The ocean has always been Earth’s quiet climate regulator. Every year, it absorbs roughly a quarter of human-produced carbon dioxide, buffering the atmosphere from even faster warming. But natural absorption alone can’t keep pace with emissions. That realization has given rise to a new field of research: ocean carbon capture, or marine carbon dioxide removal (mCDR) — the science of enhancing the ocean’s natural capacity to take in and store carbon.

At its core, ocean carbon capture encompasses a family of approaches. Some are biological, relying on the power of life itself — restoring coastal “blue carbon” ecosystems like mangroves and seagrasses, or stimulating phytoplankton growth to boost photosynthesis. Others are chemical or physical, using techniques like ocean alkalinity enhancement, where crushed minerals or alkaline substances are added to seawater to increase its ability to absorb CO₂. A more high-tech branch, known as direct ocean capture, filters dissolved carbon directly from seawater and stores it underground or converts it into stable carbonates.

The science behind these methods is grounded in the ocean’s carbon chemistry. When CO₂ dissolves in seawater, it reacts to form bicarbonate and carbonate ions — stable, long-lived forms of carbon. By altering that balance through alkalinity or biology, scientists can in theory draw more CO₂ from the air and lock it away for hundreds or even thousands of years.

What makes the idea so compelling is scale. The ocean covers 70% of the planet and already holds fifty times more carbon than the atmosphere. If harnessed carefully, even a modest enhancement of this natural process could remove billions of tons of CO₂ — a potentially crucial supplement to emission cuts. But the ocean is also a delicately balanced system, and manipulating it demands extraordinary caution.