Our history of carbon dioxide (CO2) emissions is taking its toll on the marine environment.
Our oceans take-up some of our excess CO2 leading to oceanic warming and acidification.
The environmental ramifications of our actions from yesterday can take years to appear, and may continue for years thereafter.
Scientists set out to assess how CO2 removal from the world's oceans may eliminate the long-term consequences of our actions on marine environments.
Mathesius and colleagues simulated the effects of two interventions: the first extracted ocean CO2 at a rate of 5 Giga tonnes of carbon per year and the second at 25, with CO2 emissions increasing at the current rate.
They found that, despite the CO2 removals, they would not be able to restore pre-industrial conditions in the oceans. Furthermore they found that high CO2 emission rates followed by carbon removal interventions would have long-term oceanic consequences anyway
The pH, temperature and dissolved oxygen in the world's oceans would, even after several centuries of carbon removal, leave a substantial legacy in the marine environment.
Pure water has a pH of 7.0. while seawater, because it contains many dissolved substances, is slightly alkaline with a pH near 8.2.
If the concentration of carbon dioxide in the atmosphere continues to increase, more of it will dissolve in ocean waters. As it dissolves it forms carbonic acid (H2CO3). In the water, carbonic acid breaks down to release hydrogen ions (H+) and bicarbonate ions (HCO3-). The bicarbonate ions then break down and release H+ and carbonate ions. The more free H+ ions the more acidic the water becomes (it will actually become less alkaline).
Many marine organisms require carbonate ions to build their aragonite shells and skeletons. However, counter-intuitively, an increase in dissolved carbon dioxide leads to a decrease in carbonate ions because additional H+ ions react with carbonate ions to form bicarbonate ions.
Where seawater temperatures are above 25°C (tropics) the CO2 reaches an upper threshold and aragonite, the form of calcium carbonate found in shells, spontaneously dissolves. Because aragonite is more soluble in colder waters and cold water takes up more carbon dioxide from the atmosphere than does warm water, the aragonite threshold is lower than the tropical 1,800 ppm. Consequently, it is estimated that the cold Southern and north Pacific Oceans will reach the aragonite threshold before 2070.
Unfortunately, there will be a lag of hundreds of years between the reduction of anthropogenically-generated carbon dioxide and the restoration of ocean pH and, ominously, changes in ocean pH is already being recorded.
In practical terms, the crossing of the aragonite threshold will spell biotic collapse of ocean food chains and mass extinctions of key species.