Man-made greenhouse gases constitute pollution of the oceans according to the UN Convention on the Law of the Sea, and states are obliged to take measures against such pollution, the International Tribunal for the Law of the Sea said in a landmark decision on Tuesday.
The Hamburg-based UN court was asked to issue an opinion on the extent to which states are obliged by international law to take stronger measures in the fight against climate change.
It was requested by a group of nine small island states in the Pacific and the Caribbean. The countries all see their existence threatened because sea levels are rising due to global warming.
The court said man-made greenhouse gases constitute pollution of the oceans and governments must take measures to reduce this, and other agreements - such as the Paris Agreement - do not release them from the obligation to do so.
The core issue was whether states are obliged by international law, such as the UN Convention on the Law of the Sea, to reduce their greenhouse gas emissions more than is regulated in the Paris Agreement, said the chairman of the Climate Change Commission of the group of island states, Payam Akhavan, ahead of the decision.
While the opinion is not binding, he believes it will have "a significant influence" on how courts will rule on climate issues in the future.
The 2015 Paris Climate Agreement aims to limit global warming to "well below" two degrees Celsius by the end of the century compared to the pre-industrial age, setting a 1.5-degree target.
Founded by Tuvalu, Antigua and Barbuda, the Commission of Small Island States on Climate Change and International Law (COSIS) also includes the Bahamas, Niue, Palau, St Kitts and Nevis, St Lucia, St Vincent and the Grenadines and Vanuatu.
In the UN Convention on the Law of the Sea, which sets out the basic regulations for almost all areas of international maritime law, the signatory states have undertaken to protect and preserve the marine environment. For example, measures must be taken against marine pollution. The almost 170 signatories include Germany, the EU and China, but not the US.