Changing ocean chemistry threatens the survival of marine life as much as warming temperatures. Understanding the basic chemistry of ocean acidification and the relevant consequences for people and wildlife are keys to effective journalism on an issue of growing importance and interest to media audiences.
Far-reaching implications – a threatened food supply, lost coastal protection, diminished biodiversity, and disrupted carbon cycling – arise from these chemical reactions. The story involves fundamental change within the largest living space on the planet, changes that are happening fast, and right now.
The Basic Chemistry of Ocean Acidification
1. The oceans are not acidic
Acidity or alkalinity of a solution is determined by the amount of hydrogen ions [H+], using the pH scale (pH = -log[H+]).
Pure water has pH 7; solutions below pH 7 are acidic, and above pH 7 are alkaline, or basic. More hydrogen ions lower the pH and increase acidity (decrease alkalinity). Because the pH scale is logarithmic, a one-unit decrease in pH means a 10-fold increase in [H+].
Oceans are slightly alkaline, with pH 8.06. The term ocean acidification refers to the process of moving toward the acid end of the scale, because dissolved carbon dioxide results in increased [H+] lowering the pH of seawater.
2. Oceans absorb carbon dioxide from the atmosphere
Over the past 200 years, the oceans have absorbed approximately half of the carbon dioxide (CO2) emitted by human activities, providing long-term carbon storage. Without this sink, the greenhouse gas concentration in the atmosphere would be much higher, and the planet much warmer. But absorbing the CO2 causes changes in ocean chemistry, namely lowering pH and decreasing carbonate ion (CO32-) concentrations.
3. Carbon dioxide reacts with seawater to decrease pH
Carbon dioxide reacts with seawater to form carbonic acid, which releases hydrogen ions, reducing pH. Since industrialization, the pH of ocean surface waters has declined 0.1 units, reflecting a 30 percent increase in acidity. Under the Intergovernmental Panel on Climate Change (IPCC) emission scenarios, pH by the year 2100 will decline 0.3-0.4 units from the pre-industrial values, reaching a pH in the range of 7.76-7.86.
4. Carbon dioxide reacts with seawater to reduce carbonate ions
The additional hydrogen ions released by carbonic acid bind to carbonate (CO32-) to form bicarbonate (HCO3–), decreasing the amount of carbonate in the water. Since industrialization, surface ocean carbonate concentrations have declined by 10 percent in the tropics and southern ocean.
Less carbonate makes it more difficult for corals, mollusks, echinoderms, calcareous algae and other shelled organisms to form calcium carbonate (CaCO3), their major mineral building block. Also, when carbonate concentrations fall too low, already formed CaCO3 starts to dissolve. So, marine organisms have a harder time making new shells and maintaining the ones they’ve already got.
5. The changes are big and happening fast
Increased carbon dioxide accelerates weathering (dissolving) of terrestrial rocks. As a result, calcium and carbonate are released into the ocean and, in the past, these releases counteracted acidification.
However, weathering takes thousands of years. Human activities now are leading to releases of carbon dioxide at a rate far greater than the natural cycle can balance. Under “business-as-usual” emissions, pH declines will be three times greater and 100 times faster than that observed between glacial-interglacial periods. Such large changes likely have not been seen on Earth for the past 20 million years.
6. There are multiple types of calcium carbonate
There are three mineral forms of calcium carbonate: magnesium-calcite, aragonite, and calcite. Each form has a different solubility, or tendency to dissolve, in seawater, measured by the saturation state (Ω). That rate in turn depends on the concentration of calcium, carbonate, and the depth (pressure). In the oceans, the concentration of calcium (Ca2+) is relatively constant, so it is the concentration of carbonate (CO32-) that determines formation of calcium carbonate.
Because surface oceans are supersaturated – the calcium carbonate saturation state is greater than one (Ω >1.0) – conditions favor mineral formation. Deeper waters are undersaturated (the saturate state is less than one, Ω <1.0) and calcium carbonate dissolves.
The depth where waters make a transition from supersaturated to undersaturated is called the saturation horizon. Carbon dioxide is also more soluble (dissolves more readily) in colder waters, so oceans near the poles accordingly have more CO2, less carbonate, lower saturation states, and, in general, shallower saturation horizons.
Magnesium-calcite (made by coralline algae), and aragonite (formed by corals and many mollusks), are approximately 50 percent more soluble in seawater than calcite, the form used by foraminifera and coccolithophores, microscopic shelled plankton. These latter species form the base of the food chain in marine temperate (cool) ecosystems and are also a major source of sand in the deep sea. Therefore, calcite stays in mineral form (lasts longer than the other two forms) at deeper depths and with reduced carbonate ion concentrations.
