Ocean acidification is a direct consequence of carbon dioxide (CO2) emitted into the atmosphere from the burning of fossil fuels. Approximately one third of CO2 emissions are absorbed by the ocean, where it dissolves in seawater to form carbonic acid. The current volume of CO2 emitted has outstripped the ocean's capacity to neutralise it, and since the beginning of the Industrial Revolution there has been an overall increase in ocean acidity of around 30%.

This acidification process poses a serious threat to many marine species, particularly those with skeletons and structures made from calcium carbonate such as corals and sea urchins, as well as shell-forming organisms like mussels. Ocean acidification reduces the capacity of these organisms to calcify – to produce and reinforce their shells and skeletons from calcium carbonate. It is also thought to affect immune response and metabolism, which reduces growth rates. Research into the impacts of acidification on specific marine species is ongoing, but results suggest that higher acidity may be harmful for copepods, groups of small crustaceans that form an important food source for many marine species. Many fish species could also be at risk from acidosis, increased acidity in the blood and tissues that affects health and reproduction.

Tropical coral ecosystems support the richest marine biodiversity in the world, and are particularly threatened by ocean acidification. Higher acidity levels reduce the growth rate and strength of coral reefs, leaving them vulnerable to erosion, especially during storms. A related threat to coral reefs stems from rising sea temperatures, an impact of climate change, which is responsible for coral bleaching – a stress condition that involves a breakdown of the symbiotic relationship between corals and the algae that live within their tissue and provide them with food. According to the IUCN 70% of coral reefs are under threat, and 20% of those are already damaged beyond repair. As well as the marine life that depends on healthy coral reef ecosystems, coral reefs provide food, protection from storm-surge flooding, and sources of income from tourism and fishing for hundreds of millions of people around the world.

Cold-water coral ecosystems are also vulnerable to erosion through ocean acidification as they are slow growing and fragile. They occur at depths of hundreds of metres and provide important habitats for deep ocean biodiversity, including breeding grounds for many commercially important fish species. Sadly many have already been damaged by deep sea trawler fishing, and their slow rate of growth makes them particularly vulnerable to the rapidity with which acidification is occurring. The UK has a number of cold-water coral ecosystems around its coastline including the Darwin Mounds off the coast of Scotland, which have been awarded Special Area of Conservation status.

Ocean acidification is not just a serious threat to marine ecosystems, it may also reduce the capacity of the oceans to absorb CO2 , leaving more of it in the atmosphere to drive climate change. Many scientists are warning that the danger to ocean health of climate change, alongside pollution and chronic overfishing, threatens a phase of extinction of marine species unprecedented in human history.

Under threat: organisms like mussels