World leaders are currently gathering in the glittering coastal city of Nice for the third United Nations Ocean Conference. As they’re waving banners of hope and ambition, the ocean they claim to protect is drowning in broken promises.

Climate change, plastic pollution, disappearing ecosystems, and industrial overreach are converging into a single, planetary emergency. But, perhaps the biggest threat of all is one that’s rarely mentioned: the seas are acidifying.

Why this is so devastating

The culprit behind this sea change is one we know all too well: carbon dioxide. As we burn fossil fuels, we release billions of tons of CO₂ into the atmosphere each year. The ocean, acting as a sponge, has absorbed about a third of it. This uptake slows atmospheric warming, but it comes at a hidden cost. The chemistry of seawater is shifting and turning more acidic.

When CO₂ dissolves in seawater, it forms carbonic acid, lowering the pH and reducing the availability of carbonate ions. These ions are critical for marine organisms like corals, oysters, and tiny floating snails that build calcium carbonate shells and skeletons. The key metric scientists use to track this is the “aragonite saturation state” (Ω_arag). Think of it as the ocean’s shell-building potential. The lower the number, the harder it is to make or maintain a shell.

In 2009, scientists proposed a planetary boundary for ocean acidification: if the global average Ω_arag dropped more than 20% below its pre-industrial value, we’d enter the danger zone.

Right now, we’ve just crossed that line. But that’s not even the final story.

The problem runs deep

Helen Findlay and colleagues from institutions across the UK, US, and Europe used new model-data products to take a finer look. They incorporated both surface and subsurface data, and they included error bars — crucially missing from earlier estimates. What they found was stunning: by 2020, more than 40% of the global surface ocean had already crossed that 20% decline boundary.

Worse still, the subsurface ocean — down to 200 meters, where many marine species live and feed — has crossed that threshold across 60% of its area. The ocean isn’t just sick at the surface. Its illness runs deep.

One of the study’s key advances is that it doesn’t treat the ocean as a single bathtub. Acidification isn’t uniform — it hits harder in cold waters, where CO₂ dissolves more readily. That’s why polar regions are acidifying fastest.

Corals, the backbone of the reefs that support a quarter of all marine life, are among the first to suffer from this. It’s not just bleaching from heatwaves anymore — acidification is eroding the very building blocks of reef survival. Oysters, mussels, and gastropods are also in trouble. Researchers are already finding wild animals with damage on their shells.

Why this planetary boundary is so important

The idea, first proposed in 2009, is simple but powerful: the Earth has limits. Cross them, and we risk destabilizing the entire system that sustains life. There are nine such boundaries (climate change, biodiversity loss, land use change, and so on). Ocean acidification is one of them.

Until recently, it was thought to be among the few not yet crossed. Maybe the oceans were still holding on and doing just fine, even while absorbing all this carbon dioxide. Well, we’ve crossed it. Crossing it doesn’t mean instant catastrophe, but it means we’re rolling the dice with systems that we only partly understand.

This study tells us, with hard data and biological proof, that one more planetary guardrail has failed. Ocean acidification is no longer something we might someday avoid. It’s something we have to now learn to live with — or better yet, fight to reverse.

The only thing that would address this problem is reducing our emissions. Ultimately, this study tells us that immediate, drastic action is even more necessary.

The ocean moderates the climate. It feeds billions. It produces half the oxygen we breathe. We don’t get to break it without breaking ourselves.

The study was published in the journal Global Change Biology.