THE HAGUE, Netherlands — The United Nations’ top court in a landmark advisory opinion Wednesday said countries could be in violation of international law if they fail to take measures to protect the planet from climate change, and nations harmed by its effects could be entitled to reparations.

Advocates immediately cheered the International Court of Justice opinion on nations’ obligations to tackle climate change and the consequences they may face if they don’t.

“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. He called the climate crisis “an existential problem of planetary proportions that imperils all forms of life and the very health of our planet.”

The non-binding opinion, backed unanimously by the court’s 15 judges, was hailed as a turning point in international climate law.

Notably, the court said a “clean, healthy and sustainable environment” is a human right. That 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 was led by the Pacific island nation of Vanuatu and backed by more than 130 countries.

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

Climate activists had gathered outside the crowded court with a banner that read: “Courts have spoken. The law is clear. States must ACT NOW.” They watched the ruling on a giant screen, clapping and cheering at times during the two-hour hearing.

“Today, the tables have turned. The world’s highest court provided us with a powerful new tool to protect people from the devastating impacts of the climate crisis — and to deliver justice for the harm their emissions have already caused,” former U.N. human rights chief Mary Robinson said in a statement.

After years of lobbying by vulnerable island nations who fear they could disappear under rising sea waters, the U.N. General Assembly asked the ICJ in 2023 for an advisory opinion, an important basis for international obligations.

“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 the island nation 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 1.7 inches, with parts of the Pacific rising higher still. The world has also warmed 2.3 Fahrenheit since preindustrial times because of the burning of fossil fuels.

Ralph Regenvanu, Vanuatu’s minister for climate change, called the ruling a “very important course correction in this critically important time. For the first time in history, the ICJ has spoken directly about the biggest threat facing humanity.”

He said the ruling exceeded his expectations. “I didn’t expect it to be good. It’s good. And it did go above and beyond,” he told reporters in The Hague.

Activists could bring lawsuits against their own countries for failing to comply with the decision, which ran to over 130 pages.

The United States and Russia, both of whom are major petroleum-producing states, are staunchly opposed to the court mandating emissions reductions. The Trump administration has again withdrawn the U.S. from the 2015 Paris Climate Agreement and has made it harder to find scientific assessments of how climate change endangers the U.S. and its people.

Asked to comment on the ruling, White House spokesman Taylor Rogers said: “As always, President Trump and the entire administration is committed to putting America first and prioritizing the interests of everyday Americans.”

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

Simply having the U.N. 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.