The primary cause of ocean acidification is an increase in carbon dioxide in the oceans that is caused by increasing levels of carbon dioxide in the atmosphere. Human activities—including the burning of fossil fuels, cement production, deforestation, and agriculture—release carbon dioxide into the atmosphere. Since the 1700s, atmospheric carbon dioxide concentrations have risen from approximately 280 parts per million to approximately 400 parts per million (see fig. 1). As the carbon dioxide concentration in the atmosphere increases, more carbon dioxide is absorbed by the oceans, where it reacts with water to form carbonic acid, most of which separates to form a hydrogen ion and a bicarbonate ion.the pH of the water. The resulting increase in hydrogen ion concentration is what lowers Since the 1700s, the average surface ocean pH has decreased from about 8.2 to 8.1, a change representing an approximately 26 percent increase in ocean acidity. At the current rate of carbon dioxide emissions, scientists project that, by 2100, the average pH of the ocean surface will drop to between 7.9 and 7.7. Such a drop would correspond to a rise in acidity of approximately 100 percent to 200 percent over preindustrial levels. Moreover, the current rate of acidification is believed to be faster than at any point in at least the last 20 million years. In addition to increasing acidity, the higher levels of carbon dioxide in the oceans also cause chemical reactions that reduce what is known as the “saturation state” of calcium carbonate minerals such as aragonite and calcite. As the saturation state of these carbonate minerals decreases, some marine organisms will need to use more energy to acquire the carbonate ions needed to build shells or skeletons. In addition, when the saturation state drops below 1 (i.e., undersaturated), then structures such as animal shells that are made of carbonate minerals may begin to dissolve. As the oceans absorb more carbon dioxide, scientific models predict that the saturation state of many surface waters, including areas supporting rich fisheries, will continue to decline (see fig. 2). The carbonate mineral saturation state is affected by several other factors in addition to carbon dioxide. For example, the temperature, pressure, and salinity of the ocean water at a particular location affects the saturation state. As a result, the latitude of a particular ocean location, the depth of the water there, and the extent to which it receives freshwater input from rivers all play a role in determining the saturation state. In general, saturation state is highest in warm, shallow, saline waters. Therefore, the Arctic Ocean, which has colder water and receives large amounts of fresh water from rivers and melting ice, and deepwater environments generally have naturally lower saturation states. Biological processes also affect the pH and saturation state of the oceans. During the day, marine plants, including phytoplankton and seagrasses, use sunlight and carbon dioxide to create and store energy, a process known as photosynthesis.dioxide levels, thus increasing the pH and carbonate mineral saturation states of the upper reaches of the ocean. Conversely, as organic (i.e., This activity reduces carbon carbon-based) matter decomposes, carbon dioxide is released back into the water, thus decreasing pH. Some of the decomposing organic matter sinks into deeper water, which, over time, has contributed to deeper waters naturally being lower in pH and having lower saturation states with respect to calcium carbonate minerals. The effect of biological processes on ocean chemistry also means that pH and saturation states vary more widely in coastal and estuarine waters than in the open ocean. Coastal and estuarine areas have high concentrations of plant life and receive freshwater inputs from rivers, factors that influence ocean chemistry. The resulting variation may occur both daily, because of the effect of photosynthesis, and seasonally, due to changes in the amount of available solar energy and of river flows. Although the primary cause of ocean acidification is the increase in global atmospheric carbon dioxide emissions, other factors also contribute to acidification, particularly in coastal and estuarine areas. In particular, increased nutrient pollution (e.g., of nitrate, phosphate, and iron) from agricultural fertilizers and from septic systems and sewage treatment plants results in higher than normal levels of biological growth, a condition known as “eutrophication.” In the short run, the faster growth of plants such as phytoplankton may raise the pH of coastal and estuarine waters by consuming carbon dioxide in the water and releasing oxygen. In the long run, however, when plants die, the decomposition process releases carbon dioxide and may lower the pH, sometimes substantially.addition, local sources of air pollution may also contribute to acidification in coastal and estuarine areas. For example, in addition to releasing carbon dioxide into the atmosphere, the burning of fossil fuels releases other gases such as nitrogen oxides and sulfur oxides that can be In deposited on coastal and estuarine waters, or on streams and rivers flowing into coastal waters, lowering the pH. Ocean acidification could have a variety of potentially significant effects on marine species, ecosystems, and