What’s the big deal?
1. We are losing calcium carbonate habitat
Reduced carbonate ions lower the saturation state, causing all forms of calcium carbonate to dissolve at shallower depths. Since industrialization, the saturation horizons for all mineral forms of calcium carbonate have become shallower by tens to hundreds of meters. In other words, calcifying organisms, especially those using aragonite and magnesium-calcite, are getting squeezed between the surface and rising saturation horizons. The upwelling currents off the west coast of North America, for instance, have already pushed undersaturated waters with respect to aragonite to the surface. In these regions, aragonite-shelled mollusks have just run out of habitat.
2. Calcification rates are declining
Even if the water remains supersaturated, the rate at which animals can make calcium carbonate slows as carbonate concentration decreases. The effect is to reduce growth and structural integrity of shells and exoskeletons, threatening survival of some corals, mollusks and echinoderms, with consequences for the entire food web.
Ocean acidification causes reduced growth in both corals and coralline algae, threatening reef structures and loss of habitat in one of the planet’s most diverse systems. Fewer and weaker reefs mean less coastal protection from storms, lost income and food for more one-billion of the Earth’s population dependant on coral reefs for their food supplies, and risks to the $30 billion annual coral reef tourism industry driven mostly by diving.
The reduced growth of mollusks could lead to fewer bivalves like clams and oysters, reducing prey of bivalve-eating mammals and other animals, and adversely affecting multi-billion-dollar fisheries.
The loss of small-shelled plankton at the base of the food chain, including pteropod mollusks and foraminifera, would mean the loss of important prey items for higher trophic levels, such as juvenile pink salmon, which prefer to dine on pteropods. Changes in the planktonic community may also cause more outbreaks of toxic algal blooms or other community effects.
3. Carbon uptake by the oceans may change
Phytoplankton absorb CO2 from surface waters and transform the carbon into sugar during photosynthesis. When these organisms die, their bodies sink, removing CO2 from the surface and storing it as carbon in the deep ocean. This “biological pump” allows more absorption of CO2 from the atmosphere. Predation on sinking particles and calcification rates both affect efficiency of this pump. Predation keeps the carbon from sinking, and organisms release CO2 during calcification. When calcification slows, less CO2 is released, and more can be absorbed by the oceans. However, less calcification may lead to less net sinking of carbon, thus slowing and therefore reducing CO2 uptake at the surface.
It remains unclear which effect may dominate, but either way, ocean acidification affects the capacity for oceans to absorb atmospheric carbon dioxide, with implications for future climate change.
4. All marine organisms are affected, not just those with shells
Carbon dioxide passes from air into the oceans, and from the oceans into animals, where it changes the internal chemistry of tissues and cells. Different species have different capacities to maintain acid-base balance or adjust to a new pH.
In any case, maintaining that balance requires energy, diverting resources from growth, reproduction, and immune function, with potential long-term consequences for survival of some of those species. In addition, changes in ocean temperature and oxygen levels, also consequences of climate change, may exacerbate the effects of elevated carbon dioxide on organism health or alter processes such as calcification. These synergistic effects, especially long-term consequences for organism physiology, are an active field of current research that currently receives far too little attention by researchers, funders and the media.
Recent studies on changing calcium carbonate saturation states of the oceans can be found in:
Feely et al., 2004. Impact of Anthropogenic CO2 on the CaCO3 System in the Oceans. Science 305:362.
Feely et al., 2008. Evidence for upwelling of corrosive “acidified” water onto the continental shelf. Sciencexpress. May 22, 2008. 10.1126/science.1155676.
Orr et al., 2005. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms (pdf). Nature 437:4095.
References regarding policy and legislation:
Schubert et al., 2006. “The Future Oceans: warming up, rising high, turning sour” (pdf – 123 pages) (Special Report of the German Advisory Council on Global Change (WBGU), 2006).
Hood et al., 2008, “How is ocean acidity changing?“.
Meehl et al., in Climate Change 2007: The Physical Science Basis. (pdf – 21 pages) Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge University Press, New York, 2007).
Marah Hardt is a post-doctoral research fellow at Blue Ocean Institute, NY where she researches and writes about the effects of climate change on marine life. Carl Safina, author and President of Blue Ocean Institute, writes and speaks extensively about how oceans are changing and what those changes mean for wildlife and for people.