Ocean Acidification from CO2 Overdose

We explain all about the acidification of seawater caused by the burning of fossil fuels. We take a brief look at the ocean chemistry involved, and the pH forecasts. We also explain the effects of an acidic ocean on tropical coral reefs and other marine creatures, including the problems of chemical communication and hypercapnia.
Coral Bleaching Ocean Acidification
Recent mass coral bleaching event on Great Barrier Reef, caused by ocean acidification. OSU. CC BY 2.0

In this article, we examine the dangers of ocean acidification due to carbon dioxide (CO2) absorption. We also show the very serious threat it poses to the health of the world’s oceans and marine food web, as well as the lives of billions people worldwide who rely on the ocean for food and jobs.

Our oceans are becoming acidic at an unprecedented rate, and research into the effects of global warming on oceans indicate that this process is happening faster today than at any time in the past 300 million years.

What Is Ocean Acidification?

Ocean acidification means that sea water is becoming increasingly acidic. This is bad news for creatures that live in the sea, many of which are highly sensitive to subtle changes in acidity and ocean chemistry. In the case of corals, ocean acidification significantly impairs their ability to develop their skeletons.

Acidification is happening because the sea is absorbing CO2 from the atmosphere at an unprecedented rate. When CO2 dissolves in seawater, the water becomes more acidic and the ocean’s pH – a measure of how acidic or alkaline it is – drops.

Ocean acidification is a big worry, because it’s happening ten times faster than at any time over the last 300 million years. In the past 200 years alone, the ocean has become 28 percent more acidic.1

Such a quick change in the ocean’s chemistry does not give marine animals time to adapt. In combination with other stresses caused by global warming, like marine heatwaves, and a reduction in ocean oxygen levels, ocean acidity has the potential to decimate marine ecosystems around the world. And once this happens, much of the damage will be irreversible.

Ocean acidification – not unlike ocean deoxygenation – is sometimes referred to as climate change’s evil twin, because it is a very real and harmful consequence of global warming, but we do not see or feel it because the effects are happening underwater and out of sight.

Ocean Fact
The ocean covers two thirds of the earth’s surface, and exerts a significant influence on our weather and climate systems. Oceans emit most of the water vapor that falls as rain on land. Exactly how oceans influence climate change, is an important area of research in atmospheric science and climatology.

What Is The Cause of Ocean Acidification?

Is a nutshell, we are. Since pre-industrial times, humans have been pumping greenhouse gases like carbon dioxide and methane into the atmosphere at an alarming rate, via the burning of fossil fuels like coal, oil and gas. When released into the troposphere, these gases boost the greenhouse effect, which heats up the planet – hence the climate crisis that we face today.

The point is, at least 25 percent of all anthropogenic, or human generated, carbon dioxide is absorbed by sea water.

Marine scientists used to think that this was a good thing. If the ocean can suck 25 percent of our greenhouse gas emissions out of the atmosphere, then maybe it could help us out by mitigating some of the effects of global warming. Now, they realize that slower warming comes at a cost – and that cost, is a change in the ocean’s chemistry.

In 2017, about 36 billion tonnes of anthropogenic CO2 was released into the earth’s atmosphere 2. As CO2 doesn’t just disappear, but lingers for years – it is estimated that up to two thirds of that CO2 will be absorbed by aquatic systems over the next 20 to 200 years, while the remainder will contribute significantly to global warming.

In other words, even if we stopped all emissions today – what we have already emitted will continue to harm us all for generations to come.

Giant Seabass
A recent study showed that European sea bass (Dicentrarchus labrax) lost their sense of smell when exposed to higher levels of carbon dioxide. 3 Photo: Caitlin Childs /CC BY-SA 2.0

How Acidic Is The Ocean Today?

The upper ocean’s pH is currently 8.1 units.

It has decreased 0.1 units since preindustrial times which means it has become 28 percent more acidic. If this doesn’t sound like much, consider that many chemical reactions, including those that are essential for life, are sensitive to small changes in pH. For example, if our blood pH range drops by as little as 0.2 units, it can lead to seizures, comas and death.

Now consider that by the year 2100, the surface ocean pH is expected to decline to between 8.05 and 7.75, depending on future emissions. 4. Since a difference of one pH unit is equivalent to a tenfold difference, a drop to under 7.8 means the ocean will be 150 percent more acidic than today.

Not every part of the ocean will experience the same rate of change however. The largest increases in ocean acidity are projected to occur at high latitudes like the Arctic Ocean, with smaller increases in the tropics.

