The increasing acidification of the oceans directly affects the role they’re able to play in the carbon cycle, exacerbating the effects of climate change and global warming. An ocean’s chemistry is extremely complex, so in order to understand the process of acidification we will focus on what is known as the Dissolved Inorganic Carbon (DIC) system and the carbonate buffer.
Acidity level relates to the hydrogen ions present in the water, as indicated by the pH, which measures the concentration of those ions in the liquid. A neutral solution, such as pure water [H2O], has a pH of 7, with equal concentrations of hydroxide [OH-] and hydrogen ions [H+]. Acidic solutions have an excess of hydrogen ions [H+] and a pH of less than 7: acidity increases as the concentration of [H+] increases. Alkaline or basic solutions, on the other hand, have an excess of [OH-] and a pH greater than 7.
Hydrogen ions are released within the upper layers of seawater as carbon dioxide is absorbed from the atmosphere into the ocean’s surface, in a rate directly proportional to its concentration in the air. In the Carbon Cycle this process is known as the Solubility Pump. Once in the water, CO2 may remain in its gaseous state or react with the water as follows:
[CO2] + [H2O] ==> [H2CO3] (Carbonic Acid)
[H2CO3] ==> [H+] + [HCO3 –] (Bicarbonate Ion)
Carbonic acid is generally unstable and quickly breaks down into bicarbonate ions, releasing [H+] into the water thus increasing its acidity. DIC in the ocean is comprised of three separate chemical compounds in the following approximate proportions:
Bicarbonate ions [HCO3 –] typically about 90%
Carbonate ions [CO3 -2 ] typically about 9%
Dissolved CO2 gas, or CO2(aq) typically about 1% of the total
The presence of these freed carbonate ions in the oceans’ water provide a way of trapping the excess hydrogen ions, regulating their concentration. The reaction between carbonate ions and hydrogen ions, forming bicarbonate ions (or hydrogen carbonate), acts as a buffer that protects the ocean system from any potential harm derived from the acidity that would be caused by excess hydrogen in the water. However, because of this carbonate buffer, the increase of CO2 in the atmosphere also contributes to the decrease of carbonate ions in the ocean.
[H+] + [CO3 -2 ] ==> [HCO3 –] (Bicarbonate Ion)
This is important because carbonate ions [CO3 -2] also play a part in the ocean’s calcium cycle, and its concentration is a determinant for the saturation horizon of calcium carbonate [CaCO3] that manifests as sedimentary rock – calcite and aragonite – on the ocean floor, as well as being the main component in the plates and shells of organisms which convert and use calcium, including, for example, for the construction of phytoplankton’s exoskeletons.
[CaCO3] <==> [Ca2+] + [CO3 -2 ]
The decrease in the concentration of carbonate ions slows down the rate at which animals, algae and phytoplankton can make organic calcium carbonate, regardless of which polymorph they use, calcite or aragonite.
The carbonate buffer in the DIC system is a natural process that has acted to stabilise the average pH of seawater at a level around 8.2, and has been able to keep the ocean alkalinity balanced over geological time despite periodical spikes in the amount of atmospheric CO2 caused, for instance, by great fires and volcanoes eruption. However, since anthropogenic emissions have increased dramatically post industrial revolution, this balance is increasingly threatened.
Over the past 200 years, approximately half of the sum total of anthropogenic CO2 emitted was absorbed via the process of photosynthesis that occurs within plankton colonies in the oceans as well as within coastal ecosystems such as mangroves, salt marshes and seagrass meadows. Despite this, proportions of CO2 in the planet’s atmosphere have risen, augmenting the dissolved inorganic carbon in the oceans’ water and consequently causing a 30% increase in the concentration of hydrogen ions. This has resulted in a reduction of the pH of surface seawater by 0.1 units and thus an increase in its acidification.
This decrease in pH levels is predicted to lead to a halving of carbonate ion concentration by 2100 compared with pre-industrial levels, interfering in CaCO3 saturation states and consequently affecting the process of calcification. This disrupts not only the food chain system within the ocean but also the biological pump of the carbon cycle. Changes in the oceans’ chemistry also affects some shallow water calcifying animals that play a vital role in releasing nutrients from sediments. Ocean acidification also threatens reef structures by reducing the growth and integrity of both corals and coralline algae, whose death is known as coral bleaching and without whom the whole coral reef habitat can not manage without.
Furthermore, the buffering capacity is diminished as both CO2 concentrations and the mean temperature of seawater increases. So far, conditions might not have appeared to change hugely, but because global warming and ocean acidification are already well under way, the planet’s oceans may not be able to provide the same service of up-taking CO2 as it has historically, with the resulting risk of affecting an increase in global warming at unpredictable rates.
Ocean acidification is essentially irreversible during our lifetimes. Even with a best-case scenario in which anthropogenic emissions were stabilised or reduced soon, and ecosystems were restored, it would take tens of thousands of years for ocean chemistry to return to a condition similar to those present before industrial times.