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What Is Ocean Acidification and How Does It Affect Marine Life?

May 10, 2022

Carbon emissions don't only drive climate change, they also have major effects on the health of our oceans.

It’s no secret that carbon dioxide emissions from human activity are driving global warming. What's less talked about is the direct impact that these emissions have on earth’s ocean. One important and little discussed issue  is  ocean acidification. 

What Is Ocean Acidification?

Ocean acidification is a long-term decrease in ocean pH, or in other words an increase in the ocean’s acidity. (The lower the pH of the water, the more acidic it is.)  

The measurement scale for pH runs from 0 to 14, with any number below 7 defined as acidic. Before the dawn of the industrial revolution, the average pH of the world’s oceans was around 8.2. Today, the average pH is approximately 8.1. This reduction of 0.1 pH units may not seem significant, but it translates to a 30 percent increase in the ocean’s acidity compared to less than 300 years ago.

Dubbed climate change’s “evil twin,” ocean acidification is one of the biggest environmental challenges facing our world today. And it’s set to get worse. Researchers predict that by 2100, ocean pH will have dropped by a further 0.3 to 0.4 pH units, which would lead to a more acidic ocean than any that’s existed in the past 20 million years.

What Causes Ocean Acidification?

Ocean acidification is mainly caused by carbon dioxide emissions—which are produced in massive quantities by human activity, particularly burning fossil fuels, factory farming, and destroying ecosystems to make way for animal agriculture.

The ocean is a natural carbon sink and therefore an indispensable resource in the fight against climate change, absorbing more than one-quarter of the world’s human-caused greenhouse gas emissions. But while the absorption of large amounts of carbon dioxide by seawater helps mitigate global warming, it also kicks off a sequence of chemical reactions.  

First, the carbon dioxide (CO2) breaks down in the water (H20) and makes H2CO3, a slightly acidic compound known as carbonic acid. A bicarbonate ion and a hydrogen ion then separate from the carbonic acid. This leads to a number of hydrogen ions joining to other carbonate ions and forming additional bicarbonate, which leaves fewer carbonate ions in the water for the calcifying animals and plants that need them. The result of this chemical process is a higher concentration of hydrogen ions in the water, which makes it more acidic.

A pteropod shell is shown dissolving over time in seawater with a lower pH. When carbon dioxide is absorbed by the ocean from the atmosphere, the chemistry of the seawater is changed. (NOAA)

In addition to carbon dioxide from the air being absorbed by the ocean, it is also directly released into the water as a result of nutrient pollution from animal agriculture. Manure from factory farms is commonly either discharged into waterways or allowed to enter rivers and streams via agricultural runoff from fields where it has been applied to fertilize crops. This manure, along with the chemical fertilizers that are used to grow animal feed, pollutes the water with nitrogen, which fuels the growth of algae. The resulting algal blooms, when they die and break down, not only create ocean dead zones but also emit carbon dioxide into the water, leading to acidification. 

Industrial animal agriculture contributes to ocean acidification in other ways too. One of these is through the soil erosion caused by intensive farming practices that are used to mass-produce corn and soy to feed farmed animals.  

How Does Ocean Acidification Affect Marine Life?

While small changes in ocean chemistry occur naturally over time, seawater pH is now falling so rapidly that many marine animals may not be able to adapt to the changing conditions. These changes put the entire ocean food web at risk. Researchers are concerned that ocean acidification, one of the “deadly trio” of threats facing marine ecosystems, could lead to a dramatic loss in biodiversity. 

Coral Reefs

Known as the rainforests of the sea, coral reefs are among the most vibrant, biodiverse ecosystems on the planet. In their natural state, these marine environments provide food and habitat to an abundance of underwater life, supporting at least 4,000 different fish species, 800 coral species, and possibly millions of other aquatic animal and plant species that have not yet been discovered. 

Corals build their exoskeletons using aragonite (calcium carbonate) crystals, which are formed using carbonate ions obtained from seawater. By causing a lack of carbonate ions, ocean acidification reduces the amount of aragonite that the corals are able to make. Because of this, coral exoskeletons in more acidic waters are thinner, weaker, and more easily damaged. 

Ocean acidification, together with water pollution and warming seas, poses a serious threat to the existence of coral reefs. Researchers warn that the majority of the world’s coral reefs—an estimated 70 to 90 percent—could be lost by 2040. 


For a while, many researchers thought that fish would be unharmed by more acidic seawater, but numerous studies in recent years have shown that this unfortunately isn’t the case. Ocean acidification can affect fish behavior, health, and wellbeing in a myriad of ways. The effects often vary between individuals and species, with some being more tolerant than others.

In one laboratory experiment, clownfish raised in more acidic water did not respond to the sound of predators, suggesting a loss of hearing. In a different study, raising Menidia beryllina (Inland Silverside) fish embryos in water with high concentrations of carbon dioxide resulted in high mortality rates. Among numerous other discoveries is that ocean acidification impairs the ability of fish shoals to work together to avoid predators. 

Oysters, Mussels, Urchins, and Starfish

Ocean acidification, as we have seen, causes a lack of carbonate ions. This makes it difficult, sometimes impossible, for calcifying animals such as oysters, mussels, urchins, and starfish to grow their protective shells. 

