Ocean Acidification: The Carbonate Chemistry Behind It

Q: Is seawater actually acidic?

No — surface seawater sits around pH 8.1, which is chemically basic (anything above 7 is basic on the 0–14 pH scale). "Ocean acidification" describes the direction of change, from a pre-industrial pH of about 8.2 toward lower values, not the ocean crossing into acidic territory. Because pH is logarithmic, that 0.1-unit drop already represents roughly a 26% increase in hydrogen-ion concentration, and organisms evolved for 8.2 experience 8.0 as measurable physiological stress.

Q: Why does more atmospheric CO2 make it harder for shells to form?

Extra CO2 dissolves into seawater and forms carbonic acid, which dissociates into H+ and bicarbonate. Seawater buffers that acid by reacting the H+ with carbonate ions (CO3²⁻ + H+ → HCO3⁻), but this consumes the very carbonate ions that corals, oysters, and pteropods pull from the water to build aragonite and calcite shells. As atmospheric CO2 climbs from the pre-industrial 280 ppm toward 1,000 ppm scenarios, the aragonite saturation state Ω falls, and once Ω drops below 1, shells dissolve faster than they can form.

Q: Is ocean acidification the same problem as ocean warming?

No — they share a cause (rising atmospheric CO2) but act through entirely different mechanisms. Warming is a greenhouse-gas effect: CO2 traps outgoing infrared radiation, raising temperature. Acidification is a direct chemistry effect: CO2 molecules dissolve straight into the water and react with it, independent of any greenhouse forcing. It is often called "the other CO2 problem" precisely because it would still occur even if CO2 had no warming effect at all, and it cannot be reversed on human timescales even once emissions stop.

The other CO2 problem

Most CO2 coverage is about the greenhouse effect: the gas traps outgoing infrared radiation and warms the planet. But roughly 30% of all the CO2 humans have emitted since industrialisation — on the order of 525 billion tonnes — never stayed in the air at all. It dissolved directly into the ocean. That dissolution triggers a purely chemical cascade, independent of any warming, which is why oceanographers call it the other CO2 problem. It would happen even if CO2 had zero effect on temperature.

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From CO2 gas to carbonic acid

When atmospheric CO2 dissolves into seawater (Henry's Law), it reacts with water to form carbonic acid, which immediately dissociates in two steps:

CO2(g)  ⇌ CO2(aq)                    Henry's Law
CO2(aq) + H2O ⇌ H2CO3                carbonic acid
H2CO3   ⇌ H+ + HCO3⁻                 bicarbonate, pKa ≈ 6.3
HCO3⁻   ⇌ H+ + CO3²⁻                 carbonate,   pKa ≈ 10.3

Each of those steps releases free hydrogen ions, which is what lowers pH. The ocean has always run this reaction — it's not new chemistry. What's new is the rate: current pH decline is proceeding roughly 100 times faster than any natural transition visible in the geological record, too fast for most marine ecosystems to adapt on evolutionary timescales.

A buffer that eats its own capital

Seawater resists pH change through a carbonate buffering reaction: excess H+ combines with carbonate ions already dissolved in the water, converting them to bicarbonate — CO3²⁻ + H+ → HCO3⁻. This is genuinely protective in the short term, holding pH more stable than a simple acid-water system would. But it works by spending down the ocean's stock of free carbonate ions, and those are exactly the ions that shell-building organisms pull out of the water to grow aragonite and calcite. Pre-industrial surface pH was about 8.2; today it is about 8.1 — a 0.1 drop that, because pH is logarithmic, corresponds to a 26% rise in hydrogen-ion concentration.

Saturation state: the number that actually matters for shells

Whether a mineral shell grows or dissolves isn't determined by pH directly but by the saturation state Ω, which compares the product of dissolved calcium and carbonate ions to the mineral's solubility product:

Ω_arag = [Ca²⁺][CO3²⁻] / K*sp

  Ω > 1  →  precipitation favoured  (shells grow)
  Ω = 1  →  equilibrium
  Ω < 1  →  dissolution              (shells dissolve)

As atmospheric CO2 climbs — from a pre-industrial 280 ppm through today's roughly 425 ppm toward high-emission scenarios near 1,000 ppm — dissolved carbonate keeps falling and Ω_arag keeps dropping with it. Cold water absorbs more CO2 than warm water, so polar seas acidify fastest: parts of the Arctic Ocean crossed Ω < 1 around 2023, the first ocean region in modern history to become aragonite-undersaturated. Coral reefs, which occupy just 0.1% of the seafloor but support roughly a quarter of all marine species, calcify more slowly as Ω falls, and combined with warming-driven bleaching, most tropical reefs are projected to be lost above 2°C of warming. Pteropods — small aragonite-shelled sea snails that anchor polar food webs — have been shown to visibly dissolve within 45 days at pH 7.8, conditions the Southern Ocean already reaches seasonally.

Why it can't just be reversed

Unlike atmospheric temperature, which responds within years to reduced forcing, ocean acidification is effectively a one-way ratchet on human timescales. Even in a hypothetical zero-emissions future, the roughly 30% of historical CO2 already dissolved into the ocean stays there for centuries, continuing to suppress carbonate availability. Under an aggressive-mitigation pathway (RCP 2.6) surface pH is projected to stabilise near 8.05 by 2100 — uncomfortable but survivable for most organisms; under a high-emissions pathway (RCP 8.5) it could reach roughly 7.75, a level the ocean has not experienced in 20–30 million years.

Frequently asked questions

Is seawater actually acidic?

No — surface seawater sits around pH 8.1, which is chemically basic (anything above 7 is basic on the 0–14 pH scale). "Ocean acidification" describes the direction of change, from a pre-industrial pH of about 8.2 toward lower values, not the ocean crossing into acidic territory. Because pH is logarithmic, that 0.1-unit drop already represents roughly a 26% increase in hydrogen-ion concentration, and organisms evolved for 8.2 experience 8.0 as measurable physiological stress.

Why does more atmospheric CO2 make it harder for shells to form?

Extra CO2 dissolves into seawater and forms carbonic acid, which dissociates into H+ and bicarbonate. Seawater buffers that acid by reacting the H+ with carbonate ions (CO3²⁻ + H+ → HCO3⁻), but this consumes the very carbonate ions that corals, oysters, and pteropods pull from the water to build aragonite and calcite shells. As atmospheric CO2 climbs from the pre-industrial 280 ppm toward 1,000 ppm scenarios, the aragonite saturation state Ω falls, and once Ω drops below 1, shells dissolve faster than they can form.

Is ocean acidification the same problem as ocean warming?

No — they share a cause (rising atmospheric CO2) but act through entirely different mechanisms. Warming is a greenhouse-gas effect: CO2 traps outgoing infrared radiation, raising temperature. Acidification is a direct chemistry effect: CO2 molecules dissolve straight into the water and react with it, independent of any greenhouse forcing. It is often called "the other CO2 problem" precisely because it would still occur even if CO2 had no warming effect at all, and it cannot be reversed on human timescales even once emissions stop.

Try it live

Every reaction and saturation curve above runs live in Ocean Acidification. Dial atmospheric CO2 from 280 to 1,000 ppm and watch pH, carbonate ion concentration and aragonite saturation state respond together.

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