laurence_c_smith's picture
Professor of Environmental Studies, Brown University; Author, Rivers of Power
Ocean Acidification

Ocean acidification, a stealthy side effect of rising anthropogenic CO2 emissions, is a recently discovered, little recognized global climate change threat that ought be more widely known.

Unlike the warming effect on air temperatures that rising atmospheric CO2 levels cause—which scientists have 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 surface waters of the ocean. 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. Hydrogen ion concentration defines pH (hence the “H” part of pH). Already, the concentration of H+ ions in ocean water has increased nearly 30% 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 1 unit of pH represents a ten-fold 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 impacted by this, 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 of smaller organisms like foraminifera, that comprise the bottom of the food chain for marine food webs. Nor does the problem even stop at shell and skeleton building: 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 become more easily gobbled up by predators.

How these multiple, cascading impacts 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 novel ways to try 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 that are more tolerant of low pH waters. At the extreme end of scientific tinkering, perhaps 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 impact on global rainfall patterns, but might mitigate CO2-induced air temperature warming. But because they do nothing to reduce atmospheric CO2 levels, they have zero impact on ocean acidification. At the present time, the only viable way to slow ocean acidification at the global scale is to reduce human induced CO2 emissions.