Ocean acidification

The ocean absorbs a substantial proportion of the CO₂ released by human activities. This oceanic uptake slows climate change but causes the seawater to become more acidic – a process known as ocean acidification. Ocean acidification has now gone beyond what is considered safe for marine life. A key indicator of ocean acidification is the aragonite saturation state. Aragonite is a form of calcium carbonate that many marine organisms – like corals and shellfish – use to build their shells and skeletons.

As more CO₂ enters the ocean, it forms carbonic acid, which lowers the pH but also reduces the availability of carbonate. This makes it harder for these organisms to grow and survive. Marine ecosystems are already feeling the effects. Cold-water corals, tropical coral reefs, and Arctic marine life are especially at risk as acidification continues to spread and intensify.

Control Variables

The Planetary Boundary for Ocean Acidification is defined by the aragonite saturation state of surface seawater — a measure of how easily marine organisms can build shells and skeletons from calcium carbonate. The Ocean Acidification boundary was been formally assessed as transgressed in 2025, making it the seventh Planetary Boundary to be outside of its Safe Operating Space and confirming that human CO₂ emissions have pushed ocean chemistry beyond safe levels.

Impacts

Ocean acidification affects both marine ecosystems and the planet’s ability to regulate climate. As the ocean absorbs more carbon dioxide from the atmosphere, its chemistry changes, reducing the availability of carbonate ions needed by shell-forming organisms. This disrupts marine food webs, weakens coral reefs, and even limits the ocean’s capacity to absorb CO₂, creating feedbacks that intensify climate change.