Carbonate ions are crucial in the first 24 to 48 hours of an oyster’s life, a time during which the tiny larvae start to grow their first shells. For that reason, ocean acidification can lead to high mortality rates among oyster larvae, as was seen in the Pacific Northwest between 2006 and 2008. 

Researchers have found that as waters become more acidic, the mineral composition of California mussel shells is changing. Once mostly made up of aragonite, the shells are now mostly calcite, which is less soluble in more acidic waters but is also weaker and offers less protection from predators. 

In sea urchins, ocean acidification can slow growth, harm shell development, and negatively affect reproduction. However, some species, such as purple sea urchins, may be able to adapt to tolerate low pH conditions. 

Some species of starfish can benefit from ocean acidification. For example, research suggests that ocean acidification may promote the growth of young crown of thorns starfish by making the algae they feed on easier to eat. This, however, could result in further damage to the corals that these invertebrates prey on later in life. 

Plants and Algae

Ocean acidification can have a wide range of impacts on various plants and algae. Because carbon dioxide is needed for photosynthesis, high concentrations of carbon dioxide in the water may be beneficial to some species. 

In one study of a common coastal seaweed, exposure to high concentrations of carbon dioxide appeared to promote growth but reduce tissue strength and decrease levels of protective phlorotannins. These structural changes could negatively affect herbivore marine animals who depend on the macroalgae for food and habitat. 

Coralline algae, an ecologically important type of red algae, have skeletons made of the most easily dissolved form of calcium carbonate and are among the most likely to be harmed by ocean acidification. Where carbon dioxide concentrations are high, most of these algae become less abundant and calcification slows. 


Shelled pteropods, tiny sea snails commonly known as “sea butterflies,” are one type of zooplankton under threat from ocean acidification. Despite their small size, these creatures play a massive role in maintaining marine ecosystems and are a crucial source of food for numerous other animals including pink salmon

As the Pacific Ocean becomes more acidic, pteropods’ thin calcium carbonate shells are already beginning to dissolve. As a result, the sea snails have to spend significantly more energy on maintaining their shells. This comes at the expense of other important biological functions and makes it more difficult for them to survive. 

How Does Ocean Acidification Affect Humans?

The ocean plays a central role in sustaining life on earth. It also provides us with several ecosystem services that range from regulating the climate to serving as a primary food source for indigenous populations around the globe. But as ocean waters become more acidic, most of these ecosystem services that humans depend on are increasingly under threat. 

Carbon Storage and Climate Regulation

Ocean acidification has a complex relationship with carbon storage and climate regulation. 

One of the few potential advantages of decreasing ocean pH is that by negatively affecting calcification, it may increase the amount of carbon dioxide absorbed by the ocean and help slow down climate change. At the same time, it may also have the opposite effect and intensify global warming by decreasing marine dimethylsulphide emissions, which cool the earth’s atmosphere. 

Coastal Protection 

By degrading coral reefs, ocean acidification is damaging a valuable form of coastal protection. Coral reefs act as a natural offshore barrier to storms and floods, reducing the energy and intensity of waves before they reach the shoreline. 

Each year, the flood protection that coral reefs give to coastal areas in the U.S. benefits more than 18,100 people and prevents economic losses of around $1.8 billion, according to research led by the U.S. Geological Survey (USGS). Ocean acidification could therefore put thousands of lives, homes, and businesses in danger, and cost billions of dollars. 


Ocean acidification affects the availability of marine animals commonly farmed or wild-caught and eaten by humans as seafood. The harm that ocean acidification causes to commercially important shell-forming marine animals such as oysters, mussels, lobsters, and crabs is a major problem for the aquaculture industry. 

Ocean acidification may also reduce the presence of polyunsaturated fatty acids and proteins in seafood that are often touted as healthy for those who eat fish. Not only that, but more acidic conditions can lead to an increase in shellfish toxicity. Luckily, all of the healthy properties found in aquatic animals are also available in the many plant-based foods like algae and avocados, and other seafood alternatives.


As marine ecosystems deteriorate due to ocean acidification, there is a risk that coastal areas may see a drop in visitor numbers. Popular scuba diving sites, for example, could become less attractive in the future. For the tourism industry and regional economies, this could mean significant financial losses.

How To Reduce Ocean Acidification

Conserving seagrasses is one potential way to help reduce ocean acidification. Ultimately though, we need to tackle the root of the issue: carbon dioxide emissions.

As of 2020, the global average concentration of carbon dioxide in the atmosphere was 412.5 parts per million (ppm). To prevent “significant harm” to the ocean, atmospheric carbon dioxide needs to fall to below 350 ppm, according to the International Union for Conservation of Nature.

To have the best possible chance of achieving this target, we need to drastically and immediately cut carbon dioxide emissions, a move that requires dietary change. Ending factory farming in favor of a plant-based food system is key to slashing emissions, protecting the ocean, and fighting climate change. 


Carbon dioxide emissions and nutrient pollution from factory farming are contributing to ocean acidification. As ocean pH continues to fall at an unprecedented rate, marine biodiversity and the survival of vital ocean ecosystems is increasingly under threat. The food choices we make today have the potential to impact marine life for years to come and play a key role in saving our oceans.