The UN General Assembly on Wednesday is expected to adopt a resolution calling for a top court to outline legal obligations related to climate change, an "unprecedented challenge of civilizational proportions."

Pushed for years by Vanuatu and Pacific islander youth, the measure asks the International Court of Justice (ICJ) to lay out nations' obligations for protecting Earth's climate, and the legal consequences they face if they don't.

If the resolution passes -- as is widely expected since more than half of UN member states have co-sponsored it -- Vanuatu's Prime Minister Ishmael Kalsakau told AFP he will be "elated."

"Global warming is en route to Armageddon," warned the leader, whose Pacific nation faces rising sea-levels and experienced back-to-back cyclones earlier this month.

He added that leaders must "react very quickly, urgently" to address the climate crisis.

The government of Vanuatu started lobbying for the climate resolution in 2021, after a campaign initiated by a group of students from a university in Fiji in 2019.

Co-sponsored by some 120 nations, the resolution asks the ICJ to clarify the "obligations of States under international law to ensure the protection of the climate system."

A week ago, the UN's panel of climate experts (IPCC) warned that global average temperatures could reach 1.5C above the pre-industrial era by as early as 2030-2035, underlining the need for drastic action this decade.

While nations have no legal obligation under the Paris Agreement to meet emission reduction targets, backers of the new climate resolution hope other instruments, such as the Universal Declaration of Human Rights or the UN Convention on the Law of the Sea, could offer some pathways for enforcement.

ICJ opinions are not binding, but they carry significant legal and moral weight, and are often taken into account by national courts.

The future ruling "will serve as an important accountability tool," Harjeet Singh of Climate Action Network, an international NGO, told AFP.

He hailed the resolution's apparent success as "potentially one of the biggest climate diplomacy and multilateral successes in the recent past."

His enthusiasm however is not shared by all.

"I don't see anything useful that the Court could say. On the other hand, I see scenarios where this request would be counterproductive," Benoit Mayer, a specialist in international law at the Chinese University of Hong Kong, told AFP.

He warned of a possible "disaster scenario," if the ICJ opinion is "clear and precise, but contrary to what the supporters of the request wanted."

- 'Particularly vulnerable' islands -

Another unknown factor is the position of the two largest emitters of greenhouse gasses, China and the United States.

During negotiations on the Paris Agreement, US diplomats secured the addition of a clause specifying that the text "does not involve or provide a basis for any liability or compensation."

This is a critical issue in the debate over paying for the so-called "loss and damage" costs borne by the poorest countries, which have contributed the least to global warming.

The UN resolution notably asks the ICJ to clarify the "legal consequences" for states which "have caused significant harm to the climate system and other parts of the environment."

It specifically asks the court to weigh obligations to "small island developing States," which are "particularly vulnerable" to climate change, as well as obligations to future generations.

When the UN considers the resolution on Wednesday, it will be a "test moment for states around the world to really show where they stand," said Nikki Reisch, of the Center for international environmental law (Ciel).

It will also be an emotional day for the Pacific youth, who spearheaded the initiative.

"This was an opportunity to do something bigger than ourselves, bigger than our fears, something important for our future," said Cynthia Houniuhi, president of the group Pacific Islands Students Fighting Climate Change.