AP Syllabus focus:
‘As more CO2 enters the atmosphere, the oceans absorb a large portion of it, which leads to increased acidity (lower pH) in seawater.’
Rising atmospheric carbon dioxide (CO₂) changes seawater chemistry.
As the ocean takes up CO₂ from the air, a chain of reactions increases hydrogen ions, lowering pH and making seawater more acidic.
How the ocean absorbs added CO₂
The ocean and atmosphere continuously exchange gases at the sea surface. When atmospheric CO₂ concentration rises, the gradient driving CO₂ into seawater increases.
Diffusion across the air–sea interface moves CO₂ from higher partial pressure in air to lower partial pressure in surface water.
Waves and wind increase surface mixing, speeding gas exchange.
Once dissolved, CO₂ can remain in surface waters or be transported deeper by circulation and mixing, allowing continued uptake at the surface.
Why “absorb a large portion” is realistic
The ocean acts as a major carbon sink because CO₂ is both physically dissolved and chemically transformed into other dissolved inorganic carbon forms. Chemical conversion reduces dissolved CO₂, helping maintain uptake from the atmosphere.
The chemistry that makes seawater more acidic
When CO₂ dissolves in seawater, it participates in reactions that produce hydrogen ions (H⁺).
This diagram summarizes the seawater carbonate system and how added atmospheric CO₂ shifts dissolved inorganic carbon among CO₂/H₂CO₃, HCO₃⁻, and CO₃²⁻. It highlights that the key driver of acidification is increased hydrogen ion concentration (H⁺), which lowers pH while also reducing carbonate availability. Source
More H⁺ means lower pH and higher acidity.
Dissolved CO₂ reacts with water to form carbonic acid (H₂CO₃).
Carbonic acid partially dissociates, releasing H⁺ and forming bicarbonate (HCO₃⁻).
Some bicarbonate further dissociates, releasing additional H⁺ and forming carbonate (CO₃²⁻).
The key idea is not that the ocean becomes “acidic” like vinegar; rather, seawater becomes more acidic than before because H⁺ increases and pH decreases.
Measuring acidity: pH
Ocean acidity is commonly tracked using pH, which is a logarithmic measure of hydrogen ion concentration.
pH: A logarithmic measure of acidity based on the concentration of hydrogen ions; lower pH indicates higher acidity.
Because pH is logarithmic, a small pH drop reflects a meaningful increase in H⁺.
This graph shows long-term observations linking rising atmospheric CO₂ to increasing CO₂ in surface seawater and a measurable decline in pH (ocean acidification). It provides empirical context for why CO₂ uptake leads to lower pH over time, rather than being only a theoretical equilibrium shift. Source
= measure of acidity (unitless)
= hydrogen ion concentration (mol/L)
Why added CO₂ lowers pH (the mechanism)
As more atmospheric CO₂ enters seawater, chemical equilibria shift toward producing more H⁺. This drives pH downward.
Carbonate system shift
Even without focusing on biological impacts, it is essential to understand that the dissolved carbon pool shifts among forms:
More CO₂ in water increases H₂CO₃ formation.
More H₂CO₃ increases dissociation, raising H⁺.
As H⁺ rises, it tends to combine with CO₃²⁻ to form HCO₃⁻, reducing carbonate availability in the carbonate system and reinforcing the chemical shift associated with lower pH.
Buffering and why pH doesn’t crash instantly
Seawater contains natural buffering systems, dominated by the carbonate–bicarbonate buffer, that resist rapid pH change.
Buffering capacity: The ability of a solution to resist changes in pH when acids or bases are added.
Buffering slows pH change, but it does not prevent it when additional CO₂ continues to enter the system over long periods. The ocean’s ability to buffer depends on the amounts of dissolved carbonate species and on mixing with deeper waters.
What controls how much acidification occurs where
Ocean acidification is not uniform; the size and speed of pH change depend on physical and chemical conditions that affect CO₂ uptake and reactions.
Key controls
Temperature: Colder water generally dissolves more CO₂, which can increase local chemical change for a given atmospheric CO₂ level.
Mixing and circulation: Strong mixing can transport dissolved carbon away from the surface, allowing continued uptake; weak mixing can trap changes near the surface.
Initial chemistry (alkalinity): Waters with higher alkalinity generally have greater buffering capacity.
Upwelling: Brings CO₂-rich deep water to the surface, often lowering pH relative to surrounding surface waters even before additional atmospheric uptake.
FAQ
Usually no; it typically remains >7, but becomes more acidic than before.
Surface uptake is fast; deep-ocean mixing occurs over decades to centuries.
Colder water dissolves more CO₂, increasing dissolved inorganic carbon and $[H^+]$.
Alkalinity reflects acid-neutralising capacity; higher alkalinity generally increases buffering.
Upwelling and respiration can raise local CO₂, increasing $[H^+]$ and lowering pH.
Practice Questions
Explain how increased atmospheric CO₂ can lead to a lower pH in seawater. (3 marks)
Describes ocean uptake: ocean absorbs CO₂ from the atmosphere due to air–sea gas exchange (1).
Links dissolved CO₂ to formation of carbonic acid/bicarbonate reactions (1).
States that reactions increase , therefore pH decreases (1).
Describe two factors that affect regional differences in how much seawater pH changes as atmospheric CO₂ increases, and explain each factor. (6 marks)
Identifies a valid factor (e.g., temperature, mixing/circulation, alkalinity, upwelling) (1).
Explains correctly how it changes CO₂ dissolution/transport/buffering and thus pH change (2).
Identifies a second valid factor (1).
Explains correctly how it changes CO₂ dissolution/transport/buffering and thus pH change (2).
