The ocean is absorbing our carbon emissions, but the chemical cost is rising rapidly

Nature · Published 3 October 2026 Swipe more like this →

In short: As the world burns fossil fuels, the ocean acts as a massive carbon sink, absorbing roughly a quarter of human-generated emissions. This service slows global warming but fundamentally alters seawater chemistry. The result is a rapid, unprecedented increase in acidity that threatens the survival of shell-building marine life everywhere.

Since the Industrial Revolution, human activity has driven atmospheric carbon dioxide from 280 parts per million to nearly 400 ppm. Because the ocean maintains a chemical balance with the air above it, it has absorbed vast quantities of this excess gas. When carbon dioxide dissolves in seawater, it triggers a series of reactions that produce carbonic acid and increase the concentration of hydrogen ions. This shift lowers the water's pH, a process known as ocean acidification.

The scale of this change is profound. While seawater remains alkaline, the average surface pH has dropped from 8.2 to approximately 8.1. Because the pH scale is logarithmic, this seemingly small shift reflects a 26 percent rise in acidity over the last 250 years. This rate of change is 100 times faster than anything marine ecosystems have endured in tens of millions of years. For much of the deep ocean, these chemical changes are expected to lag behind surface shifts by several centuries.

The primary victims are marine calcifiers—organisms like corals, oysters, clams, and plankton that rely on carbonate ions to build their skeletons and shells. As acidity rises, carbonate ions become less available, and existing structures may begin to dissolve. The consequences extend beyond these species; research indicates that acidification can impair the sensory abilities of fish, such as their capacity to detect predators. With one billion people relying on ocean-based food and livelihoods, the potential for widespread ecological and socioeconomic disruption is significant.

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