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A historic climate ruling by the world's highest court could make it legally riskier for fossil fuel companies to do business and embolden lawsuits against oil and gas expansion, experts say.

The International Court of Justice's first-ever advisory opinion on climate change contained a particularly strong position on fossil fuels that surprised even veteran observers of environmental law.

The Hague-based court declared that states had an obligation under international law to address the "urgent and existential threat" of climate change, a decision hailed as a milestone by small islands most at risk.

The unanimous decision went further than expected, with the court spelling out what responsibility states have to protect the climate from planet-warming emissions from burning fossil fuels.

Failing to prevent this harm "including through fossil fuel production, fossil fuel consumption, the granting of fossil fuel exploration licenses or the provision of fossil fuel subsidies—may constitute an internationally wrongful act" by that state, the court added.

"It's really significant," said Sophie Marjanac, an international climate lawyer and director of legal strategy at the Polluter Pays Project, a campaign group.

"It goes further than I expected, and it really makes some pretty groundbreaking findings," she told AFP.

ICJ advisory opinions are not legally enforceable, but such opinions are rare, and seen as highly authoritative in steering national courts, legislation and corporate behavior around the globe.

Litigation against fossil fuel projects is growing, but so too are legal challenges by states and companies using the courts to block or unwind action on climate change.

Legal risks

Jorge Vinuales, who helped draft the request for the court's opinion, said the fossil fuels language in the final opinion "went as far as one could expect the court to go, which is no small feat".

He said this interpretation of liability for climate harm would probably be picked up in domestic and global courtrooms.

"If so, it could have far-reaching effects," Vinuales, a professor of law and environmental policy at the University of Cambridge, told AFP.

Fossil fuel companies and oil- and gas-producing nations could ignore the ICJ "but that raises legal and litigation risks of its own", he added.

Its opinion could be used in a lawsuit against expanding a coal mine, a private dispute between an investor and a state, or a contract negotiation involving a fossil fuel financier, said Marjanac.

"It could come up in all sorts of ways, all over the place. The influence is unlimited, really," she said.

This could particularly be the case in countries that can adopt international law directly into their constitutions and legal frameworks, though this would depend on national context and take time to trickle down.

In these countries, which include France, Mexico, and the Netherlands, courts may have to take the ICJ opinion into account when hearing a case against an oil and gas venture.

Even in so-called "dualist states" where international law is not automatically incorporated, constitutional courts and other national legislatures often respected and adopted aspects of ICJ opinion, experts said.

The ruling "opens the door to challenges to new fossil fuel project approvals and licensing," said Marjanac, and "makes the operating environment much more difficult" for oil and gas majors.

Line of defense

The court also "provided stricter measures surrounding the business of fossil fuels" and underscored that governments could not avoid blame for polluting companies within their jurisdiction, said Joy Reyes from the London School of Economics.

"Countries will have to be more circumspect when it comes to licensing permits and broader policies around fossil fuels, because it may open them up to liability in the future," Reyes, a climate litigation specialist, told AFP.

It could also empower smaller states to pursue compensation from big polluters, and give countries threatened with legal action by fossil fuel companies a stronger line of defense.

And it could be harder now for oil and gas companies "to claim they have a legitimate expectation to be able to operate a fossil fuel project without impediment," Lorenzo Cotula, an international legal expert, told AFP.

"It's now clear that states have a legal duty to take action in this space, and if they're able to articulate this in possible proceedings, I think that will be a strong legal argument to make," said Cotula, from research institute IIED.

© 2025 AFP