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A “clean, healthy and sustainable environment” is a human right and, if countries fail to take “appropriate action to protect the climate system”, they could be in violation of international law, according to the UN’s top court.

Judges at the International Court of Justice (ICJ) have just delivered a long-awaited advisory opinion on nations’ climate obligations and the consequences they face if they don’t fulfil them.

“Failure of the state to take appropriate action to protect the climate system from GHG (greenhouse gas) emissions…may constitute an internationally wrongful act which is attributable to that state,” ICJ President Yuji Iwasawa said, including through fossil fuel production, fossil fuel consumption or the provision of fossil fuel subsidies

The court affirmed that a “clean, healthy and sustainable environment” is a human right.

It also said that countries harmed by climate change could also be entitled to reparations, but what they are owed must be decided on a case-by-case basis.

“Today, the tables have turned,” Mary Robinson, Member of the Elders, First woman President of Ireland and former UN High Commissioner for Human Rights, said in a statement.

“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.”

Climate change is an ‘urgent and existential’ threat

ICJ President Yuji Iwasawa called climate change an “existential problem of planetary proportions that imperils all forms of life and the very health of our planet”.

He said that, building on the latest science and decisions at UN climate conferences, the target of limiting global warming to 1.5 °C above pre-industrial levels must be understood to be the internationally agreed-upon objective of climate policies.

Iwasawa added that countries have a duty to cooperate on preventing harm caused by climate change and must make sure their national climate targets represent the highest possible ambition.

The ICJ described the climate system as an “integral and vitally important part of the environment and which must be protected for present and future generations”.

What does the ICJ opinion mean for global climate action?

Though the 500-page advisory opinion is non-binding and the court can’t force countries to act, it forms an important basis for international climate obligations. All UN member states, including emitters such as the United States and China, are parties to the court.

“When a court like the ICJ recognises new connections between conduct and legal norms, like the idea that failing to curb fossil fuels-related emissions can violate international legal obligations, it does not stop there. That recognition opens the door for further legal claims,” explains Sebastien Duyck, senior attorney at the Center for International Environmental Law.

“If states have legal duties to prevent climate harm, then victims of that harm have a right to redress. In this way, the ICJ advisory opinion does not only clarify existing rules, it creates legal momentum. It reshapes what is now considered legally possible, actionable, and ultimately enforceable.”

Sarah Mead, co-director of the Climate Litigation Network, adds that the ruling validates what a majority of people around the world expect from their governments – meaningful climate action.

“Nearly all countries’ climate plans are falling short of what’s needed to keep everyone safe, which is why more and more people are turning to courts to hold their governments to account,” she says.

“Today, the law stands firmly on their side, confirming leaders do in fact have a duty to protect the planet and safeguard everyone’s future.”

‘The world’s smallest countries have made history’

The ruling comes after years of lobbying from vulnerable island nations led by Vanuatu, which led to the ICJ being asked by the UN General Assembly in 2023 to deliver an advisory opinion.

“Today, the world’s smallest countries have made history,” says Vishal Prasad, Director of Pacific Islands Students Fighting Climate Change.

“The ICJ’s decision brings us closer to a world where governments can no longer turn a blind eye to their legal responsibilities. It affirms a simple truth of climate justice: those who did the least to fuel this crisis deserve protection, reparations, and a future.

“This ruling is a lifeline for Pacific communities on the frontline.” 

A panel of 15 judges was tasked with answering two questions. Firstly, what obligations do states have under international law to address climate change for both current and future generations? Second, what are the legal consequences for states that fail to meet these obligations, causing serious climate harm?

At hearings in December last year, the ICJ heard from more than 100 countries and organisations, with written statements or comments from around 150 more, making it the largest case the top UN court has ever seen.

Ahead of the ruling, activists gathered outside the court holding banners 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. When it was over, others emerged from the courtroom laughing and hugging.