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.
Figure 1: Changes in Sea-Surface pH from Anthropogenic CO2 Emissions (pre-industrial to 1990s)
Note: Lower pH indicates greater acidity (see Box 1: Understanding the pH Scale)
Source: Pacific Science Association, 2007
Oceans and the Global Carbon Cycle
The ocean plays a critical role in the global carbon cycle: the amount of carbon stored in the ocean is roughly 50 times greater than that in the atmosphere (see Figure 2). At the surface, the ocean interacts constantly with the atmosphere to absorb and release carbon dioxide. Once absorbed, a carbon atom will remain in the ocean for hundreds of years, circulating from the ocean's surface to its depths and back to the surface again. A small amount of this absorbed carbon will descend to the ocean floor in the form of dead marine organisms, where it is then trapped within deep ocean sediments. Overall, the ocean acts as a carbon sink, with a net intake of approximately two billion metric tons of carbon per year, equivalent to one-third of annual anthropogenic emissions (Royal Society, 2005).
Figure 2: Annual Carbon Flows and Storage (billion metric tons)
Source: NOAA Earth System Research Laboratory, 2007
CO2 Emissions and Ocean Acidification
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 (see Figure 3).
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Box 1: Understanding the pH Scale The pH scale, ranging from zero to 14, is used by scientists to measure the acidity or alkalinity (a.k.a. basicity) of a solution, which is determined by the concentration of hydrogen ions, where more H+ indicates greater acidity. Solutions with a value of seven are considered neutral (such as pure water), with lower values being more acidic and higher values being more alkaline. The pH of pristine seawater ranges between 8 and 8.3, indicating that the ocean is naturally somewhat alkaline, although deeper and colder water tends to be more acidic. Due to the nature of the pH scale, a 30 percent increase in ocean acidity corresponds to a decrease of only 0.1 pH units. |
Figure 3: Past and Projected Oceanic pH Levels
Source: EUR-OCEANS, 2007
Potential Impacts on Marine Organisms
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.
Figure 4: Oceanic Aragonite Saturation Levels (1765-2100)
Source: Ocean Acidification Network, 2006
Uncertainties Highlight Need for Additional Research
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.
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RELATED LINKS:
The Ocean Acidification Network
NASA Oceanography: The Ocean and the Carbon Cycle
"The Dangers of Ocean Acidification" (Doney, 2006)
"Ocean Acidification - the other half of the CO2 problem" (EUR-OCEAN, 2007)
Intergovernmental Oceanographic Commission
EarthTrends
Map: Coral Bleaching Events and Sea Surface Temperature Anamoly Hot Spots, 1997-1998
"Coral Reefs: Assessing the Threat"
Other Publications
Ocean Acidification Due to Increasing Atmospheric Carbon Dioxide (Royal Society, 2005)
