News about CO2 pollution is commonplace, but what you may not know is what happens with the CO2 that is released into the atmosphere. During the past 200 years the amount of anthropogenic (originating by human activity) CO2 released into the atmosphere has increased greatly. The excess CO2 in the atmosphere is absorbed into the oceans. Half of the excess CO2 from the atmosphere is in the upper 10 % of the oceans due to the slow mixing processes of the oceans. This results in chemical changes in the ocean which has caused an estimated 0.1 pH decrease in the oceans.
As the CO2 is absorbed by the ocean, it bonds with the seawater to form carbonic acid. The acid releases a bicarbonate ion and a hydrogen ion. The hydrogen ion bonds with free carbonate ions in the water which then forms a bicarbonate ion. Marine animals would normally use the carbonate for making calcium carbonate, a main component in shells and skeletons. After the chemical changes that take place due to the hydrogen ions there is less carbonate available to the marine animals.
Studies have shown that pre-industrial CO2 emissions have doubled to 401ppm, leading to a 40% decrease in coral calcification and growth. This is because of the inhibition of aragonite formation due to the decrease in carbonate-ion concentrations. Carbonate accretion will decrease greatly when CO2 in the atmosphere approaches 480 ppm and carbonate ion concentrations will drop below 200 μmol kg−1 in the oceans causing aragonite saturation values of 3.3 inoceans. Fossil records prove similar behaviors in calcified organisms in the Triassic time period where the atmospheric CO2 levels reached levels five times as high as today's. The concern for today's organisms is that theymay not have the capacity to adapt fast enough to our rapidly changing environment. If things continue at current rates, reef erosion will likely cause the diversity of corals to decline, leading to reduced habitat complexity and loss of biodiversity, including certain fish and invertebrates associated with coral.
There are many synergies connected to the decline of coral reefs. A synergy is when an outcome is dependent on the interaction of multiple factors and this outcome is greater than the separate effects added together. Some examples include reef-dependent organisms becoming rare or extinct, and effects on macroalgae and phytoplankton. There will also be effects on coastal protection, fisheries, and tourism. In order for the coral to maintain their growth rate or physical extension, they may reduce their skeleton densities. This can increase the erosion from grazing animals and storm damage.
The erosion can cause loss of structural complexity reducing habitat quality and diversity, and the ability to absorb wave energy. With the deterioration of the reef fisheries and tourism is affected because it is the coral and the fish that people like to see. As the amount of CO2 emissions increases the consequences on the reef, previously explained, become more extensive and unmanageable meaning they may become irreversible. Decreasing rates of reef accretion, increasing rates of bioerosion (biological breakdown of calcareous reef materials), rising sea levels, and intensifying storms may cause a wide range of coastal barriers to decline.