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Equally Evil Twin of Climate Change: Ocean Acidification

When we think about climate change, we automatically think of global warming as the main culprit. We think about the melting ice and rising sea levels as the consequences of global warming as these issues severely inconvenience us who live on land. More than 70% of our world comprises the oceans. We often forget that the increased carbon in our atmosphere is also behind the rapid acidification of our world’s oceans, as we don’t experience its effects directly. Ocean acidification is often known as “climate change’s equally evil twin” as it is also a very significant and harmful consequence of the excess carbon dioxide in our atmosphere.

Corals before and after ocean acidification.

Our oceans are a huge carbon sink, they absorb about 25 percent of the carbon dioxide we produce every year. This process occurs naturally as carbon is usually transferred back and forth between the atmosphere and oceans. This exchange of carbon usually occurs very slowly, generally over thousands of years. Humans have disturbed this exchange. Since the beginning of the industrial era, we have added 400 billion tons of carbon dioxide to the atmosphere due to the amount of fossil fuels we’ve burnt for energy, deforestation, and more. As a result, the ocean has absorbed around 525 billion tons of carbon dioxide from the atmosphere, including around 30 percent of all the extra carbon dioxide emitted due to human activity. It is a good thing for the atmosphere as it slows down global warming. But it is bad news for the oceans.

The natural exchange of carbon dioxide between the atmosphere and the sea. More carbon dioxide is dissolved in the ocean than released back into the atmosphere.

Oceans have recorded to have a 0.1 pH unit drop since the start of the Industrial Revolution, meaning the ocean has become more acidic. While 0.1 units might sound like a very minor change, it is consequential: Because the pH scale is logarithmic, this small 0.1 decrease means that the water is 28 percent more acidic than it was before. But how exactly does ocean acidification work?

The carbon dioxide (CO2) that is absorbed and dissolved in the ocean does not remain as floating carbon dioxide molecules. Instead, as the water (H2O) and carbon dioxide molecules mix, they react to form carbonic acid (H2CO3). This compound is an acid, meaning that in water, it releases hydrogen ions (H+) during dissociation (where compounds split reversibly into its ions). Although carbonic acid is a weak acid, which means it dissociates slower than other stronger acids such as hydrochloric acid, this still means that there is an increase in hydrogen ion concentration due to the dissociation. This means that the seawater has a lower pH by definition - pH is the scale used to determine the acidity of a solution by depending on the concentration of hydrogen ions. The more hydrogen ions present, the lower the pH of the seawater is which means it becomes more acidic. This doesn’t mean the ocean itself is acidic, even though a lot of carbon dioxide is being dissolved. However, the increasing acidity of the ocean has widely impacted the underwater ecosystem, especially shelled organisms.

The carbon dioxide that is dissolved in the seawater, reacts with water molecules to form carbonic acid. Carbonic acid readily dissociates into hydrogen and carbonate ions. The accumulation of hydrogen ions due to more carbon dioxide being dissolved is what makes the ocean more acidic.

Many essential chemical reactions are sensitive to pH. A small change in pH can have harmful effects on marine organisms, impacting chemical communication, reproduction, and growth. It has the biggest effect on the building of skeletons in shelled marine organisms such as corals, oysters, mussels, and more. A key component of their exoskeleton is calcium carbonate (CaCO3). To make calcium carbonate, shell-building marine animals combine a calcium ion (Ca2+) with a carbonate ion (CO3 2-) from the surrounding seawater, releasing carbon dioxide and water in the process. However, hydrogen ions also tend to combine with carbonate ions to form bicarbonate ions (HCO3-). They have a greater attraction to carbonate ions than calcium. But shelled-organisms can’t extract the carbonate ions they need from bicarbonate ions which prevents them from building new shells or from building their skeleton (corals). The hydrogen ions essentially hog up most of the carbonate ions, making it harder for the shelled marine organisms and corals to build their shells and skeletons. It forces them to spend more energy in this process, taking away resources from other important processes such as reproduction. If there are too many hydrogen ions and not enough molecules such as carbonate ions to bind with, they can even encourage existing calcium carbonate ions to break apart. This results in shells that already exist to dissolve, or in the case of corals, make their exoskeleton brittle, easier for it to dissolve.

As the concentration of hydrogen ions (H+) increases, the more it tends to form bicarbonate ions (HCO3-) by combining with carbonate ions (CO3 2-). This often involves binding with carbonate ions that would've otherwise been used to build the shells and skeletons of corals and shelled marine organisms, resulting in smaller populations of healthy corals and fewer, smaller shelled marine organisms.

