Sour seas: How does carbon dioxide affect our oceans?

Between 25 and 50% of carbon dioxide (CO₂) emissions produced by the burning of fossil fuels has been absorbed by the oceans, and it’s thought that this process has considerably slowed the rate of global warming. This might sound like good news, but unfortunately all that CO₂ doesn’t just conveniently disappear when it goes into the water! The problem is that CO₂ combines with water molecules to make a weak acid called carbonic acid:

Using this equation, we can predict that that the more CO₂ the oceans absorb, the more acidic they will become. We measure acidity using the pH scale. Remember, LOWER pH means HIGHER acidity.

Typical representation of the pH scale. pH 0 is highly acidic and pH 14 highly alkaline.

But do the observations reflect this theory?

The answer is yes: Sea surface pH has decreased from 8.25 to 8.14 since the Industrial Revolution. This doesn’t sound like much, but the pH scale is not linear – it is logarithmic. So this seemingly tiny change in pH actually translates to a 30% increase in H⁺ ions in our seas. We call this phenomenon ocean acidification.

This iconic dataset shows that the pH of seawater has decreased since measurements began in the 1990s, as CO2 levels in the atmosphere and oceans have increased. (Image credit: Doney, S. C., et al. Ocean acidification: The other CO2 problem. Ann. Rev. Mar. Sci.1, 169-192 (2009)).

Biologists are very worried about the effect this will have on marine life. The problem is that a vast number of marine organisms build their skeletons and shells from calcium carbonate. But calcium carbonate dissolves in acidic solutions! Many researchers have showed that when these organisms are exposed to high levels of CO₂, they cannot build their shells/skeletons properly. You can prove this to yourself by doing a simple experiment at home – click here.

Below are some examples of research showing the effect of increasing CO₂ on various sea life.

The dissolution of a pteropod shell over 45 days in seawater of increased acidity (Image credit: David Littschwager, National Geographic Society).

This example shows the drastic effect of increased acidity on coral growth. Image A shows an example of a coral grown under normal conditions, while Image B shows the same species grown in more acidic water. (Image credit: Fine, M. and Tchernov, D. Scleractinian coral species survive and recover from decalcification. Science 315, 1811-1811 (2007)).

This set of graphs shows how different types of sea creatures react to increasing CO2 levels in seawater. (Image credits: Graphs from Wittmann, A. C. and Portner, H.-O. Sensitivities of extant animal taxa to ocean acidification. Nature Clim. Change 3, 995-1001 (2013). Image credits (left-right): T. Hudson, I. Skipworth, P. Randall, A. Shaw, A. Green)). — This set of graphs shows how different types of sea creatures react to increasing CO2 levels in seawater. (Image credits: Graphs from Wittmann, A. C. and Portner, H.-O. Sensitivities of extant animal taxa to ocean acidification. Nature Clim. Change 3, 995-1001 (2013). Images (left-right): T. Hudson, I. Skipworth, P. Randall, A. Shaw, A. Green)).

Ocean acidification threatens the future of many marine creatures, and will have a knock-on effect for entire ecosystems. These ecosystems deserve to be protected not only for their beauty and uniqueness, but because we are a part of them – millions of people around the globe rely on the sea for food and income. This threat is another huge motivation to reduce our carbon emissions and take responsibility for our planet. Yet outside of the scientific community, not many people have heard of ocean acidification. Public awareness is key to shaping climate change policies. So now that you’re in the loop, spread the word!

Sour seas: How does carbon dioxide affect our oceans?

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