Ocean acidification and coral reef decline: the crisis and what it means

01What ocean acidification is and what causes it

Ocean acidification is the ongoing decrease in the pH level of the Earth's oceans, caused by the absorption of carbon dioxide from the atmosphere. Ocean acidification is the ongoing decrease in the pH of the Earth's ocean. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05. Carbon dioxide emissions from human activities are the primary cause of ocean acidification, with atmospheric carbon dioxide levels exceeding 422 ppm. CO2 from the atmosphere is absorbed by the oceans. This chemical reaction produces carbonic acid which dissociates into a bicarbonate ion and a hydrogen ion. The presence of free hydrogen ions lowers the pH of the ocean, increasing acidity. Marine calcifying organisms, such as mollusks and corals, are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.

When atmospheric CO2 dissolves in seawater, it forms carbonic acid, which releases hydrogen ions and lowers the water's pH. Since the start of the Industrial Revolution, ocean surface pH has dropped by approximately 0.1 units — a 30% increase in acidity when measured on the logarithmic pH scale.

The process is sometimes called the evil twin of climate change because it is driven by the same greenhouse gas emissions but affects marine chemistry rather than temperature directly. While climate change gets most of the public attention, acidification may prove equally destructive to ocean life.

02How CO2 changes ocean chemistry

The chemistry is straightforward but consequential. Carbon dioxide is a chemical compound with the chemical formula CO2. It is made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at normally-encountered concentrations it is odorless. As the source of carbon in the carbon cycle, atmospheric CO2 is the primary carbon source for life on Earth. In the air, carbon dioxide is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater. When CO2 dissolves in seawater it reacts with water to form carbonic acid (H2CO3), which dissociates into bicarbonate (HCO3-) and hydrogen ions (H+).

The increase in hydrogen ions is what lowers pH. But there is a second effect: the added hydrogen ions combine with carbonate ions (CO3 2-), reducing the concentration of carbonate available in the water. This is the critical consequence for marine life.

Many marine organisms — corals, mollusks, some plankton — build their shells and skeletons from calcium carbonate. They need carbonate ions to do this. When carbonate concentration drops, shell-building becomes harder and more energy-intensive. At extreme levels, existing shells can begin to dissolve.

03The impact on coral reefs and marine life

A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Coral reefs are built by polyps that secrete calcium carbonate skeletons, and acidification directly impairs this process. Slower growth means reefs are less able to recover from storms, bleaching events, and physical damage.

The effects cascade beyond corals. Pteropods — tiny swimming snails that form a critical food source for fish, whales, and seabirds — have shown shell damage in acidified waters. Oysters, clams, and urchins face similar challenges. Species that depend on these organisms for food are affected in turn.

Reefs are sometimes called the rainforests of the sea because they support roughly 25% of all marine species despite covering less than 1% of the ocean floor. When reef health declines, the biodiversity that depends on it declines with them.

04Which regions are most affected

The impacts are not uniform. Polar waters absorb CO2 more readily because cold water dissolves gases more easily, making Arctic and Antarctic regions acidify faster than tropical zones. The Pacific Northwest of North America has already seen oyster larvae die-offs linked to acidified upwelling water.

Tropical reef systems face a compounded threat: warming causes mass bleaching, and acidification slows recovery. The Great Barrier Reef, the Caribbean, and Southeast Asian reef systems have all experienced significant coverage loss. Some Caribbean reefs have lost over 80% of their coral cover since the 1970s.

Coastal communities in developing nations are most vulnerable because they depend on reef fisheries for protein and reef tourism for income. When reefs degrade, the economic and food security consequences fall hardest on those with the fewest alternatives.

05The cascade effect through marine food webs

Ocean acidification does not act in isolation. It interacts with warming, deoxygenation, overfishing, and pollution to produce effects that are difficult to predict. A species that tolerates acidification alone may not tolerate acidification combined with heat stress.

The loss of calcifying organisms — the ones that build shells — ripples through food webs. If pteropods decline, the fish that eat them decline. If those fish decline, the larger predators that eat them decline. The cascading effect can restructure entire ecosystems.

Some species may benefit from acidification conditions — certain seagrasses and algae grow faster with more CO2 available. But the overall pattern is one of reduced biodiversity and simplified ecosystems, which are less resilient to further stress.

06What the economic impact looks like

The economic stakes are substantial. Coral reefs generate an estimated $375 billion per year in ecosystem services, including fisheries, tourism, coastal protection, and biodiversity support. Losing reef systems means losing these services.

Shellfish aquaculture is directly threatened. The U.S. oyster industry, concentrated in the Pacific Northwest, has already invested in monitoring systems to detect acidified water before it reaches hatcheries. Fisheries that depend on calcifying species face declining yields.

Coastal protection is a less obvious cost. Healthy reefs absorb wave energy, reducing storm damage to shorelines. When reefs degrade, that buffer is lost, and communities face higher costs for seawalls, disaster response, and infrastructure repair.

07What can be done to slow acidification

The only permanent solution is reducing CO2 emissions. Because acidification is driven by atmospheric carbon dioxide, no local intervention can fully protect ocean chemistry while global emissions continue. The Paris Agreement targets, if met, would slow but not stop the decline.

Local and regional measures can buy time. Reducing nutrient runoff from agriculture and wastewater decreases local acidification hotspots. Restoring seagrass beds and kelp forests can locally raise pH by absorbing CO2. Marine protected areas reduce compounding stressors like overfishing.

Research into assisted evolution and selective breeding of heat- and acid-tolerant coral strains is ongoing. These approaches may help preserve reef biodiversity while the broader emissions problem is addressed. But they are interventions, not solutions — they cannot replace the fundamental need to reduce atmospheric CO2.