Fossil fuel combustion and industrial processes release over six billion metric tons of carbon into the atmosphere each year. The consequences of these greenhouse gas emissions are often discussed in terms of rising global temperatures, but global warming is not the only threat from increased atmospheric concentrations of carbon dioxide (CO2). Ocean acidification, which occurs when CO2 in the atmosphere reacts with water to create carbonic acid, has already increased ocean acidity by 30 percent (Doney, 2006). Although the chemistry of this effect is well understood and not much debated, the full consequences of ocean acidification for marine ecosystems and human well-being are only beginning to be revealed.
With the rise of atmospheric CO2 concentrations from the pre-industrial level of 280 parts per million to 379 parts per million in 2005 (IPCC, 2007), the amount of carbon in the ocean has increased substantially and rapidly. Global data collected over several decades indicate that the oceans have absorbed at least half of the anthropogenic CO2 emissions that have occurred since 1750 (Sabine et. al. 2004). This carbon dioxide has combined with water to form carbonic acid, which, like all acids, releases hydrogen ions (H+) into solution, making ocean surface water 30 percent more acidic on average. Depending on the extent of future CO2 emissions and other factors, the Intergovernmental Panel on Climate Change (2007) predicts that ocean acidity could increase by 150 percent by 2100.
A 150 percent increase in ocean acidity would be undetectable to the average human, but certain marine organisms including mollusks, crustaceans, reef-forming corals and some species of algae and phytoplankton are particularly vulnerable to small changes in pH. These species, known as "marine calcifiers," all create skeletons or shells out of calcium carbonate. The essential building block for this process is the carbonate ion, but when combined with hydrogen ions released by carbonic acid, it is rendered useless for shell-building organisms. The concentration of carbonate ions is expected to decline by half during this century due to increased atmospheric carbon dioxide levels (Orr et. al., 2005).
Marine calcifiers face a second challenge: their calcium carbonate shells dissolve in environments that are too acidic. In fact, some deep, cold ocean waters are naturally too acidic for marine calcifiers to survive, meaning that these organisms only exist above a certain depth known as the "saturation horizon." With ocean acidification, the saturation horizon is expected to shift closer to the surface by 50 to 200 meters relative to its position during the 1800s (Doney, 2006). The Southern and Arctic oceans, which are colder and therefore naturally more acidic, may become entirely inhospitable for organisms with shells made from aragonite--one of the weaker mineral forms of calcium carbonate--by the end of this century (EUR-OCEANS, 2007).
Potential impacts on harvested species like fishes and squids are more uncertain. One area of concern is acidosis, or the build-up of carbonic acid in body fluids, which can disrupt growth, respiration and reproduction. An indirect but perhaps more certain consequence is that many species will suffer from the loss of marine calcifiers, which provide essential food and habitat (including coral reefs) for countless ocean dwellers.
Scientists are still unclear about the full consequences of ocean acidification. Several lab studies that have investigated the effects of increased acidity on marine calcifiers have found concerning results, but theories regarding impacts at the ecosystem level remain speculative. Effects on human well-being, both through lost fisheries and recreational potential, are also unknown.
Despite our lack of knowledge, the trend of ocean acidification is undeniably concerning, especially considering the devastating consequences that acid rain had on freshwater ecosystems during the 20th century. Furthermore, the ocean is currently undergoing other potentially dangerous changes, including warming, sea level rise, pollution and overfishing. The rapid pace at which these changes are occurring, and the fact that they are happening simultaneously, threatens to disrupt the ocean's well-balanced physical, chemical and biological processes faster than they can adapt.
Once the ocean's pH has been lowered, it will take thousands of years to reverse. Thus, reducing carbon dioxide emissions will be critical to minimizing future ocean acidification. Even if emissions are reduced, however, the ocean will inevitably continue to undergo significant human-induced changes throughout this century. To prepare for these changes, we will need scientific research to enhance our knowledge of complex ocean processes and ecosystem interactions. Furthermore, ocean resource and fisheries managers, with the support of improved scientific understanding, must be alert to early warning signs of ecosystem decline and take precautionary measures to protect vulnerable species.
This article was part of the EarthTrends Update: September 2007 "Ocean Acidification, the Other Threat of Rising CO2 Emissions" by Crystal Davis. You can subscribe to their newsletter on the EarthTrends Webpage.