Pacific nations applaud 'lifeline' climate ruling

"For young people and for future generations, this opinion is a lifeline and an opportunity to protect all that we hold dear," said Vishal Prashad, the director of a group of island law students who spearheaded the campaign with the backing of the Vanuatu government.

"Today is historic for climate justice and we are one step closer to realising this."

Vanuatu's government had been engaging the United Nations Framework Convention on Climate Change for 30 years before the decision, the nation's climate change adaptation minister Ralph Regenvanu said.

"Today's ruling confirmed states do have legal obligations to act on climate change,"  Mr Regenvanu told reporters.

"And these obligations are grounded in international law, they're grounded in human rights law, and in the duty to protect our environment."

Australia's response to the ruling would dramatically impact its relationships with Pacific neighbours, La Trobe University anthropology and development lecturer Aidan Craney said. 

"We can either show solidarity or lose all legitimacy," he said.

The legality of Australia's continued support for and subsidisation of fossil fuel projects could come under scrutiny after the decision, Grata Fund founder Isabelle Reinecke said.

"This historic ruling by the world's highest court officially marks the end of an era of states ducking and weaving legal responsibility for climate harm," she said

The international court's opinion would pave the way for exchanges between climate change-impacted states and large polluting nations over reparations, Australian National University international law expert Donald Rothwell said.

"If settlements are not reached there is now a clearer pathway forward for international climate litigation by the specially impacted states," he said.

Importantly, all states - not just signatories to the Climate Change Convention, the Kyoto Protocol and the Paris Agreement - would face obligations under the court's advice.

While no judgment had been issued, advisory opinions could still be very influential in the interpretation and development of international law.

"In this way, the ICJ advisory opinion does not only clarify existing rules, it creates legal momentum," said Sebastien Duyck, senior attorney at the Center For International Environmental Law.

ActionAid Australia executive director Michelle Higelin said the ruling was clear and Australia must play its part to keep global heating to 1.5C.

"This is not a choice, this is an obligation to take stronger and more urgent action," she said.

ActionAid wants the government to "urgently" transition away from fossil fuels and support climate adaptation efforts in low-income countries.

Oxfam climate change policy lead Nafkote Dabi agreed.

"Rich countries have to increase their financing to Global South countries to help them reduce emissions and protect their people from past and future harm," Ms Dabi said.

"This is not a wish-list – it is international law."

Global science and policy institute, Climate Analytics, which has an Australia-Pacific region office, said the finding foreshadowed potentially serious legal penalties.

Action could be taken under customary international law if countries climate targets undershoot the Paris Agreement to limit global warming to 1.5C above pre-industrial levels.

Australia's commitment to the agreement includes reducing greenhouse gas emissions by 43 per cent below 2005 levels by 2030 and achieving net-zero emissions by 2050.

"The degradation of the climate system and of other parts of the environment impairs the enjoyment of a range of rights protected by human rights law," presiding judge Yuji Iwasawa said, handing down the court's opinion.

The court decision "confirms that states' obligations to protect human rights require taking measures to protect the climate system ... including mitigation and adaptation measures," Judge Hilary Charlesworth, an Australian member of the court, said in a separate opinion.

A response was sought from the federal government.