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Ocean acidification is the process by which the ocean absorbs increasing amounts of carbon dioxide (CO2) from the atmosphere, leading to a decrease in pH levels and a change in the ocean’s chemistry.
The ocean plays a major role in reducing carbon dioxide emissions from the atmosphere and, in turn, also helps mitigate climate change and global warming. However, because it absorbs carbon dioxide, there are direct impacts on ocean chemistry. In simple terms, ocean acidification is the lowering of ocean water pH and carbonate saturation that is an outcome of the increasing carbon dioxide emissions in the atmosphere.
The ocean has absorbed around a third of the carbon dioxide emitted into the atmosphere by human activities such as deforestation, burning fossil fuels, agriculture, cement production, and more since the industrial revolution. This can have negative impacts on marine organisms such as coral reefs, shellfish, and certain species of fish, as well as on the overall ocean ecosystem.
According to the Intergovernmental Panel on Climate Change, the global ocean means pH today is around 8.1 percent. It is estimated to have decreased from 8.2 by 0.1 units over the past 250 years- it is equal to a 30 percent increase in ocean acidification.
Ocean acidification has potential and indirect ecological and biological impacts due to the chemical changes occurring in the ocean. This is happening today and will likely increase in the future depending on emissions.
When the ocean absorbs carbon dioxide, and it dissolves in the water to produce aqueous (CO2(aq)) (a solution that is solvent in water), it also produces carbonic acid (H2CO3). Carbonic acid quickly splits apart to form bicarbonate ions (HCO3). Further, these ions split apart into carbonate ions (CO32). Both these reactions produce protons. This lowers the pH of the water- the seawater thus becomes more acidic than it was before.
It is important to remember that pH is the negative logarithm of the proton activity or concentration. When carbon dioxide dissolves in ocean water, it does not fully split apart into carbonate ions, and the number of hydrogen ions formed is relatively smaller. This happens as the ocean has a natural capacity to buffer against any changes in pH.
When carbon dioxide gets neutralized by the reaction with carbonate ions to produce bicarbonate ions- the bicarbonate ions then partly dissolve. They release protons and thus decrease the pH- this is how ocean acidification occurs. However, the decrease in pH is actually much smaller than for an unbuffered system.
The total dissolved inorganic carbon (DIC) in seawater is the sum of aqueous, bicarbonate ions, and carbonate ions. The ocean surface water today has a pH of about 8.1. This means that bicarbonate ions are the dominant carbonate species in seawater. They represent around 90 percent of DIC. Carbonate ions are the next most dominant carbon species representing 10 percent of DIC. Aqueous represents less than 1 percent of DIC.
Without large amounts of carbon dioxide making chemical changes in the water, all the carbon species generally remain equal, However, today, as increasing carbon dioxide gets added to seawater, the pH will gradually decrease, and the carbon species will not be equal anymore. This all occurs due to human activities. Thus, humans are the cause of ocean acidification.
Ocean acidification severely affects marine flora and fauna. It decreases the amount of carbonates, a prime building block in seawater. This situation makes it increasingly difficult for marine organisms, such as plankton and coral reefs, to form their skeletons and shells. Even their existing shells may begin to dissolve due to ocean acidification. Human activities, such as the burning of fossil fuels and deforestation, are the main cause of ocean acidification. Reducing carbon emissions and transitioning to renewable energy sources can help slow or halt the process of ocean acidification.
Ocean acidification greatly impacts calcifying organisms like mollusks, corals, and more. Their shells and skeletons may become brittle, less strong, and dense. Coral reefs may become much more vulnerable to storm damage. Ocean acidification may also slow their recovery rate.
The current pH of seawater is always changing. A single organism can handle the changes in different pH levels during its lifetime. However, the issue with ocean acidification is the sustained nature of the change, as the risks come from lifetime exposure to lower pH levels. As carbon dioxide emissions and, in turn, the rate of ocean acidification increases, it will influence the extent to which calcifying organisms will be able to adapt or survive.
Ocean acidification does not have the same effects on all marine species. Some species of seagrass and algae may benefit from the carbon dioxide in the seawater, as this helps them increase their growth rates and photosynthesis.
Marine floral and fauna may also experience modifications in their development, growth, survival, and abundance in response to ocean acidification. Most marine organisms are much more vulnerable to acidification in their early stages of life. For instance, young fish may have trouble locating suitable habitats to live in. Thus, the frightening question now is: will ocean acidification cause the extinction of marine species?
Coral reefs, crabs, oysters, urchins, shrimps, plankton, lobsters, gastropods, starfish, and bivalves are a few of the marine organisms dying due to ocean acidification.
Even though ocean acidification affects marine organisms in different ways, both positively and negatively- it will still cause significant changes in marine ecosystems in the 21st century, according to research. The increasing impacts of climate change and global warming will further worsen the changes in ocean ecosystems. For instance, the decrease in ocean oxygen levels- is also known as ocean deoxygenation. This is currently affecting marine organisms in certain regions.