Global warming: what are the five key questions the International Court of Justice must rule on?

The opinion, expected at 3 p.m. in Paris, is expected to run to several hundred pages, clarifying nations' obligations to prevent climate change and the consequences for polluters who fail to comply. Here are five key points to be decided in the opinion.

This is the heart of the matter. The ICJ judges will seek to bring together different strands of environmental law into a definitive international standard.

Major polluters argue that this is unnecessary, since the United Nations Framework Convention on Climate Change, and its annual COPs, are the political body already tasked with taking action.

But the plaintiffs want to see a broader framework adopted, including human rights and the law of the sea. Vanuatu urged the judges to consider "the entire body of international law" in their opinion. The ICJ is "the only international court with general jurisdiction in all areas of international law, which allows it to provide such a response," the small Melanesian country said.

This is the second, controversial question that the judges considered: what are the legal consequences for countries whose pollution has contributed to the climate crisis?

Countries like the United States argue that the 2015 Paris Agreement, adopted by virtually every country in the world, is the key text, which does not provide for direct compensation for past damage. Major polluters believe it is impossible to attribute responsibility for a global phenomenon to specific countries anyway.

On the other hand, a fundamental principle of international law is invoked: "ubi jus, ubi remedium": where there is right, there is a remedy. In legal jargon, this would translate as cessation, non-repetition, and reparation. This could potentially mean stopping fossil fuel subsidies, reducing emissions, and so on.

Vulnerable countries are demanding financial reparations - unacceptable to rich countries, which have always rejected the principle - and recognition of past wrongdoing, as well as grace periods for repaying debts linked to climate disasters.

Harm or not?

Another issue is that of "transboundary" law, often known as the "no-harm rule." This principle of international law means that a state must not allow activities on its territory that are likely to cause harm to another state.

Does this rule apply to greenhouse gas emissions? Polluters argue no, since there is no single, specific source that can be identified as causing damage to another state.

One of the central questions at the December hearings was when governments became aware that greenhouse gas emissions were warming the planet.

The United States responded in the late 1980s. Switzerland argued that the link could not be established until scientific studies in that decade.

Nonsense, respond the plaintiffs, citing studies dating back to the 1960s. This point is crucial because it could affect when potential reparations would take effect.

The concept of "intergenerational equity" is another fundamental demand of climate justice activists. "The impact of climate change is not time-limited," Namibia asserts, with the most severe effects hitting populations decades, even centuries later.