Initially, deeper waters will experience less of an increase, with models forecasting a decline in pH of between 0.2 and 0.5 at a depth of about 1’000 meters, depending on the emissions scenario and location.

Did you know: The ocean plays an important role in the carbon cycle, in fact it is earth’s largest carbon sink with huge reserves of blue carbon, some dating back thousands of years.

The Chemistry Behind Ocean Acidity

The ocean is constantly exchanging gases with the atmosphere – dissolved gases evaporate from the ocean into the atmosphere and atmospheric gases dissolve into the ocean, in a process called the ocean-atmospheric exchange.

The rate of exchange is almost constant – meaning, the amount of gas in the atmosphere and ocean is almost equal. The more CO2 there is in the atmosphere, the more the ocean will try to absorb to reach equilibrium.

When CO2 enters the ocean it dissolves in the water. This dissolution triggers a series of chemical reactions which produces carbonic acid and ultimately hydrogen ion (H+) – leading seawater to become more acidic.

In the process, it also causes carbonate ions to be relatively less abundant. Carbonate ions are important building blocks of structures for marine creatures who use calcium carbonate in their exoskeletons and shells. These creatures include: crustaceans such as crabs and lobsters, certain sponges, shelled molluscs, including oysters, mussels, snails, clams, chitons and nautilus, and some shelled zooplankton; as well as coral reefs.

The changes in ocean chemistry can affect the behavior of non-calcifying organisms as well.

The Ocean’s Ability to Absorb CO2 is Likely to Diminish

Even though the sea is huge, we have to assume that it has a limited capacity to absorb carbon dioxide. Rather like a sponge can only absorb so much liquid. A recent study 5 concluded that the ocean’s ability to absorb carbon dioxide will diminish. At the same time, researchers say, acidification in the ocean will accelerate.

The Effect of Ocean Acidification on Marine Life

A small change in the pH of seawater can have harmful effects on marine life’s reproduction, growth and even how they talk to each other. 6 Even if a sea creature is not directly harmed by acidification it may be affected indirectly through changes in its habitat or changes in the food web. The main impacts on marine life include:

1. Calcifying Species

A more acidic environment has a dramatic effect on species that build their shells with calcium carbonate such as clams, sea urchins, coral reefs and marine snails. The picture below shows what happens to a marine snail called a pteropod, when it is exposed to ocean water adjusted to sea water chemistry projected for the year 2100. Its shell melted after about 45 days. This is similar to the way acid rain corrodes limestone buildings.

When shelled organisms are at risk, the entire marine food web is at risk. For example, a 10 percent drop in the pteropod population could result in a 20 percent drop in the body weight of pink salmon, one of its main predators. Whales, krill and many other smaller fish also rely on pteropods for their source of food.

In recent years there has been a near total failure of oyster harvests in both natural ecosystems and aquaculture facilities along West Coast, America. The sea is becoming so acidic that it is dissolving the shells of baby oysters. No wonder the weakening of calcifer’s structures is sometimes called the osteoporosis of the sea.

Another vulnerable group is phytoplankton. One species in particular, foraminifera, is responsible for the sequestration of 25-50 percent of the carbon absorbed by the oceans and thus plays a vital role in limiting atmospheric carbon dioxide levels. Foraminifera also generate about 50 per of the oxygen we breathe through photosynthesis, form the basis of marine food webs and fuel the biological pump that transports carbon from the surface of the ocean to the deep interior. Now, researchers have discovered that foraminifera shells are being significantly thinned by ocean acidification.

2. Corals

Increasing ocean acidification has been shown to significantly reduce the ability of coral reefs to produce their skeletons. Corals depend on a specific pH balance to extract calcium from the seawater, a need which is particularly important for young polyps, as they settle onto rock and start building their skeletons. Studies have shown a 50-70 percent decline in larvae settlement in waters with lower pH levels.

Research indicates that, by 2100, coral reefs will erode faster than they can be rebuilt. Ocean warming and ocean deoxygenation don’t help either. This puts the long-term viability of these ecosystems and estimated one million species that depend on coral reef habitat, in danger.

Bleached Acropora Coral
Bleached Acropora coral, with normal coral in the background. Coral reefs are among the most diverse of all known marine ecosystems. As many as 1 in 4 of all ocean species depend on reefs for food and shelter, at some point in their lives. Rising sea temperatures and ocean acidity are putting coral reefs in danger. According to the IPCC’s Special Report on Global Warming of 1.5°C (2018), 1.5 degrees of warming will wipe out 70-90 percent of all warm water coral reefs, and 2 degrees will wipe out 99 percent. Image: Creative Commons CC-SA-3.0

3. Chemical Communication

Most marine species use chemical cues to determine what to eat, and when to fight, run or mate with other creatures. A sea animal can assess gender, social status, and even the reliability of a mate’s sperm, through signals from info-chemicals. When the chemistry of the water changes, it upsets this delicate balance.

