commission of Small Island States on Climate Change and International Law (COSIS), a coalition representing nine Caribbean and Pacific Islands nations. The government of Antigua and Barbuda spearheaded the effort.

In presenting the advisory opinion at the ITLOS office in Hamburg, Germany, in May 2024, ITLOS president Tomas Heidar said the court anchored its decision in the latest climate science, recognizing the “authoritative assessments” put together by the Intergovernmental Panel on Climate Change (IPCC).

Nikki Reisch, the energy and climate program director at the US-based Center for International Environmental Law, welcomes the “lucid and succinct” way the report was written. By issuing such direct advice, she says, the court quashed attempts by some UNCLOS member states to suggest that there is uncertainty around the science of climate change. The advisory opinion, she says, lays to rest “any doubt that there is enough scientific certainty to … define what states must do and refrain from doing.”

Sarah Cooley, the director of climate science at the US-based nonprofit Ocean Conservancy and a lead chapter author of a recent IPCC report, spoke as an expert witness for COSIS during the hearing. She says she felt that her role in communicating climate science had succeeded.

“Being an American, I’ve seen all sorts and stripes of climate science denial, and it was so refreshing to see this international tribunal was really accepting the IPCC’s products as a legitimate, cutting-edge synthesis of what we know about climate science,” she says.

Beyond recognizing the causes and consequences of climate change, the tribunal laid out several obligations for UNCLOS member states. The court stressed that wealthier nations must shoulder a bigger portion of the burden of cutting greenhouse gas emissions, opening the door for further discussions about what constitutes a fair share. Developing states, the 21-judge panel wrote, must also be supported with funding and technical assistance.

Shobha Maharaj, the science director of the Hawai‘i-based reforestation company Terraformation and the lead chapter author of a recent IPCC report, gave evidence on behalf of COSIS during the proceedings. Maharaj hopes the court’s decision will bear tangible results. A good start, she says, would be to see small island states get better access to the data from the most advanced climate models, which are run by wealthy nations.

“Some of the most vulnerable places and peoples in the world do not have the right data to make projections, which are needed to inform their adaptation and resilience planning,” says Maharaj. “It’s almost like flying blind.”

To determine the responsibilities of UNCLOS member states, the court considered the consequences of climate change on human populations and the natural world. Member states, the tribunal said, must protect and preserve rare or fragile ecosystems, as well as the habitats of depleted, threatened, and endangered species, from climate change and ocean acidification.

The tribunal’s advisory opinion is not legally binding, but Reisch expects it will be referenced in future climate change–related lawsuits. ITLOS’s decision is also expected to encourage states to submit more ambitious national emission pledges during the next round of climate talks in Azerbaijan in November 2024, says Isabela Keuschnigg, a legal officer at Opportunity Green, a UK-based NGO that submitted evidence to the tribunal.

Although ITLOS is the first international court to render an opinion on the duty member states have to address climate change, two other prominent courts—the Inter-American Court of Human Rights and the International Court of Justice—are weighing similar cases. ITLOS’s approach, says Reisch, “sends a very strong signal to [these] two courts about the starting point in terms of being guided by science and helping to define what the law requires.”

Maharaj, who has since contributed expert scientific input to the other two international courts, says giving evidence to the tribunal was “one of the most gratifying experiences” in her professional life.

“I’ve been looking for hope for some time,” Maharaj says. The court’s decision gives her hope that “science plus law can actually be used as a catalyst to move things forward a bit faster.”