Discussions of climate change tend to fix on the atmosphere, and understandably so: rising air temperatures produce the droughts, floods and heatwaves that dominate the news cycle. But roughly a quarter to a third of the carbon dioxide that human activity releases each year does not stay in the atmosphere at all — it dissolves into the ocean, and in doing so sets off a chain of chemistry with consequences that are, in their own way, as far-reaching as anything happening on land, even though they remain largely invisible to anyone not equipped with a titration kit.
The chemistry itself is simple enough to state. Carbon dioxide dissolved in seawater reacts with water to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions. It is this increase in free hydrogen ions that lowers the ocean's pH — hence 'acidification', though the term is slightly misleading, since the ocean remains mildly alkaline overall and is merely becoming less alkaline than it was. Since the beginning of the industrial era, average ocean surface pH has fallen by about 0.1 units, which sounds modest until one recalls that the pH scale is logarithmic, so this shift represents roughly a 30% increase in the concentration of hydrogen ions — a change of a magnitude and speed that the oceans, going by the evidence of the geological record, have not experienced in at least the last several million years.
The biological stakes of this shift centre on a group of processes collectively called calcification. Corals, oysters, certain plankton and a great many other marine organisms build their shells and skeletons out of calcium carbonate, extracted from ions dissolved in seawater. As seawater becomes more acidic, the availability of the carbonate ions that these organisms need for this process declines, and at sufficiently low concentrations, the seawater can even become corrosive to existing calcium carbonate structures, dissolving them faster than the organism can rebuild. The pteropod, a small swimming sea snail that forms an important part of the diet of fish such as salmon, is often cited as an early warning sign: laboratory studies have shown its shell beginning to dissolve within weeks of exposure to seawater chemistry projected for the end of this century, and researchers have since found pteropods with visibly pitted and etched shells in parts of the Southern Ocean already.
The consequences of a decline in calcifying organisms are not confined to the organisms themselves. Coral reefs, though they cover a tiny fraction of the ocean floor, support an estimated quarter of all marine species at some point in their life cycle, and their physical structure, not merely their biological activity, provides this habitat; a reef that stops growing, or begins net dissolving, is a shrinking apartment block for an enormous and disproportionate share of ocean life. Similarly, the small calcifying plankton at the base of many marine food webs cannot decline without consequences rippling upward through the fish, seabirds and marine mammals that ultimately depend on them, even if those consequences take decades to become fully apparent.
What makes ocean acidification an unusually difficult problem to communicate, let alone to solve, is precisely its invisibility and its indifference to any fix that operates only on the atmosphere. Even in the optimistic scenario where humanity succeeds in stabilising atmospheric carbon dioxide, the ocean, having absorbed decades of excess carbon, will continue to acidify and will take centuries, not years, to re-equilibrate. The only durable solution, in other words, lies not with the ocean at all, but with the same source that produces the atmospheric problem in the first place.
The chemistry itself is simple enough to state. Carbon dioxide dissolved in seawater reacts with water to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions. It is this increase in free hydrogen ions that lowers the ocean's pH — hence 'acidification', though the term is slightly misleading, since the ocean remains mildly alkaline overall and is merely becoming less alkaline than it was. Since the beginning of the industrial era, average ocean surface pH has fallen by about 0.1 units, which sounds modest until one recalls that the pH scale is logarithmic, so this shift represents roughly a 30% increase in the concentration of hydrogen ions — a change of a magnitude and speed that the oceans, going by the evidence of the geological record, have not experienced in at least the last several million years.
The biological stakes of this shift centre on a group of processes collectively called calcification. Corals, oysters, certain plankton and a great many other marine organisms build their shells and skeletons out of calcium carbonate, extracted from ions dissolved in seawater. As seawater becomes more acidic, the availability of the carbonate ions that these organisms need for this process declines, and at sufficiently low concentrations, the seawater can even become corrosive to existing calcium carbonate structures, dissolving them faster than the organism can rebuild. The pteropod, a small swimming sea snail that forms an important part of the diet of fish such as salmon, is often cited as an early warning sign: laboratory studies have shown its shell beginning to dissolve within weeks of exposure to seawater chemistry projected for the end of this century, and researchers have since found pteropods with visibly pitted and etched shells in parts of the Southern Ocean already.
The consequences of a decline in calcifying organisms are not confined to the organisms themselves. Coral reefs, though they cover a tiny fraction of the ocean floor, support an estimated quarter of all marine species at some point in their life cycle, and their physical structure, not merely their biological activity, provides this habitat; a reef that stops growing, or begins net dissolving, is a shrinking apartment block for an enormous and disproportionate share of ocean life. Similarly, the small calcifying plankton at the base of many marine food webs cannot decline without consequences rippling upward through the fish, seabirds and marine mammals that ultimately depend on them, even if those consequences take decades to become fully apparent.
