The International Court of Justice delivered an advisory opinion on Wednesday (early Thursday AEST) in a landmark case about nations’ obligations to tackle climate change and the consequences they may face if they don’t, calling it an “urgent and existential” threat to humanity.

“Failure of a state to take appropriate action to protect the climate system … may constitute an internationally wrongful act,” court President Yuji Iwasawa said during the hearing.

The court also said countries harmed by climate change could be entitled to reparations for the damage they have suffered from rising global temperatures, but what they are owed should be determined on a “case-by-case” basis.

The non-binding opinion, which runs to more than 500 pages, is seen as a potential turning point in international climate law.

The court said a “clean, healthy and sustainable environment” is a human right. Enshrining a sustainable environment as a human right paves the way for other legal actions, including states returning to the ICJ to hold each other to account as well as domestic lawsuits, along with legal instruments like investment agreements.

The case is led by the Pacific island nation of Vanuatu and backed by more than 130 countries.

All UN member states including major greenhouse gas emitters like the United States and China are parties to the court.

Outside the court, climate activists gathered with a banner that read: “Courts have spoken. The law is clear. States must ACT NOW.” 

The courtroom, known as the Great Hall of Justice, was packed.

A panel of 15 judges was tasked with answering two questions: What are countries obliged to do under international law to protect the climate and environment from human-caused greenhouse gas emissions? Second, what are the legal consequences for governments when their acts, or lack of action, have significantly harmed the climate and environment?

“The stakes could not be higher. The survival of my people and so many others is on the line,” Arnold Kiel Loughman, attorney-general of Vanuatu, told the court during a week of hearings in December.

In the decade up to 2023, sea levels rose by a global average of around 4.3 centimetres, with parts of the Pacific rising higher still. The world has also warmed 1.3 degrees since preindustrial times because of the burning of fossil fuels.

Vanuatu is one of a group of small states pushing for international legal intervention in the climate crisis, but it affects many more island nations in the South Pacific.

Activists could bring lawsuits against their own countries for failing to comply with the decision.

“What makes this case so important is that it addresses the past, present, and future of climate action. It’s not just about future targets — it also tackles historical responsibility, because we cannot solve the climate crisis without confronting its roots,” Joie Chowdhury, a senior attorney at the Centre for International Environmental Law, told AP.

The United States and Russia, both of whom are major petroleum-producing states, are staunchly opposed to the court mandating emissions reductions.

But those who cling to fossil fuels could go broke doing it, the UN secretary-general told The Associated Press in an exclusive interview this week.

Simply having the court issue an opinion is the latest in a series of legal victories for the small island nations. Earlier this month, the Inter-American Court of Human Rights found that countries have a legal duty not only to avoid environmental harm but also to protect and restore ecosystems.

Last year, the European Court of Human Rights ruled that countries must better protect their people from the consequences of climate change.

In 2019, the Netherlands’ Supreme court handed down the first major legal win for climate activists when judges ruled that protection from the potentially devastating effects of climate change was a human right and that the government has a duty to protect its citizens.