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A short class with Sarah and her students. The other ways are under More.
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When we burn fossil fuels, the CO₂ does not all stay in the air. Roughly a quarter of what humans have released has ended up somewhere else. Where?
In the ocean. It dissolves at the surface.
Right. Once it is dissolved, CO₂ reacts with water. What does it make?
Carbonic acid, H₂CO₃.
Good. Carbonic acid is weak, but some of it splits into H⁺ and HCO₃⁻. What happens to the pH when hydrogen ions increase?
The pH goes down, so the water becomes less alkaline.
Exactly. That fall in pH is what we call ocean acidification.
Seawater contains carbonate ions, CO₃²⁻. What do you think happens when extra hydrogen ions meet them?
They join together to make hydrogencarbonate, HCO₃⁻.
Yes. That soaks up some of the H⁺, which stops the pH falling even further. But what is lost?
Carbonate ions. And corals and oysters need those for their shells.
Exactly. Shells are calcium carbonate, made from calcium ions and carbonate ions. Fewer carbonate ions means building costs more energy, and in very corrosive water shells can even start to dissolve.
So the ocean is not like a strong acid eating shells straight away. It is more that the building blocks are running out.
That is a very good way to put it.
Surface ocean pH has dropped from about 8.2 to about 8.1. Is a change of 0.1 worth worrying about?
It sounds tiny.
Remember that pH is logarithmic. Each whole unit is a factor of ten in hydrogen ion concentration. A drop of 0.1 multiplies it by 10 to the power 0.1. What is that?
About 1.26, so roughly 26% more hydrogen ions.
Right. And is the ocean acidic now?
No. At 8.1 it is still alkaline. Acidification means it is moving towards the acidic side.
Perfect. The word describes the direction of change, not the end point.
more CO₂ in the air means more dissolves in seawater, especially in cold water.
CO₂ reacts with water to form H₂CO₃, which releases H⁺ ions and lowers the pH.
extra H⁺ combines with carbonate ions, so carbonate becomes scarcer.
organisms need carbonate ions to build calcium carbonate shells and skeletons.
a fall of 0.1 in pH means about 26% more hydrogen ions.
surface seawater has a pH of about 8.1; acidification is a shift towards acidic, not a change into an acid.
Since the industrial revolution, the average pH of surface seawater has fallen from about 8.2 to about 8.1.
Pteropods, tiny swimming sea snails with thin aragonite shells, are among the first organisms whose shells dissolve as acidification increases.
Increasing acidity lowers the survival of young oysters, which matters for shellfish farms as well as for wild populations.
Breathing out through a straw into water containing bromothymol blue indicator turns it from blue to yellow, a classroom model of carbon dioxide forming an acid in water.
CO₂ goes in, carbonic acid is made; H-plus goes up and carbonate starts to fade.
On a coral building site, carbonate ions are the bricks. Each day trucks of CO₂ arrive and release hydrogen ions, which grab bricks and turn them into bicarbonate. The builders can still work, but they have to search harder for every brick.
Cold seas catch more CO₂: colder water dissolves more gas, so polar oceans acidify first.
D.A.R.C. - CO₂ Dissolves, carbonic Acid forms, H⁺ is Released, Carbonate is used up.
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Which acid forms when carbon dioxide dissolves in seawater?
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Why does dissolved CO₂ lower the pH of seawater?
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Which ion becomes scarcer as the ocean acidifies?
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How much has surface ocean pH fallen since the industrial revolution?
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Why are polar seas affected early?
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Has the ocean become acidic?
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The ongoing decrease in the pH of seawater caused by the absorption of carbon dioxide from the atmosphere.
H₂CO₃, a weak acid formed when carbon dioxide reacts with water.
HCO₃⁻, also called the bicarbonate ion; formed when carbonic acid ionises or when H⁺ combines with carbonate.
CO₃²⁻, the ion that marine organisms combine with calcium ions to build calcium carbonate.
The process by which organisms build shells and skeletons of calcium carbonate.
A form of calcium carbonate used by corals and many molluscs; it dissolves more easily than calcite.
A system that resists changes in pH when acid or alkali is added.
The rising of deep, cold water to the ocean surface, bringing up dissolved nutrients and carbon dioxide.
This lesson explains the chemistry behind ocean acidification: how carbon dioxide from the air dissolves in seawater, forms carbonic acid, releases hydrogen ions and uses up the carbonate ions that shell-building organisms need.
Ocean acidification happens in a chain of steps. Carbon dioxide dissolves at the sea surface, reacts with water to form carbonic acid, and the acid releases hydrogen ions that lower the pH. Many of those hydrogen ions then combine with carbonate ions, so carbonate becomes scarcer and corals, molluscs and some plankton find it harder to build calcium carbonate shells and skeletons. The lesson also shows how small pH numbers hide large changes, why seawater is still alkaline, and why cold polar seas and upwelling zones are affected first.
Ocean acidification is the ongoing fall in the pH of seawater caused by the ocean absorbing carbon dioxide (CO₂) from the atmosphere. Since the industrial revolution the ocean has taken up roughly a quarter of the CO₂ released by human activities. This slows global warming, but it changes the chemistry of seawater through a chain of reactions.
Step 1: carbon dioxide dissolves CO₂(g) ⇌ CO₂(aq) The more CO₂ there is in the air, the more dissolves at the sea surface. Cold water dissolves more gas than warm water, so polar seas take up CO₂ readily.
Step 2: carbonic acid forms CO₂(aq) + H₂O(l) ⇌ H₂CO₃(aq) Carbonic acid is a weak acid: only part of it ionises.
Step 3: hydrogen ions are released H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq) Each carbonic acid molecule that ionises releases a hydrogen ion and a hydrogencarbonate (bicarbonate) ion. More hydrogen ions means a lower pH.
Step 4: carbonate ions are used up H⁺(aq) + CO₃²⁻(aq) → HCO₃⁻(aq) Much of the extra H⁺ combines with carbonate ions already present in seawater. This buffering limits how far the pH falls, but it removes carbonate ions. The overall change can be summarised as CO₂ + H₂O + CO₃²⁻ → 2HCO₃⁻.
Step 5: building shells becomes harder Corals, molluscs, sea urchins and some plankton build shells and skeletons from calcium carbonate: Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s). With fewer carbonate ions available, building and repairing these structures costs more energy. If carbonate falls low enough, the water becomes corrosive to calcium carbonate and unprotected shells start to dissolve. Aragonite, the form of calcium carbonate used by corals and many molluscs, dissolves much more easily than calcite, so aragonite builders feel the change first.
How big is the change?
Where the change is fastest
Acidification and warming share a cause, rising CO₂, but they are different processes. Coral bleaching is triggered mainly by unusually warm water; acidification adds a second stress by making it harder for corals to build their skeletons. Because both come from CO₂, ocean acidification is sometimes called "the other CO₂ problem".