A more acidic ocean won’t destroy all the life in the ocean, but the rise in the seawater acidity by 28 percent is already seen to be affecting some ocean organisms. Acidification is limiting coral growth by corroding their pre-existing skeletons and at the same time slowing the growth of the new skeletons, making them very vulnerable to erosion. Branching corals are especially vulnerable to acidification because of their more fragile and delicate structures. This is evident in the reefs in Papua New Guinea for example, whereas natural carbon dioxide has seeped, the delicate branching corals have disappeared, and instead, big boulder colonies have taken over, as they are more susceptible to dissolving. According to a recent study in the journal Science, if we keep increasing the concentration of carbon dioxide in the atmosphere, by around 2080, our oceans will be so acidic that even healthy coral reefs will be eroding faster than they can rebuild (Harvey). As many organisms depend and live in coral reefs, this change is likely to harmfully impact them too as they would be losing their habitat, their source of food. This in turn also affects us, humans, as many of the fish we hunt and eat are dependent on those corals, so losing coral reefs would also mean that we and many other terrestrial species would be losing a food source. As coral reefs are at the base of many ocean ecosystems, acidification will also bring down the rest of these marine ecosystems.

As branching corals are more delicate and fragile, they struggle to live in acidified waters where natural carbon dioxide seeps in, as their skeletons are more easily dissolved and broken.

Other than coral reefs, as mentioned before, ocean acidification is impacting marine shelled-organisms such as oysters, mussels, clams, urchins, and starfish. Mussels and oysters are expected to grow less shell by 25 and 10 percent respectively by the end of the century. Urchins and starfish aren't as well studied, but it is known that they build their shell parts from high-magnesium calcite, a type of calcium carbonate that dissolves more quickly than the calcium carbonate corals use (also known as aragonite). This results in their shells being a lot weaker, meaning they can be easily crushed or eaten. Some of the impacts go beyond shell-building. Mussels’ byssal threads (a bundle of filaments secreted by them), with which they famously cling onto rocks with, can’t hold on as well in acidic waters. Oyster larvae can’t even start building their shells, as they have a massive growth spurt in the first 48 hours where they build their shells quickly so that they can start feeding. However, the acidic seawater dissolves the shells before they even properly form, having already caused massive oyster death in the US Pacific Northwest. Zooplankton, which are tiny drifting animals that also build their shells of calcium carbonate, are also dissolving quickly in the oceans. Fish, which don’t have shells, are also impacted by acidification. Because the surrounding water has a lower pH, a fish’s cells often come into balance with the seawater by intaking carbonic acid. This leads to the fish having a condition called acidosis where the intake of carbonic acid changes the pH of the fish’s blood. More energy is then used to remove the excess acid in its blood, reducing the energy the fish can use for other tasks such as catching food, escaping from predators, etc. The impact of acidification on each species of marine organisms is unknown, but so far the shifts seen in the oceans have proved to be harmful and have the potential to cause a major impact on the food web and human fisheries.

A sea snail shell is dissolved over the course of 45 days in seawater, adjusted to an ocean chemistry projected for the year 2100. Similarly, other shelled marine organisms are currently becoming more incapable of building their shells such as mussels and oysters.

It is obvious what we need to do: Cut carbon emissions. We burn fewer fossil fuels, plant more trees, and mangroves that can also absorb carbon dioxide. We need to do everything we can to reduce our carbon footprint, to avoid any further destruction of the natural environment that we are so dependent on. We need to slow down the rate of ocean acidification in addition to the other consequences of excess carbon dioxide such as global warming because these processes have a huge potential to destroy the environment and ecosystems around the world that we are so heavily reliant on. If we don't act now, the destruction of the marine ecosystem also will drag the human race and the rest of life down with them.

Works Cited

Borunda, Alejandra. "Ocean Acidification." National Geographic, 7 Aug. 2019,

Chelsea Harvey, E&E News. "Corals Are Dissolving Away." Scientific American, 23 Feb. 2018,

"Effects of Ocean Acidification on Corals." Oceana USA, 3 Dec. 2014,

"Global Warming’s Evil Twin: Ocean Acidification." Climate Reality, 6 Nov. 2019,

Ocean Portal Team. "Ocean Acidification." Smithsonian Ocean, 20 June 2019, Accessed 16 Oct. 2020.

Walsh, Bryan. "Ocean Acidification Will Make Climate Change Worse.", 26 Aug. 2013,

Images Used

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