Reading passage
Roughly a quarter of the carbon dioxide that humanity releases into the atmosphere does not stay there. It dissolves into the surface waters of the ocean, where it sets off a chain of chemical reactions that is steadily changing the seas. Since the beginning of the industrial revolution, the average pH of surface ocean water has fallen by about 0.1 unit. Because the pH scale is logarithmic, this apparently small shift represents an increase in acidity of around thirty percent, a rate of change probably not experienced for millions of years.
The chemistry is straightforward and uncontroversial. When carbon dioxide dissolves in seawater, it forms carbonic acid, which quickly releases hydrogen ions. These hydrogen ions make the water more acidic, but they also combine with carbonate ions, the raw material that many marine organisms use to build shells and skeletons of calcium carbonate. As more carbon dioxide enters the ocean, carbonate ions become scarcer, and the mineral forms of calcium carbonate, particularly the variety called aragonite, become harder to form and easier to dissolve.
The consequences are already visible in commercial shellfish farms. Between 2006 and 2009, hatcheries in the Pacific Northwest of the United States suffered catastrophic losses of young oysters when naturally acidic deep water welled up along the coast during seasonal winds. The larvae, which must build their first shells within days of hatching, simply failed to survive. Only after hatchery operators began to monitor incoming water and chemically buffer it during upwelling events did production recover, an early demonstration that adaptation is possible but rarely free. The episode is now regarded as a turning point, because it showed that acidification was not a distant threat but an immediate economic problem for an industry worth hundreds of millions of dollars a year.
Corals face a different but related threat. Reef structures are built from calcium carbonate, and laboratory experiments show that many corals grow more slowly, or produce weaker skeletons, as carbonate ions decline. Field studies at natural volcanic seeps, where carbon dioxide bubbles through the seabed, offer a preview of future conditions: as acidity rises, complex coral communities give way to simpler assemblages dominated by seaweeds and sponges, with losses of the species diversity that supports fisheries and tourism.
Effects are not confined to creatures with shells. Experiments on young fish indicate that acidic water can interfere with smell and hearing, the senses that larvae use to find suitable habitat and avoid predators. Clownfish raised under conditions projected for the end of this century, for instance, were attracted to the odour of predators instead of avoiding them. Some of these behavioural findings have proven difficult to replicate, however, and the extent to which they apply in the wild remains an active area of research and debate.
The pace and severity of acidification vary from place to place. Cold water absorbs more carbon dioxide than warm water, so polar seas are acidifying fastest, and parts of the Arctic Ocean are expected to become corrosive to aragonite within decades. Wind-driven upwelling brings naturally acidic deep water to the surface along the west coasts of continents, compounding the problem for ecosystems there. Tropical regions are changing more slowly, but even there the chemistry is shifting measurably from decade to decade. Nor does acidification act alone: it interacts with ocean warming and falling oxygen levels, and experiments that combine these stressors often find effects worse than any single factor would predict.
Responses fall into three broad categories. Scientists are expanding monitoring networks so that changes in seawater chemistry can be tracked in near real time. Aquaculture industries are adapting through water treatment, careful siting and the selective breeding of shellfish strains that tolerate lower pH. Some researchers are even exploring whether certain seagrasses might locally raise pH by absorbing carbon dioxide during photosynthesis. Ultimately, however, every credible assessment reaches the same conclusion: the only durable solution is to cut the emissions that drive the chemistry in the first place.