The world’s top oceans court ruled last week that greenhouse gas emissions count as marine pollution. What does that mean?

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It means that countries – including New Zealand – must take action to address climate change under the international law of the sea.

The unanimous decision comes after nine Pacific and Caribbean small island nations asked the Oceans Court (officially, the International Tribunal for the Law of the Sea) to clarify whether greenhouse gas emissions count as marine pollution under an important international oceans treaty.

The United Nations Convention on the Law of the Sea lays out everyone’s rights and responsibilities when it comes to the ocean. It says that states must “prevent, reduce and control pollution of the marine environment”. Human-made greenhouse gas emissions harm the marine environment by causing acidification and rising temperatures. But back when the treaty was adopted in 1982, climate change wasn’t really on anyone’s radar. So, it’s been unclear whether planet-heating greenhouse gases count as “pollution”.

This was the crux of the request posed by the team of small island states: do greenhouse gas emissions count as pollution, and do countries have an obligation to stop them? “The tribunal’s answer was an emphatic yes,” writes Karen Scott, law professor from the University of Canterbury.

The Oceans Court also ruled that simply complying with existing climate agreements won’t necessarily be enough to satisfy the obligations under the law of the sea. This means countries may have to take action beyond what’s required by the Paris Agreement.

The court’s decision, while not legally binding, is “a major milestone for international climate justice,” says Dalila Gharbaoui, climate crisis research fellow at the University of Canterbury. It “could be replicated as precedent for other cases and likely to inform future climate justice cases in national, regional and international courts,” she says.

Because it is non-binding, practical implementation – especially by geopolitically powerful countries still reliant on fossil fuels – is uncertain, Gharbaoui says. But the decision still carries diplomatic weight, and is a “critical step” in acknowledging the “interconnectedness of climate change and ocean health”.

It’s also a big win for small island states, who have contributed little to global heating but are disproportionately impacted by climate change. “This is a historic moment for small island developing nations in their quest for climate justice; an important first step in holding the major polluters accountable, for the sake of all humankind,” said H.E. Eselalofa Apinelu, Tuvalu high commissioner to Fiji.

The nine island nations who brought the case to the Oceans Court are Antigua and Barbuda, Tuvalu, Palau, Niue, Vanuatu, St Lucia, St Vincent and Grenadines, St Kitts and Nevis, and the Bahamas.