Why Scientists Call Ocean Acidification the 'Evil Twin of Climate Change'

In 2025, ocean acidification officially breached its planetary boundary. That makes it the seventh Earth system process now outside the “safe operating space for humanity.” But do we really understand what ocean acidification is? 

It’s often called the ‘evil twin of climate change’. Unlike warming, it’s about chemistry. They absorb about 30% of our CO₂ emissions and 90% of the excess heat. This makes them the most critical long‑term shield against climate change. 

The deep ocean stores far more carbon than forests, soils, or the atmosphere making it the largest carbon sink on earth. Tiny plankton help absorb CO₂, and when they die, their remains sink, locking carbon away for centuries. Cold water also absorbs CO₂ and sinks, carrying it into the depths. For hundreds to thousands of years, this system has kept our planet in balance. 

But acidification is changing that balance. It reduces the carbonate ions that corals, shellfish, and plankton need to build their shells and skeletons. Without enough carbonate, these organisms weaken. Their shells thin, their survival rates drop, and less carbon gets carried to the deep ocean. 

As acidification harms marine ecosystems, the ocean’s ability to absorb and store carbon decreases. More CO₂ stays in the atmosphere, driving climate change. And that, in turn, worsens acidification. The ocean still absorbs massive amounts of CO₂, but the system is becoming stressed and less sustainable. 

Forests and soils are vulnerable to deforestation, fires, and land‑use change. Oceans, by contrast, have always been our strongest ally, locking carbon away for centuries. Now, that strength is at risk. Instead of being a protective buffer, the ocean is becoming a strained system that may lose its effectiveness as the planet’s largest carbon sink. 

Can ocean acidification be reversed? Not completely. But it can be slowed. The most effective solution is rapid decarbonization—cutting emissions from fossil fuels, deforestation, and industry.

Transitioning to renewable energy and improving efficiency directly reduces the amount of CO₂ entering the ocean. Restoring ecosystems like mangroves, seagrasses, and salt marshes can also help buffer acidity locally. 

Ocean acidification is a warning sign. The ocean has carried the weight of our emissions for centuries. If we want it to keep protecting us, we must act now to protect it. 

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