What makes ocean acidification an unusually difficult problem to communicate, let alone to solve, is precisely its invisibility and its indifference to any fix that operates only on the atmosphere. Even in the optimistic scenario where humanity succeeds in stabilising atmospheric carbon dioxide, the ocean, having absorbed decades of excess carbon, will continue to acidify and will take centuries, not years, to re-equilibrate. The only durable solution, in other words, lies not with the ocean at all, but with the same source that produces the atmospheric problem in the first place.
Question 1
Single correct option
Which of the following best expresses the central idea of the passage?
- A. Ocean acidification is a minor side-effect of climate change that will resolve itself once atmospheric carbon dioxide stabilises.
- B. Ocean acidification, driven by the ocean's absorption of carbon dioxide, threatens calcifying marine organisms and the ecosystems that depend on them, and cannot be reversed by fixes aimed only at the atmosphere.
- C. Coral reefs are the only marine ecosystems meaningfully affected by rising carbon dioxide levels.
- D. The pH of the ocean has changed so little since the industrial era that its biological effects are negligible.
Show answer and solution
Answer: (B) Ocean acidification, driven by the ocean's absorption of carbon dioxide, threatens calcifying marine organisms and the ecosystems that depend on them, and cannot be reversed by fixes aimed only at the atmosphere.
The passage explains the chemistry of acidification, its effects on calcifying organisms (corals, pteropods, plankton) and cascading ecosystem effects, and closes by stressing that even if atmospheric CO2 is stabilised, 'the ocean... will continue to acidify' and take centuries to re-equilibrate — so an atmosphere-only fix is not sufficient. Option B reflects this; the others contradict specific statements in the passage.
Question 2
Single correct option
The passage notes that average ocean surface pH has fallen by about 0.1 units since the industrial era. Why does the passage describe this as representing roughly a 30% increase in hydrogen ion concentration?
- A. Because pH and hydrogen ion concentration are unrelated, and the 30% figure is measured independently.
- B. Because the pH scale is logarithmic, so a small numerical change in pH corresponds to a much larger proportional change in hydrogen ion concentration.
- C. Because ocean temperature changes amplify the effect of any pH change threefold.
- D. Because scientists round all pH measurements to the nearest whole number before calculating percentage change.
Show answer and solution
Answer: (B) Because the pH scale is logarithmic, so a small numerical change in pH corresponds to a much larger proportional change in hydrogen ion concentration.
The passage explicitly states the reason: 'the pH scale is logarithmic, so this shift represents roughly a 30% increase in the concentration of hydrogen ions.' A logarithmic scale means small differences in the stated value correspond to large multiplicative differences in the underlying quantity.
Question 3
Single correct option
It can be inferred from the passage that stabilising atmospheric carbon dioxide levels alone would be insufficient to quickly reverse ocean acidification because:
- A. atmospheric carbon dioxide has no chemical relationship to ocean acidity.
- B. the ocean would continue to acidify from decades of already-absorbed carbon and would take centuries to re-equilibrate.
- C. ocean acidification is caused primarily by plastic pollution rather than carbon dioxide.
- D. coral reefs would regrow instantly once atmospheric carbon dioxide stopped rising.
Show answer and solution
Answer: (B) the ocean would continue to acidify from decades of already-absorbed carbon and would take centuries to re-equilibrate.
The passage states that 'the ocean, having absorbed decades of excess carbon, will continue to acidify and will take centuries, not years, to re-equilibrate,' even under an optimistic scenario of stabilised atmospheric CO2 — this is why atmosphere-only solutions are insufficient in the short term.
Question 4
Single correct option
According to the passage, why are pteropods cited as an 'early warning sign' of ocean acidification?
- A. Because their population size is easiest for researchers to count accurately.
- B. Because laboratory studies showed their shells beginning to dissolve within weeks under projected future seawater chemistry, and pitted shells have since been observed in parts of the Southern Ocean.
- C. Because pteropods are the only calcifying organisms found in the Southern Ocean.
- D. Because pteropods do not appear anywhere in the marine food chain, making them a neutral indicator species.
Show answer and solution
Answer: (B) Because laboratory studies showed their shells beginning to dissolve within weeks under projected future seawater chemistry, and pitted shells have since been observed in parts of the Southern Ocean.
The passage states that laboratory studies showed the pteropod's 'shell beginning to dissolve within weeks of exposure to seawater chemistry projected for the end of this century', and that 'researchers have since found pteropods with visibly pitted and etched shells in parts of the Southern Ocean already' — making it an early, observable warning sign.
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