Why the oceans are becoming more acidic
Acid ionization of dissolved carbon dioxide · Acid–base · Equilibrium
The ocean has soaked up about a quarter of our CO2 emissions, and shell-building animals are paying for it.
Reactants approach
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ΔH = +7.6 kJ · endothermic, absorbs heat
ΔH per mol CO2.
Meet the molecules
- Reactantdissolved in water
CO2
dissolved carbon dioxide
- Reactantliquid
H2O
water
- Productdissolved in water
HCO3−
hydrogen carbonate ion
- Productdissolved in water
H3O+
hydronium ion
What you notice
The ocean absorbs roughly a quarter of the carbon dioxide people release by burning fossil fuels. That slows climate change but alters seawater chemistry: average surface pH has fallen from about 8.2 before the industrial era to about 8.1 today, which means roughly 30% more hydronium ions. The extra acid also lowers the concentration of carbonate ions, which corals, oysters, clams and tiny swimming snails called pteropods need to build their calcium carbonate shells and skeletons. In experiments at the pH expected later this century, some of these shells grow thinner or begin to dissolve.
Try it
Pour red cabbage juice into two glasses of water and blow gently through a straw into one for a minute; the CO2 in your breath shifts its colour toward pink compared with the other glass.
Step by step
- 1
CO2 crosses into the sea
At the surface, atmospheric CO2 dissolves until the water is in balance with the air above it. More CO2 in the air means more in the water.
- 2
A weak acid forms
A small fraction of the dissolved CO2 combines with water to form carbonic acid, H2CO3.
- 3
A proton moves
Carbonic acid passes a proton to a water molecule, giving HCO3– and H3O+. Adding these steps together gives the equation shown.
- 4
Carbonate is used up
Much of the new H3O+ reacts with carbonate ions (CO3^2– + H3O+ → HCO3– + H2O), leaving less carbonate for shell-building animals.