Tuesday, October 06, 2015

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What is Ocean Acidification (OA)?

Another emerging concern about increasing carbon dioxide (CO2) emissions is ocean acidification.  Ocean acidification primarily describes a change in ocean chemistry, including a reduction in pH, which results from an imbalance between the accelerating uptake of atmospheric CO2, and the ocean’s ability to resist this change. Other chemical additions to or subtractions from the ocean can also be important at local and regional scales (e.g. sulfur dioxide, excess nutrient input). The world’s surface oceans are tightly linked with the atmosphere and absorb huge amounts of CO2 each year. This exchange, in part, helps to regulate the planet’s atmospheric CO2 concentrations but it comes at cost for the oceans.  Over the last 250 years, the atmospheric concentration of CO2 has increased from 280 parts per million to over 400 parts per million due to the burning of fossil fuels (e.g. coal, gas, oil) and land use change (for instance, conversion of natural forest into crop production). Were it not for ocean uptake of CO2, atmospheric levels would be increasing at an even greater rate than they are now.

Indeed, roughly a third of all the CO2 related to human activities since the 1700’s has been absorbed by the oceans (Sabine et al., 2004).  Each year, approximately 2.5 billion metric tonnes of additional carbon enters the ocean in the form of CO2.  That’s equivalent to 11 million railroad hopper cars filled with coal.  Such a train would encircle the earth 14 times!  CO2 is an acid gas: when it dissolves in water, including seawater, it forms carbonic acid. Thus, as CO2 dissolves into the oceans, it fundamentally alters ocean chemistry, making it more acidic. This acid addition is where the name “ocean acidification” comes from and is why ocean pH is declining world-wide.  Already, ocean pH has declined globally by about 0.1 units and current emissions trajectories suggest it could decrease 0.7 units by the year 2300 (Zeebe et al., 2008).

The chemistry behind Ocean Acidification

Increases in CO2 in the atmosphere drive corresponding increases in dissolved CO2 with the surface oceans.  This dissolved CO2 reacts with the seawater to form carbonic acid (H2CO3). Carbonic acid nearly completely dissociates to form bicarbonate ions (HCO3-) and hydrogen ions (H+). The increase in the concentration of hydrogen ions from these reactions causes the seawater to become more acidic and hence the term “ocean acidification”.  However, seawater is naturally buffered to resist large changes in pH by reacting some of this excess H+ with carbonate ions (CO32-). Additionally, this increase in hydrogen ions favors the formation of bicarbonate ions over carbonate ions, making carbonate ions relatively less abundant.  Carbonate ions are an important part of calcium carbonate (CaCO3) structures, such as sea shells and coral skeletons. Decreases in seawater carbonate ions can make building and maintaining shells and other calcium carbonate structures difficult for calcifying organisms such as coral, plankton, and shellfish.