Other pollution in the sea, such as microplastics, are also disrupting communication. After floating for a while, plastic begins to send out a chemical signal called DMS (dimethylsulphide). DMS acts as a foraging cue making many marine species like penguins, sharks and albatrosses think that the plastic is food.

4. Hypercapnia

Hypercapnia is a condition of elevated CO2 levels in internal fluids. 7 In certain marine animals like squid, hypercapnia can affect mobility, survival and the ability to reproduce. Fish must continually maintain a optimum internal pH, which differs from the pH of the surrounding water. If there are changes in pH in the surrounding water, the fish must work harder to maintain their internal pH level, meaning it has less energy to expend on growth and reproduction.

It is not known yet, given the rapid rise of CO2 levels in the sea, whether fish are approaching their upper limit to fight off hypercapnia. Embryos and larvae tend to be more more vulnerable to environmental stress than adults. So, even if elevated CO2 levels in aquatic environments are tolerable for adults, fish populations may still be affected through the decreased survival of eggs and larvae.

Ocean Fact
Scientists are beginning to realize how important microscopic marine microbes like bacteria and viruses are to the marine food web. However their sheer numbers of species makes it almost impossible to determine the effect of rising temperatures and global warming on their activities and numbers.

How Does Ocean Acidification Impact Humans?

The effects of climate change on humans are reasonably well known, but we tend to be less well aware of what is happening in our oceans. Perhaps, because unlike bushfires and other extreme weather events, ocean changes are less visible.

Yet, ocean acidification is affecting the entire world’s oceans, estuaries and waterways. It is impacting the economies who are dependent on fish and shellfish and the three billion people worldwide who rely on food from the ocean as their primary source of protein. 8

Nearly 50 percent of all U.S. seafood comes from Alaska, a sensitive region where ocean warming and ocean acidification are already damaging marine habitats. In the West coast region, ocean acidification has caused the near-total failure of the $100 million a year oyster industry over the past 5 years. 9

Acidification can be worse in coastal areas due to hypoxia, which is considered a driver for ocean acidification. Where there are more humans, there is more pollution – increased sewage and nutrients such as nitrate and phosphate flowing off the land. This causes eutrophication, literally excessive growth of blooms and marine algae. This in turn causes a lack of oxygen in the water which can cause many aquatic species to die.

TIP: Discover why is phosphorus important to life?

Beyond the loss of biodiversity, and the effect on fisheries and aquaculture, and food food security for millions of people, ocean acidifications threatens coastal tourism and other sea-related economies.

How Can We Stop Ocean Acidification?

Although scientists understand the chemistry behind acidification, just how exactly it will impact biological life is not yet known. Scientists have been tracking ocean pH for more than 30 years – ever since more accurate spectrophotometric pH measurement methods were introduced – but studies into biological impact really only started in 2003, when the rapid shift caught their attention and the term ocean acidification was first used. What we do know, is that the ocean will look very different in years to come. Some organisms will survive and adapt, but many more will struggle and go extinct.

Meantime, we know that ocean acidity is a direct consequence of man-made CO2 emissions. Which means, if we want to help our oceans, we must cut those emissions drastically. To avoid serious harm, atmospheric levels of CO2 need to fall to around 320-350 ppm, compared to present-day levels of about 410 ppm. 10

Climate change mitigation technologies such as carbon capture and storage may contribute towards this goal, as well as geoengineering schemes like planting more mangroves and seagrasses to suck carbon out of the water and bury it deep in their roots. A recent study, for example showed that seagrass meadows can increase surrounding sea pH levels by 0.38 units. That would give coral reefs about an extra 18 percent growth rate.


  1. Acidification chemistry and impacts on ocean life. Smithsonian []
  2. Global Carbon Budget 2018 []
  3. Near-future CO2 levels impair the olfactory system of a marine fish []
  4. Ocean indicator assessment, European Environment Agency []
  5. Surface ocean pH and buffer capacity: past, present and future – Jiang, L., Carter, B.R., Feely, R.A. et al. 2019 []
  6. How do fish talk to each other. British Council. 2017. []
  7. Physiological effects on fishes in a high‐CO2 world. 2005, Atsushi Ishimatsu et al []
  8. WHO – Availability and consumption of Fish []
  9. Ocean acidity, NOAA []
  10. Oceans are acidifying fast, IUCN[]
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