Jun 13th 2025|Nice
Nice, a tourism hotspot long considered the capital of the French Riviera, has been home for the past week to more than 10,000 extra visitors. As delegates of the third United Nations Ocean Conference (UNOC), their minds have been focused less on the setting and more on the task before them: to “accelerate action…to conserve and sustainably use the ocean”.
Fresh in the delegates’ minds were the conclusions of the One Ocean Science Congress, held in the city the preceding week, at which some 2,000 ocean scientists made recommendations for the UNOC. These included calls to “acknowledge the deep ocean’s critical role, and the scale of what we still do not know” and to “pause potentially risky interventions” until those uncertainties are reduced. The talks in Nice made a decent stab at achieving the first of these goals; they only inched towards achieving the second.
As well as providing humanity with food and livelihoods, the ocean has long helped protect it from the worst consequences of global warming. It has absorbed at least 25% of the carbon dioxide human industry has emitted over the past two centuries and continues to take in more each year. It has also soaked up more than 90% of the excess heat trapped by greenhouse gases since 1970. Both have meant that Earth’s surface temperatures have remained much lower than they would otherwise have been (though there are now serious concerns about how much more heat the oceans will be able to absorb overall). “In the long run, the climate is a slave to the ocean”, notes François Houllier, a biologist and the co-organiser of the science congress.
NASA estimates that the ocean as a whole holds more than 38,000 gigatonnes of carbon—over 50 times more than the atmosphere—making it the largest active carbon reservoir on Earth. Because of how ocean organisms sink after death, and cold carbon-rich water becomes denser, over centuries or millennia a large proportion of that carbon is thought to end up towards the bottom.
These deep ocean processes have, in the past, been very hard to observe. Recent advances are beginning to change that. New technologies, such as Deep Argo floats, which can conduct autonomous surveys down to 6,000m, have begun to replace expensive and infrequent submersible dives. A small but growing number of fixed sensors are also helping scientists monitor deep ocean currents and chemistry. Nevertheless vast swathes remain totally unmapped and unmonitored.
Those lacunae make it far harder to predict the consequences of activities in the deep. Deep-sea mining is one such currently in the spotlight, partly because of Donald Trump’s declaration earlier this year that his government would begin issuing commercial permits for areas of seabed, including those outside American jurisdiction. Though the seabed contains many rare-earth metals thought to be necessary for the green transition, many scientists are worried about how mining could stir up sediment and release ancient stores of carbon.
They are also concerned about ocean geoengineering, any of a family of proposed large-scale modifications aimed at increasing the amount of carbon the ocean sequesters. Many have been suggested—including growing and then sinking seaweed; fertilising surface waters with iron to stimulate phytoplankton blooms; and pumping dissolved carbon directly into deep waters—but few have been trialled at any meaningful scale.
Before approving any attempts, the majority of the scientists in Nice wanted to understand how long the carbon would remain sequestered, as well as any potential effects on the deep sea. These may not outweigh the benefits, warns Callum Roberts, a marine-conservation biologist and oceanographer at the University of Exeter. “We’ve just got to keep doing the research”, he says.
In the meantime, scientists are worried that much of the high seas—the three-fifths of global ocean that sits outside national jurisdictions—is a free-for-all. Less than 3% have any sort of protection, and experts say that what exists is not effectively enforced. This has led to increased scrutiny on the International Seabed Authority, which is currently trying to draw up rules to regulate deep-sea mining. More of UNOC’s time was spent trying to drum up support for the UN’s “high seas” treaty which, among other things, will require environmental assessments to be done ahead of geoengineering projects. Unlike preceding treaties, however, it accepts that such projects may be necessary.
Though the high seas treaty was passed in 2023, as of the week before UNOC began it had only been ratified by 31 countries, well short of the 60 needed for it to come into force. The pressure for delegations to make a public announcement in Nice seems to have helped: the tally is now at 56, with at least 12 more ratifications expected over the next few months.
This is good news, although it will take time for the ratifications to translate into new protections on the open ocean. Most of the progress in Nice may come down to attitude rather than action. As the text of the meeting’s final declaration makes plain, there is now growing political recognition of what scientists have long said: that the ocean plays an essential role in moderating global warming, and needs to be protected on this basis. ■
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