Concentrations of carbon dioxide (CO2) are rising rapidly in the atmosphere, due to the burning of fossil fuels and deforestation. About 25% of the extra CO2 added to the atmosphere through human activities is being absorbed by the oceans.
When atmospheric CO2 dissolves in seawater, it first forms carbonic acid and triggers a cascade of other chemical changes. The concentrations of hydrogen ions increase and carbonate ions decline. In fact, the concentrations of hydrogen ions have already increased by 30% in the seawater compared with pre-industrial times 250 years ago, increasing the level of ocean acidity. This change in the seawater chemistry is called “Ocean Acidification”.
The surface ocean pH has declined by about 0.1 so far, and is predicted to further decline by 0.2–0.4 by the end of this century, depending on future CO2 emissions. Geological records show that the present rate of change in the seawater chemistry is 10 to 100 times faster than it has been for many millions of years.
Although some scientists had recognised more than 50 years ago that rising CO2 concentrations will affect seawater chemistry and cause ocean acidification, this phenomenon has only recently emerged as one of the big knowledge gaps in marine science, and has now become a global research priority.
There is still very little known about how the ongoing changes in the seawater chemistry will affect marine ecosystems. Experimental studies suggest ocean acidification will profoundly affect the physiology and behaviour of some marine organisms. For example, ocean acidification makes it harder for some marine animals to form their shells and skeletons, but it may make it easier for some marine plants to do photosynthesis. A lower pH in the seawater may even lead to behavioural changes in fishes and invertebrates such as a reduced ability to recognise and avoid predators. It is unclear how natural marine ecosystems, such as coral reefs, will respond to such profound global changes.