Leaders of small island states turned to the UN maritime court on Monday to seek protection of the world's oceans from catastrophic climate change which threaten the very existence of entire countries.

The nine island states are asking the International Tribunal for the Law of the Sea (ITLOS) to determine if carbon dioxide emissions absorbed by the oceans can be considered pollution, and if so, what obligations countries have to prevent it.

"This is the opening chapter in the struggle to change the conduct of the international community by clarifying the obligation of states to protect the marine environment," said the prime minister of Antigua and Barbuda, Gaston Browne.

"The time has come to speak in terms of legally binding obligations rather than empty promises that go unfulfilled," he said, addressing the court in Hamburg, Germany.

Ocean ecosystems create half the oxygen humans breathe and limit global warming by absorbing much of the carbon dioxide emitted by human activities.

But increasing emissions can warm and acidify seawaters, harming marine life.

At the heart of the case is the international treaty UNCLOS that binds countries to preventing pollution of the oceans.

The UN treaty defines pollution as the introduction by humans of "substances or energy into the marine environment" that leads to harm to marine life.

But it does not spell out carbon emissions as a specific pollutant, and the plaintiffs argue that these emissions qualify.

Marine heatwave

The push for climate justice won a big boost when the UN General Assembly in March adopted a resolution calling on the International Court of Justice to lay out nations' obligations on protecting Earth's climate and the legal consequences they face if they fail to do so.

The ICJ's advice is still pending but the action has opened up a new front to bind countries to pledges on reducing emissions.

The move at the UN had been led by Vanuatu, which also counts among the islands that had brought Monday's case before the ITLOS.

Small islands like Vanuatu are particularly exposed to the impact of global warming, with seawater rises threatening to submerge entire countries.

"Just a few years — this is all we have before the ocean consumes everything my people built across centuries," Tuvalu's Prime Minister Kausea Natano told the court.

"If international law has nothing to say about an entire country going underwater… then what purpose does it serve?" he asked, pleading for a clear direction from the court.

Across the two-thirds of the planet covered by seas, nearly 60 percent of ocean surface waters experienced at least one marine heatwave in 2022, according to the annual State of the Climate report led by scientists from the US National Oceanic and Atmospheric Administration.

This is 50 percent more than pre-industrial levels and "the highest in the modern atmospheric record and in paleoclimate records dating back as far as 800,000 years", the report published this month noted.

The world's oceans also set a new temperature record in August. Average sea surface temperatures reached an unprecedented 21 degrees Celsius (69.8 degrees Fahrenheit) for over a week, according to the EU's Copernicus Climate Change Service, after months of unusually high temperatures.

Other island states joining the ITLOS case include The Bahamas, Niue, Palau, St Kitts and Nevis, St Lucia as well as St Vincent and the Grenadines.

Thirty-four other state parties will also participate in the court hearing, with sessions scheduled through to 25 September.