Asked by maribel
Explain how burning fossil fuels can lead to a lower ocean pH.
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Answered by
GPT-5 mini
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Short answer: burning fossil fuels releases extra CO2 to the atmosphere. Much of that CO2 dissolves into the surface ocean and reacts with seawater to form carbonic acid, which releases hydrogen ions (H+). More H+ means lower pH (more acidic).
Step‑by‑step process
- Combustion of coal, oil and gas adds CO2 gas to the atmosphere.
- CO2 from the air dissolves into the surface ocean: CO2(g) ⇌ CO2(aq).
- Dissolved CO2 reacts with water to form carbonic acid, which dissociates:
- CO2 + H2O ⇌ H2CO3
- H2CO3 ⇌ H+ + HCO3−
- HCO3− ⇌ H+ + CO3^2−
- Each dissociation releases H+ into seawater, increasing [H+] and thus lowering pH.
Why pH falls and why it matters
- pH is a logarithmic measure of [H+], so a small drop in pH represents a noticeable increase in acidity. Since pre‑industrial times the average surface ocean pH has fallen by ~0.1 units (roughly a 25–30% increase in H+ concentration).
- The added H+ also shifts carbonate chemistry toward bicarbonate, reducing the concentration of carbonate ions (CO3^2−). Lower carbonate makes it harder for shell‑ and skeleton‑forming organisms (corals, mollusks, some plankton) to build calcium carbonate (CaCO3) structures, because carbonate availability and saturation states decrease.
Other notes
- The ocean has absorbed roughly 25–40% of human CO2 emissions, which helps limit atmospheric warming but causes ocean acidification.
- Other pollutants (e.g., NOx, SO2) can contribute locally via acid deposition, but the global ocean pH decline is driven mainly by dissolved CO2 from fossil fuel burning.
Step‑by‑step process
- Combustion of coal, oil and gas adds CO2 gas to the atmosphere.
- CO2 from the air dissolves into the surface ocean: CO2(g) ⇌ CO2(aq).
- Dissolved CO2 reacts with water to form carbonic acid, which dissociates:
- CO2 + H2O ⇌ H2CO3
- H2CO3 ⇌ H+ + HCO3−
- HCO3− ⇌ H+ + CO3^2−
- Each dissociation releases H+ into seawater, increasing [H+] and thus lowering pH.
Why pH falls and why it matters
- pH is a logarithmic measure of [H+], so a small drop in pH represents a noticeable increase in acidity. Since pre‑industrial times the average surface ocean pH has fallen by ~0.1 units (roughly a 25–30% increase in H+ concentration).
- The added H+ also shifts carbonate chemistry toward bicarbonate, reducing the concentration of carbonate ions (CO3^2−). Lower carbonate makes it harder for shell‑ and skeleton‑forming organisms (corals, mollusks, some plankton) to build calcium carbonate (CaCO3) structures, because carbonate availability and saturation states decrease.
Other notes
- The ocean has absorbed roughly 25–40% of human CO2 emissions, which helps limit atmospheric warming but causes ocean acidification.
- Other pollutants (e.g., NOx, SO2) can contribute locally via acid deposition, but the global ocean pH decline is driven mainly by dissolved CO2 from fossil fuel burning.
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