When we talk about carbon dioxide and the climate, the conversation usually centers on the atmosphere. But the ocean plays a huge and often overlooked role. It has absorbed a large share of the carbon dioxide humans have released, softening the pace of warming in the air. That service comes at a cost. As carbon dioxide dissolves into seawater, it changes the ocean's chemistry, a process known as ocean acidification. Sometimes called the other carbon problem, it is reshaping conditions for life throughout the sea.
The Chemistry Behind the Change
When carbon dioxide from the air dissolves into seawater, it reacts with the water to form a weak acid. This reaction increases the acidity of the ocean and, importantly, reduces the availability of certain dissolved minerals that many marine organisms depend on. Chief among these are the carbonate building blocks that creatures use to construct shells and skeletons.
Scientists measure acidity on the pH scale, where lower numbers mean more acidic. Since the start of the industrial era, the average pH of surface ocean water has dropped measurably. That may sound small, but the pH scale is compressed, so even a modest drop represents a substantial increase in acidity. The change is happening faster than at almost any point in the deep geological past, giving ecosystems little time to adjust.
Who Is Most at Risk
The organisms most vulnerable are those that build hard parts from carbonate minerals. As those minerals become scarcer, building and maintaining shells and skeletons takes more energy, and in severe conditions existing structures can begin to dissolve. Groups at particular risk include:
- Corals, whose stony skeletons form the reefs that shelter a quarter of all marine species.
- Shellfish such as oysters, clams, and mussels, which are also important to fisheries.
- Tiny floating snails called pteropods, a key food source in cold ocean regions.
- Certain plankton whose shells anchor the base of many marine food webs.
Because so many of these species sit near the bottom of the food chain, trouble for them can ripple upward, affecting the fish and larger animals, and the human communities, that ultimately depend on them.
Effects Beyond Shell-Building
Acidification does more than weaken shells. Research suggests it can affect the behavior and senses of some fish, interfere with the development of larvae and young animals, and combine with other pressures such as warming water and reduced oxygen to stress marine ecosystems in overlapping ways. A reef weakened by acidification, for example, is less able to withstand a marine heat wave, and the combined blow can be far worse than either alone.
The economic stakes are real. Coastal communities that rely on shellfish farming have already seen impacts when acidified water reaches hatcheries, and some have adapted by monitoring water chemistry closely and adjusting when they raise young shellfish.
What Can Be Done
The root cause of ocean acidification is the same as that of climate change: rising carbon dioxide. That means the most direct solution is reducing emissions. Alongside that, scientists and coastal managers pursue several approaches:
- Monitoring ocean chemistry closely so industries and communities can prepare for changing conditions.
- Protecting and restoring habitats like seagrass meadows and kelp forests that can locally buffer chemistry and support biodiversity.
- Supporting shellfish hatcheries in adapting their operations to more acidic water.
- Continuing research to identify which species and ecosystems are most resilient.
Ocean acidification is a quieter companion to climate change, unfolding largely out of sight beneath the waves. But because the ocean feeds billions of people and supports an astonishing web of life, understanding and addressing it is essential. The chemistry is clear, and so is the connection to the choices we make about carbon.