Questions: Carbon Dioxide Solubility and Ocean Circulation

Question 1

Climate models project that the thermohaline circulation could weaken significantly due to freshwater input from melting ice sheets. What effect would this have on atmospheric CO₂ levels?

• A. Atmospheric CO₂ would decrease, because more surface water would be exposed to the atmosphere for gas exchange
• B. Atmospheric CO₂ would increase, because deep water formation would slow, reducing transport of CO₂-rich surface water into the deep ocean
• C. Atmospheric CO₂ would be unaffected, because ocean chemistry controls CO₂ levels, not circulation
• D. Atmospheric CO₂ would decrease, because slower circulation means carbon stays near the surface longer and re-releases to the atmosphere

Answer: (B)

The solubility pump works by having cold, CO₂-saturated surface water sink into the deep ocean, physically removing carbon from contact with the atmosphere. If deep water formation slows, less CO₂-rich water descends, the deep-ocean carbon reservoir fills more slowly, and more CO₂ remains in the atmosphere. Paleoclimate records support this: glacial periods with stronger ventilation of deep Southern Ocean waters corresponded to lower atmospheric CO₂ than interglacials. A weakened thermohaline circulation would reduce this carbon sink.

Question 2

Why do high-latitude oceans tend to absorb CO₂ from the atmosphere while tropical oceans tend to release it?

• A. High-latitude organisms photosynthesize more, consuming CO₂, while tropical organisms respire more
• B. Cold polar waters dissolve more CO₂ than warm tropical waters, making polar surface water undersaturated relative to the atmosphere and tropical water supersaturated
• C. Trade winds push CO₂ from the tropics to the poles, forcing it into the water at high latitudes
• D. High-latitude ocean currents are slower, giving more time for CO₂ to dissolve before water moves away

Answer: (B)

CO₂ solubility in seawater follows the same temperature-dependence as in any solvent: cold water dissolves more gas. Polar surface waters, cooled by contact with cold air and sea ice, are undersaturated in CO₂ relative to the atmosphere and absorb it. Tropical waters, warmed at the surface, become supersaturated and release CO₂. This temperature-driven gradient is the physical heart of the solubility pump — the fact that CO₂ is absorbed where water is about to sink is what makes the pump effective at transferring carbon to the deep ocean.

Question 3

Cold seawater dissolves more CO₂ than warm seawater at the same pressure.

• T. True
• F. False

Answer: True

Gas solubility in liquids decreases with temperature — the same principle that makes a warm soda go flat faster than a cold one. For CO₂ in seawater, this effect is significant: polar waters (~2°C) can hold roughly twice as much dissolved CO₂ as tropical surface waters (~25°C) at equivalent atmospheric CO₂ concentrations. This temperature-solubility relationship is the physical basis for the solubility pump and explains why the high-latitude ocean is the primary entry point for atmospheric CO₂ into the deep carbon reservoir.

Question 4

As the ocean absorbs more anthropogenic CO₂, its capacity to absorb additional CO₂ in the future increases, because more carbonate chemistry is available to buffer the incoming gas.

• T. True
• F. False

Answer: False

This reverses the reality. When CO₂ dissolves in seawater, it reacts with carbonate ions (CO₃²⁻) to form bicarbonate (HCO₃⁻), consuming carbonate and reducing alkalinity. As carbonate ion concentrations fall, the ocean's buffering capacity decreases — each additional mole of CO₂ acidifies the ocean more and is absorbed less efficiently. This is a positive feedback on atmospheric CO₂: as humans emit more CO₂, the ocean becomes progressively less able to absorb the next increment, so a larger fraction stays in the atmosphere. The fraction of annual emissions absorbed by the ocean is expected to decline over time.

Question 5

Explain why the ocean's capacity to absorb anthropogenic CO₂ is expected to decline over time, even setting aside the effect of rising ocean temperatures.

Model answer: When CO₂ dissolves in seawater, it reacts with carbonate ions (CO₃²⁻) to form bicarbonate (HCO₃⁻). This reaction consumes carbonate ions, reducing the ocean's alkalinity and buffering capacity. As carbonate ion concentrations fall, the thermodynamic resistance to further CO₂ uptake increases — each additional molecule of CO₂ encounters less buffering and drives the equilibrium further toward atmospheric retention. The Revelle factor quantifies this: a high Revelle factor means the ocean must change its dissolved CO₂ concentration a lot for a given atmospheric change, indicating reduced uptake efficiency.

This is distinct from the temperature effect (warmer water holds less CO₂) but acts in the same direction. Both effects together — reduced solubility from warming and reduced buffering from acidification — mean the ocean's current role as absorbing ~25% of annual emissions will likely decline as atmospheric CO₂ rises. This creates a self-amplifying dynamic: more CO₂ in the atmosphere makes the ocean absorb proportionally less, leaving even more CO₂ in the atmosphere.