coastal communities, according to the six summary reports we reviewed. However, the scientific understanding of these effects is still developing, and there remains uncertainty about their scope and severity. Calcifying species—that is, species that produce shells and skeletons composed of calcium carbonate minerals—are expected to be among the most vulnerable types of species to ocean acidification. Not all calcifying species, however, are expected to respond the same way to reductions in carbonate ion concentrations resulting from ocean acidification, and some are expected to be more at risk than others. For example: Mollusks. Ocean acidification could negatively affect the survival and growth of some mollusks—including bivalve species such as oysters and mussels—by making calcification more difficult, according to the summary reports we reviewed. Among bivalve species that have displayed a negative response to ocean acidification, larval and juvenile bivalves may be more susceptible to harm than adult bivalves. For example, according to a 2013 study of Pacific oyster (Crassostrea gigas) larvae, during the period of initial shell formation in the early days of an oyster’s life, oyster larvae rely primarily on the energy derived from their egg reserves to build their shells because the oysters have not yet developed their primary feeding organ. When exposed to an environment with a lower carbonate ion concentration, however, the larvae have to expend more energy to build their shells. This increased energy expenditure during the period of initial shell formation makes it more difficult for the oysters to develop their feeding organs and reduces the probability that the oysters will survive. Figure 3 compares Pacific oyster larvae raised in acidified conditions (characterized by reduced pH levels and carbonate mineral saturation states) with those raised under more favorable ocean conditions. The figure shows that the Pacific oyster larvae raised under acidified conditions experienced impaired shell development, and their shells had various deformities compared with the larvae raised under more favorable conditions. Even in instances where oysters are able to survive their larval stage under acidified conditions, the greater energy required to produce their shells could contribute to decreased growth in later life stages. For example, a 2012 study examined the effects of ocean acidification on the Olympia oyster (Ostrea lurida) and found that larvae raised in water with a low pH (7.8) exhibited a slower shell growth rate that continued even once the oysters became juveniles. Not all mollusk species have responded negatively to ocean acidification conditions in research experiments, according to the reports we reviewed. One of the reports noted that roughly half of the mollusk species examined have displayed no effects from ocean acidification, and that a few mollusk species have displayed positive effects. For example, one study cited in the report found that mortality rates declined for juveniles of one clam species (Ruditapes decussatus) kept in seawater with artificially lowered pH levels compared with juveniles kept in seawater at higher pH levels. Corals. Corals were also identified in the summary reports we reviewed as being among the calcifying species most at risk for harm from ocean acidification. Ocean acidification may make it more difficult for some corals to grow their skeletons due to the reduced carbonate ion concentration and may affect coral reproduction. In addition, ocean acidification could potentially increase the rate at which coral skeletons dissolve and erode. However, not all coral species are expected to be negatively affected by ocean acidification, according to the reports we reviewed. and Reduced pH: Variable Responses to Ocean Acidification at Local Scales?” Journal of Experimental Marine Biology and Ecology, vol. 396 (2011). could cause behavioral changes in some fish species by interfering with the functioning of sensory systems, such as the ability to smell. Such changes in sensory systems could, in turn, produce behavioral changes that alter predator-prey interactions, among other things. For example, one study cited in two of the summary reports we reviewed found that clownfish larvae raised in an elevated carbon dioxide environment became attracted to the scent of predators rather than avoiding this odor, whereas clownfish larvae raised under carbon dioxide conditions similar to the current environment exhibited a strong avoidance to the odor of predators. Similarly, another study found that juvenile damselfish and cardinalfish living in ocean waters with naturally high carbon dioxide levels—caused by nearby volcanic vents that emit carbon dioxide and lower the pH of surrounding waters—were attracted to predators’ odors and exhibited bolder behavior than fish from ocean waters that were unaffected by the volcanic vents.behavioral changes cause species to be at a higher risk from predators, such changes could potentially lead to increased mortality rates for affected species. However, according to the reports we reviewed, the extent to which other fish species—including commercially important fish such as salmon—are vulnerable to behavioral effects caused by changes in the functioning of sensory systems due to ocean acidification is unclear.