- As carbon dioxide (CO2) dissolves in sea water, it forms carbonic acid, decreasing the ocean’s pH, a process collectively known as ocean acidification.
- Present ocean acidification occurs approximately ten times faster than anything experienced during the last 300 million years, jeopardising the ability of ocean systems to adapt to changes in ocean chemistry due to CO2.
- Ocean acidification has the potential to change marine ecosystems and impact many ocean-related benefits to society such as coastal protection or provision of food and income.
- Increased ocean temperatures and oxygen loss act concurrently with ocean acidification and constitute the ‘deadly trio’ of climate change pressures on the marine environment.
- To combat the worst effects of the deadly trio, CO2 emissions need to be cut significantly and immediately at the source.
- Sustainable management, conservation, restoration and strong, permanent protection of at least 30% of the oceans are urgently needed.
Present ocean acidity change is unprecedented in magnitude, occurring at a rate approximately ten times faster than anything experienced during the last 300 million years. This rapid timeline is jeopardising the ability of ocean systems to adapt to changes in CO2 – a process that naturally occurs over millennia. Changes in ocean pH levels will persist as long as concentrations of atmospheric CO2 continue to rise. To avoid significant harm, atmospheric concentrations of CO2 need to get back to at least the 320-350 ppm range of CO2 in the atmosphere.
Compared to other similar events in Earth's history, ocean acidification, over hundreds of years, has been happening very fast. However, its recovery has been very slow due to the inherent time lags in the carbon and ocean cycles.
Ocean acidification has the potential to change marine ecosystems and impact many ocean-related benefits to society such as coastal protection or provision of food and income. Although more knowledge on the impacts of ocean acidification on marine life is needed, changes in many ecosystems and the services they provide to society can be extrapolated from current understanding. Some of the strongest evidence of the potential effects of ocean acidification on marine ecosystems stems from experiments on calcifying organisms.
Increased sea water acidity has been demonstrated to affect the formation and dissolution of calcium carbonate shells and skeletons in a range of marine species, including corals, molluscs such as oysters and mussels, and many phytoplankton and zooplankton species that form the base of marine food webs.
Changes in species growth and reproduction, as well as structural and functional alterations in ecosystems, will threaten food security, harm fishing industries and decrease natural shoreline protection. They will also increase the risk of inundation and erosion in low-lying areas, thereby hampering climate change adaptation and disaster risk reduction efforts.
Increased ocean temperatures are likely to have direct effects on the physiology of marine organisms and influence the geographical distribution of species. Some species such as reef-forming corals, already living at their upper tolerance level, will have more difficulties ‘moving’ fast enough to new areas. Drastic changes in ocean temperature can also lead to coral bleaching events, where corals expel the symbiotic algae living in their tissues, causing them to turn completely white. The role of coral reefs in buffering coastal communities from storm waves and erosion, and in supporting income generation (fisheries and tourism) for local communities and commercial businesses, is jeopardised. The potential recovery of such bleaching events is hampered due to the declining calcification rates on reefs caused by ocean acidification.