The world's highest court has declared states have a legal obligation to tackle climate change and that failing to do so is a "wrongful act" that could open the door to reparations.
Advocates for stronger action to curb carbon emissions have hailed the landmark advisory opinion from the International Court of Justice (ICJ) as a turning point for future climate talks in favour of smaller nations vulnerable to the impact of climate change.
The ICJ said climate change was an "urgent and existential threat" to humanity and countries had a "duty to cooperate" on addressing it.
"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 legal consequences resulting from the commission of an internationally wrongful act may include … full reparations to injured states in the form of restitution, compensation and satisfaction."
The court added that a "sufficient direct and certain causal nexus" had to be shown "between the wrongful act and the injury".
In its non-binding opinion, which runs to more than 500 pages, the ICJ said a "clean, healthy and stable environment" is a human right.
The finding will potentially pave the way for nations most affected by climate change to litigate against major polluting countries.
Vanuatu's special envoy on climate change, Ralph Regenvanu — whose country led a global campaign for the ICJ to hear the case — has not ruled out launching litigation against large polluting countries, including Australia, in the wake of the findings.
"According to the advisory the ICJ handed down today, Australia is committing international wrongful acts because it is sponsoring, it is subsidising, fossil fuel production and excessive emissions," he told the ABC.
'Course correction' on climate
Mr Regenvanu earlier hailed the court's decision as a "landmark milestone".
"It's a very important course correction in this critically important time," he said.
"Even as fossil fuel expansion continues under the US's influence, along with the loss of climate finance and technology transfer, and the lack of climate ambition following the US's withdrawal from the Paris Agreement, major polluters, past and present, cannot continue to act with impunity and treat developing countries as sacrifice zones to further feed corporate greed."
He said Vanuatu would now take the ruling back to the UN General Assembly to "pursue a resolution that will support implementation of this decision".
Advocates cheered as the court hand down its opinion in the Hague, which is in the Netherlands.
The case began in 2019 when a group of Pacific Islander law students in Vanuatu decided to seek the ICJ's opinion on the legal obligations of states to prevent climate change.
Vanuatu's government supported their cause and provided diplomatic support, rallying a global coalition that succeeded in convincing the UN General Assembly to refer the case to the ICJ in 2023.
Vishal Prasad, the director of Pacific Islands Students Fighting Climate Change and one of the students who initiated the case, said the advisory opinion would play a major role in holding polluters accountable.
"The ICJ's decision brings us closer to a world where governments can no longer turn a blind eye to their legal responsibilities," he said.
"It affirms a simple truth of climate justice: those who did the least to fuel this crisis deserve protection, reparations, and a future."
Mr Prasad told the ABC the advisory opinion gave hope to young Pacific Islanders that there was a chance to address the climate crisis.
"It's a huge tool in our toolkit now to carry the fight for climate justice forward," he said.
"Our calls for justice are much stronger and the demand for action and accountability are now even stronger."
Two questions posed to ICJ judges
The court's 15 judges were asked, for the first time, to give an opinion about the obligations of nations to prevent climate change — and the consequences for them if they failed.
ICJ president Yuji Iwasawa said the climate "must be protected for present and future generations" and the adverse effect of a warming planet "may significantly impair the enjoyment of certain human rights, including the right to life".
The case was the biggest the ICJ had handled, with judges poring over tens of thousand of pages of submissions from countries and organisations around the world, including Australia.
Outside the court in the Hague, about a hundred demonstrators waved flags and posters bearing slogans like "No more delay, climate justice today".
They watched proceedings on a big screen, clapping and cheering at times during the 2-hour hearing.
When it was over, others emerged from the courtroom laughing and hugging.
Courts have become a key battleground for climate action as frustration has grown over sluggish progress toward curbing planet-warming pollution from fossil fuels.
The Paris Agreement struck through the UN Framework Convention on Climate Change (UNFCCC) has rallied a global response to the crisis, but not at the speed necessary to protect the world from dangerous overheating.
All members of the United Nations are party to the ICJ, including major emitters like the United States and China.
More than 100 nations and groups gave submissions at a mammoth two-week hearing in December in the Great Hall of Justice in the Hague.
Many hailed from distant Pacific Island nations and delivered impassioned appeals in the sober arena while dressed in colourful traditional attire.
The debate pitted major wealthy economies against the smaller, less-developed states most at the mercy of a warming planet.
Big polluters including the United States and India, as well as Australia, argued that legal provisions under the UNFCCC were sufficient in outlining the responsibilities of states regarding climate change.
But smaller states rejected this, saying the UN framework alone was inadequate to mitigate climate change's devastating effects and that the ICJ's opinion should account for a broad set of international laws.
These states also urged the ICJ to impose reparations on historic polluters, a highly sensitive issue in global climate negotiations.
They also demanded a commitment and timeline to phasing out fossil fuels, monetary compensation when appropriate, and an acknowledgement of past wrongs.
