Ocean acidification, a stealthy side effect of rising anthropogenic CO2 emissions, is a recently discovered, little recognized global climate-change threat that should be more widely known.
Unlike the warming effect on air temperatures that rising atmospheric CO2 levels cause—which scientists understood theoretically since the late 1800s and began describing forcefully in the late 1970s—the alarm bell for ocean acidification was rung only in 2003, in a brief scientific paper. It introduced the term “ocean acidification” to describe how some of the rising CO2 levels are absorbed and dissolved into the ocean’s surface waters. This has the benefit of slowing the pace of air-temperature warming (thus far, oceans have absorbed at least a quarter of anthropogenic CO2 emissions) but the detriment of lowering the pH of the world’s oceans.
In chemistry notation, dissolving carbon dioxide in water yields carbonic acid (CO2 + H2O ↔ H2CO3), which quickly converts into bicarbonate (HCO3-) and hydrogen (H+) ions. H+ concentration defines pH (hence the “H” part of pH). Already, the concentration of H+ ions in ocean water has increased nearly 30 percent relative to pre-industrial levels. The pH of the world’s oceans has correspondingly dropped about 0.1 pH unit and is expected to fall another 0.1 to 0.3 units by the end of this century. These numbers may sound small, but the pH scale is logarithmic, so one unit of pH represents a tenfold change in hydrogen-ion concentration.
The increased abundance of bicarbonate ions leads to decreased availability of calcite and aragonite minerals in ocean water, depriving marine mollusks, crustaceans, and corals of the primary ingredients from which they build their protective shells and skeletons. Highly calcified mollusks, echinoderms, and reef-building corals are especially sensitive. Low pH ocean water can also corrode shells directly.
Of the familiar organisms most affected by these conditions, oysters and mussels are especially vulnerable, but the detrimental effects of ocean acidification go far beyond shelled seafood. They threaten the viability of coral reefs and smaller organisms, like Foraminifera, that comprise the bottom of the food chain for marine food webs. Nor do the problems stop there: New research shows that small changes in ocean water pH alters the behavior of fish, snails, and other mobile creatures. They become stunned and confused. They lose sensory responsiveness to odors and sounds and can thus be more easily gobbled up by predators.
How these multiple cascading effects will play out for our planet’s marine ecosystem is unknown. What we do know is that as some species suffer, other, more tolerant species will replace them. Spiny sea urchins may do better (up to a point), for example. Jellyfish are especially well positioned to flourish in low pH. In a world of acidifying oceans, we can envision beaches awash not with pretty shells but with the gelatinous stinging corpses of dead jellyfish.
Scientists are now investigating new ways to mitigate the ocean-acidification problem. Certain species of sea grass, for example, may locally buffer pH. Planting or reintroducing sea grasses could provide some relief in protected estuaries and coves. Selective breeding experiments are underway to develop strains of aquatic plants and animals more tolerant of low pH waters. At the extreme end of scientific tinkering, new genetically engineered marine organisms may be on the horizon.
While some of these ideas hold promise for particular locations and species, none can stabilize pH at the global scale. Genetic engineering raises a whole other set of ethical and ecological issues. Furthermore, some of the more hopeful geoengineering solutions proposed to combat air-temperature warming work by increasing the Earth’s reflectivity (e.g., blasting sulfate aerosols into the stratosphere to reflect incoming sunlight back into space). These strategies have problems of their own, like unknown effect on global rainfall patterns, but they might mitigate CO2-induced air-temperature warming. However, because they do nothing to reduce atmospheric CO2 levels, they will have zero effect on ocean acidification. Today the only viable way to slow ocean acidification on a global scale is to reduce human-induced CO2 emissions.