Our oceans are a major carbon sink. 30% of CO2 from anthropogenic sources has been taken up since the industrial revolution by our oceans, and this has used up about one-third of the storage capacity of the ocean surface. Prior to the industrial revolution absorption and release of CO2 from the ocean was in approximate equilibrium, but uptake since this time of human induced CO2 emissions is leading to acidification of our surface ocean waters.
The ocean’s PH is currently 8.2, and is maintained by a carbonate and bicarbonate ion buffer system. However, dissolved CO2 forms carbonic acid altering the ratio of the carbonate/bicarbonate ions, resulting in a decrease in pH. This acidification process, already clearly observable in the colder waters of high latitudes, will severely impact the tropics by 2030-2050 under current scientific trajectories.
The vulnerability of different taxa to acidification depends on the form of carbonate they secrete as this affects the carbonates solubility. Coral skeletons are made of an aragonite form of calcium carbonate, and shallow tropical seawater is supersaturated with aragonite. However as atmospheric CO2 levels have risen over the last century, aragonite saturation levels have fallen significantly.
Pre industrial revolution atmospheric CO2 levels were 280 parts per million (ppm), today they are 387ppm, and they are estimated to increasing 100 times faster than has naturally occurred for at least the last 650,000 years. When atmospheric CO2 reaches 560ppm most ocean surface waters pH will have reduced by about 0.24 units to just under 7.9, and will be adversely undersaturated with respect to aragonite. Leaving only a few parts of the Pacific with adequate levels of aragonite saturation for coral growth. Under current rates of increase this will occur sometime in the third quarter of this century.
Even before this point, when CO2 levels reaches approximately 450ppm, calcification of important reef-building coral species will be reduced by up to 50%, without factoring in stresses by climate–related synergies. Over all reef building processes will be severely diminished or will cease altogether because as much as 90% of the coral skeleton is removed by normal erosion processes, so only 10% of coral growth actually contributes to the reef structure even under ideal reef-building conditions. On the Great Barrier Reef a decrease growth rate of 14% since 1990, unprecedented in the past 400years, has already been observed. This is most likely a response to acidification combined with elevated temperature stress.
Calcification, however, is not the only biological process which will be impacted by ocean acidification. Photosynthesis, oxygen exchange and reproduction are all highly likely to be impacted by relatively subtle changes in dissolved CO2 and pH. The biological effects of ocean acidification is complex because reef-building organisms have evolved unique and complex ecologies with tight interdependencies between key species making them extremely vulnerable to environmental changes, particularly changes which disrupt the pathways of the carbon system they are so totally dependent on.
EDGE – Evolutionarily Distinct & Globally Endangered. Web site.
