6.7B - Ocean Acidification
Ocean acidification, as a result of its role as a carbon sink, is increasing due to fossil fuel combustion and risks crossing the critical threshold for the health of coral reefs and other marine ecosystems that provide vital ecosystem services.
Ocean acidification involves a decrease in the alkalinity/pH of oceans, caused by the uptake of carbon dioxide from the atmosphere, released by burning fossil fuels. It occurs because oceans are a carbon sink (absorbs more carbon from the atmosphere than it releases). (The carbon sink function is the precursor to a particular environment becoming a carbon store.)
Up until the early 19th century, the average ocean pH was 8.2 but this had fallen to 8.1 by 2015. This may seem a minuscule change, but the mean values disguise the fact that there has been a large fall in the pH of surface waters.
There has been a decrease everywhere since the 1700s and 1990s, which generally increases with distance from the equator, (though there is less of a change directly next to Antarctica and the Arctic). The greatest change is of less than -0.1 in an arc between north Scotland and southern Greenland.
Coral reefs, an important component of ocean life, stop growing when the pH is less than 7.8.
The situation is now approaching the point that there is a real risk of some marine ecosystems and their goods and services passing the critical threshold of permanent damage. In the case of coral reefs, they are also being threatened by the rise in surface water temperatures. The widespread bleaching of the Great Barrier Reef of Australia is a clear indication that this threat has become a reality.