Ocean Acidification

The ocean absorbs carbon dioxide and other gases from the atmosphere.

The amount of carbon dioxide in the atmosphere is increasing due to human activities, such as burning fossil fuels.

The ocean has absorbed as much as 40 percent of the extra carbon dioxide produced by humans.

Almost half of all the carbon absorbed by the ocean has taken place in the Southern Ocean.

When carbon dioxide dissolves in seawater, chemical reactions cause the ocean to become more acidic.

Ocean acidification makes it difficult for corals to build their skeletons, and for marine organisms to build and maintain their shells. 

The polar regions are particularly vulnerable to the effects of ocean acidification. 

Acidification threatens Antarctic ecosystems, with potential impacts on global ocean health and human industries.

pH chart
Image credit: Karen Balliet and Hans Kirkendoll.

What is Ocean Acidification?


Ocean acidification is a chemical process that makes seawater more acidic. Acidity is measured on the pH scale from 0 to 14, with 7 being neutral. Lower numbers represent a higher acidity and vice versa. 

The ocean is naturally slightly alkaline (the opposite of acidic). Before the Industrial Age, the pH of the ocean was around 8.29.

carbon cycle
Image credit: NOAA

What causes ocean acidification?


Ocean acidification occurs when the ocean absorbs excess carbon dioxide from the atmosphere. While it is normal for the ocean to absorb some carbon dioxide, humans have released huge amounts of this greenhouse gas very quickly. Since the Industrial Revolution the amount of carbon dioxide in the atmosphere has almost doubled. It builds up in the atmosphere and the ocean is working overtime absorbing the excess. 

sea ice under microscope


How does ocean acidification happen?

When carbon dioxide is dissolved in seawater, the ocean (H2O) absorbs carbon dioxide (CO2), carbon dioxide dissolves and is converted into carbonic acid (H2CO3), which makes the ocean more acidic.

Photo credit: Sea Ice by J. Data Imagery on Foter

smog city skyline
City skyline with smog

The ocean is becoming more acidic


Before the Industrial Age, ocean pH was around 8.29. Since then it has dropped to 8.1. While this change might sound small, it makes a big difference. This is because the pH scale is logarithmic, so a change from 8 pH to 7 pH means a tenfold increase in acidity. 

This process is speeding up

In the 13 years between 1994 and 2007, the ocean absorbed four times more carbon dioxide than it did in the almost 200 years between 1800 and 1994. 


Impacts of ocean acidification

The current increase in acidity is the fastest known change in ocean chemistry in the past 50 million years, a rate of change that makes it difficult for marine ecosystems to adapt. This rapid transition to a more acidic ocean threatens to destabilize the entire Antarctic marine ecosystem.


Shells dissolving

One of the problems with a more acidic ocean is that it eats away at a mineral in the ocean called calcium carbonate.

Just as calcium is important for human bones to grow, many marine organisms with shells (called calcifiers), such as Antarctic clams and sea snails (pteropods) use calcium carbonate as the building blocks for their shells. 

Ocean acidification reduces the amount of calcium carbonate in the water, making it hard for calcifiers to build their shells. 

More acidic waters also dissolve existing shells, exposing delicate invertebrate bodies to predators and the harsh environment around them. 

As the Southern Ocean continues to become more acidic, the impacts will be felt across the ecosystem, from Antarctic clams to penguins, seals, and human fisheries.

Image source: Rogers.A.D et.al. PLoS Biology

coldwater coral

Corals threatened

As sea water becomes more acidic it becomes more difficult for cold-water corals to build their skeletons.

Cold-water corals play a vital role in marine ecosystems across the Southern Ocean, providing shelter and breeding grounds for fish and shellfish.

Like calcifiers, corals rely on calcium carbonate to build their skeletons. As ocean acidification makes calcium carbonate less available, the impacts on coral will disrupt marine ecosystems, with impacts on fisheries and the rate of global heating. 

Image credit: Simon Pietro Canese, PNRA GEOSMART Project

Under ice algae

Trace metals changing

Trace metals are essential for marine life to survive, and are present in tiny concentrations across the ocean. 

As the ocean becomes more acidic, concentrations of trace metals such as iron, zinc and nickel will change. 

These small changes could have large impacts for Antarctic life, particularly microscopic plants like algae, which are the foundation of the Antarctic food web. 

Durvillea antarctica

Carbon sink

Like plants on land, ocean plants such as kelp, microalgae and other forms of phytoplankton consume carbon dioxide through photosynthesis to create energy for life.

Across the Southern Ocean, undersea ‘forests’ of these photosynthetic organisms draw tonnes of carbon out of the atmosphere each year. 

In a more acidic ocean, changes in trace metals could affect plant growth and reduce the amount of carbon they can draw down from the atmosphere. 

This will lead to a faster increase in carbon dioxide in the atmosphere and more rapid heating.

Researchers are continuing to investigate acidification in the Southern Ocean, building upon existing data to better predict the changes to come. 

Adélie penguins

Polar oceans are worst affected


Polar oceans are particularly vulnerable to ocean acidification. Around half of the carbon dioxide taken up by the whole ocean is absorbed by the Southern Ocean, as more gas dissolves in cold water than warm water. 

In addition, calcium carbonate is naturally low in polar oceans compared to the rest of the world.  Species in the polar regions have adapted to the limited supply, but ocean acidification makes calcium carbonate even less available, making an already scarce resource even more hard to find. 

Gentoo underwater


Into the unknown

According to researchers, if we continue with ‘business as usual’ carbon dioxide emissions, by 2300 the ocean will be more acidic than it has been in around 20 million years, and the rate of change could be faster than any time in history.

We are entering uncharted territory, racing towards an ocean with a chemical composition we have never seen before.


Related reading


Ocean pump

Antarctica helps regulate global ocean currents.

stormy sea

Ocean Heat

The Southern Ocean is warming up.


Life on Ice

Reduced sea ice is impacting Antarctic life.

stormy southern ocean

Southern Ocean

The Southern Ocean flows around Antarctica.

whale tail

Life at Sea

Warmer oceans are affecting ecosystems.

Now that you’ve learned about how carbon dioxide is making the Southern Ocean more acidic, read on to learn more about extraordinary Antarctica.

Southern Ocean


humpback whale
Humpback whale
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