An advisory opinion like the one issued by the International Court of Justice (ICJ) this week is not binding and so is not directly enforceable, but that does not mean it has no weight.

By saying that all countries were firmly bound to a swathe of legal obligations under existing laws and treaties, experts say the ruling will influence courts, climate negotiations and policy decisions across the world.

The ICJ "couldn't have been clearer" on the binding nature of a range of climate duties, said Cesar Rodriguez-Garavito, Professor of Law and Director of the Climate Law Accelerator at New York University.

He said the ruling, which was responding to questions on countries' climate responsibilities from the UN General Assembly, was "as strong as we could have imagined".

"The consensus among the judges is fully behind the conclusion that international law establishes clear and binding obligations for states not to cause massive harm to the environment in general and not to harm the climate system in particular," he told AFP.

These included ensuring national climate plans reflect the highest possible ambition to stay within the Paris agreement's safer global warming cap of 1.5C above pre-industrial times -- a level that the world could reach this decade.

If those obligations are not met, the court said states may be obliged to repair damaged infrastructure or ecosystems -- and if that is not possible, they could face compensation claims.

This will ripple into future litigation, said Markus Gehring, professor of European and international law at the University of Cambridge.

"We are a far cry from a contentious case between two countries, where someone is demanding liability for past and present climate change damage, but in theory, the court lays out an avenue towards such claims," he said.

- 'Wrongful act' -

Major petrostates like the United States may take little heed of the court's warning that expanding production of oil, gas and coal could constitute an "internationally wrongful act".

But Gehring said countries could choose to ignore ICJ advisory opinions "at their peril".

He cited the court's 2019 advice that the United Kingdom should end its occupation of the Chagos islands.

After Britain initially rejected the ruling, a UN General Assembly resolution demanded it cede the islands to Mauritius, which it eventually did in 2024.

Gehring said that while the court's climate decision is not directly binding on individual states, it would be indirectly binding through subsequent domestic or international court action and through UN institutions.

The move by US President Donald Trump to withdraw from the Paris deal also would be unlikely to absolve the country from its duties, Gehring added, because the obligation to address climate change is now "crystal clear in international law".

"So even leaving the Paris Agreement and the climate treaty regime does not eliminate those obligations," he said.

ICJ judge Sarah Cleveland said countries' "significant responsibilities" to protect the climate system may also affect interpretation of international investment law.

- 'Shield and sword' -

The ruling was "a decisive legal vindication" for Vanuatu -- which spearheaded the push for an ICJ opinion -- the country said in a legal analysis of the decision.

The Pacific island nation, which is at risk from rising seas, said the court's conclusions would strengthen its hand in global climate negotiations, helping it demand greater climate ambition and attract financial support for countries suffering climate loss and damage.

It could also open the way for legal action against countries and possibly companies that have by their actions and omissions caused climate harm, the statement said.

"For Vanuatu, the opinion is both shield and sword: a shield affirming its right to survival and a sword compelling the world's major emitters to act in line with science and justice," it added.

Manuel Pulgar-Vidal, former president of the UN COP20 climate conference in Peru and now WWF Global Climate and Energy lead, said he expects the ICJ ruling to "move the needle strongly".

"The timing is so fantastic because we are in difficult times in the climate debate, so to have that opinion in the current time, it is showing that we should never lose our hope," he told AFP.