Bleached coral

It is thought that coral bleaching can be made worse by increasing carbon dioxide levels, a change that can affect the health of an entire reef © Damsea/ Shutterstock

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What is ocean acidification?

Ocean acidification is often described as the evil twin of climate change.

Prof Richard Twitchett explains what ocean acidification is and what it could mean for the planet.

Earth's oceans are changing due to rising levels of carbon dioxide in the atmosphere.

Museum palaeontologist Prof Richard Twitchett explains, 'Ocean acidification is a change in the pH of seawater, which is normally around neutral'.

The pH scale measures a substance's acidity or alkalinity, spanning from zero to 14 with seven meaning neither acidic nor alkaline. Substances with a pH lower than seven are acidic, while those higher are classed as alkaline.

For millions of years, the ocean maintained a slightly alkaline state, with an average pH of around 8.2.

Richard says, 'Carbon dioxide that is present in the atmosphere dissolves into the ocean. This lowers the pH, making it more acidic.'

A group of sea urchins on a rock

Ocean acidification can affect any animal that has a calcium carbonate exoskeleton or shell, including sea urchins [pictured] sea snails, clams and oysters © IWA Design/ Shutterstock

Although the increase in acidity has only caused the ocean's pH to drop to 8.1, this actually represents a 25% increase in acidity in the last two centuries alone.

When did ocean acidification start?

It can be difficult to study ocean acidity from a long time ago.

However, scientists know that the Industrial Revolution of the 1800s triggered an escalation of carbon dioxide levels in the atmosphere, which has continued to climb ever since.

The gas is being produced faster than nature can remove it, meaning increasing amounts are being absorbed by the ocean.

Ocean acidification is now thought to occur faster than it has been in the last 20 million years. In the past, similar changes in pH have happened naturally, but over much longer periods of time.

If carbon dioxide emissions continue to accumulate at this rate, the ocean will keep absorbing more of the gas each year.

What causes ocean acidification?

Carbon dioxide is an invisible, usually odourless gas. It is emitted through natural processes, but also largely through human industry. The more carbon dioxide in the atmosphere, the more the ocean absorbs.

The main culprit for this is the burning of fossil fuels.

Other industrial processes also contribute to atmospheric carbon dioxide levels. Cement production, for example, accounted for around 8% of the gas released globally in 2015.

Additionally, deforestation reduces the number of plants absorbing the gas.

An open area in the middle of a rainforest where many trees have been cut down.

Deforestation contributes to ocean acidification. The more trees that are felled, the more carbon dioxide there is in the atmosphere, the more gas that the ocean can absorb - increasing acidity. © Rich Carey/ Shutterstock

Plants turn carbon dioxide into organic tissue such as wood and leaves. When trees are felled this carbon is turned back into carbon dioxide as the tree decays or is burnt.

But in coastal areas, logging can also displace acidic soil into waterways, gradually contributing to the lowering pH in the ocean.

'There is a natural aspect as well', says Richard. 'Naturally there are very acidic parts of the ocean, mainly around underwater hydrothermal vents and cold seeps.'

These openings occur on the sea floor and are caused by underground volcanic activity.

'The gases that come out of these and bubble up through the water contain carbon dioxide, so there are some areas of the ocean that are naturally even more acidic than the surface of the ocean.'

What effect is the change having on marine life?

Marine life is at risk from rising acidity. Organisms with shells or skeletons made from calcium carbonate are already being affected, their shells beginning to dissolve.

Richard says, 'Some Italian colleagues of mine compared populations of snails living beside volcanic vents with those living a distance away.

'What we found, unsurprisingly, was that those living near to the more acidic vents had dissolving shells.

Diagram of a projection of a pteropod shell dissolving in 2100

Laboratory experiments can show what the extent of erosion of marine animal shells will be in the future if ocean acidity continues to increase © NOAA Environmental Visualisation Laboratory (EVL)/ Wikimedia Commons

'This means that they have to spend a lot of energy trying to repair their homes. They then aren't able to put that energy into growth or reproduction.'

Experiments have also shown that if the water is more acidic, other types of animals thicken their shells and survive that way, but this also takes additional energy.

Richard says, 'Some are able to survive and reproduce, although it becomes more efficient for them to be smaller, and this could have a big impact on the food chain.'

Why does the acidity of the ocean matter?

Ocean acidification could have an impact on food supplies.

Richard says, 'There was concern that the shelled animals we eat, like oysters, may eventually all dissolve and die.

'A lot of experiments have been carried out looking at this issue, and the results have shown that the response to increased acidity varies between species.'

Studies of prehistoric acidification periods show that many groups of animals were able to survive - which is likely also the case for animals affected now.

Richard says, 'By looking at the effects of acidification in the past and the long-term trajectory for the future, some marine biologists are now moving on to look at other important issues, such as the ocean's decreasing oxygen concentration.

'Animals may be able to withstand acidic conditions, but it isn't possible for them to survive without oxygen.'

What can we do to stop ocean acidification?

Halting ocean acidification is practically impossible.

A giant clam on a reef

Giant clams such as Tridacna gigas [pictured] are a big part of the framework of reefs. But with shells made of calcium carbonate, even these large species could begin to dissolve. © Sergey Ognjannikov/ Shutterstock

Richard says, 'Realistically, there isn't actually anything that can be done beyond what is already happening, which is cutting down emissions.

'Carbon dioxide could be drawn out of the atmosphere and put into long-term storage, but apart from that there is no way that we can actually affect the pH of the ocean in any way.'

By looking back at how life coped with earlier periods of ocean acidification, scientists may be able to understand how it may cope with the current lowering of pH.

Although it may not be possible to stop acidification, by reducing carbon emissions - which many countries are pledging to do - slowing the rate it's occurring at may be within our grasp.