The following article is from Coral Reef Watch's introduction to Ocean Acidification (the original of which can be found here):
Over the past two centuries, humans have dramatically altered the composition of the earth’s atmosphere through deforestation and the burning of fossil fuel. Atmospheric carbon dioxide (CO2) concentrations have increased to the highest level experienced on Earth for at least the past 650,000 years. The rate of increase has also been faster than at any time during that period. The global oceans are the largest natural reservoir for much of this excess CO2, absorbing approximately one-third of that attributed to human activities each year [Sabine et al., 2004]. As a result, dissolved CO2 in the surface ocean will likely double over its pre-industrial value by the middle of this century, representing perhaps the most dramatic change in ocean chemistry in over 20 million years.
Changes in atmospheric CO2 over the past 400,000 years and projected to 2100. The top graph calculates the corresponding change in sea surface pH. Paleo Volstok Icecore and Mauna Loa Observatory data are obtained from the Dept. of Energy’s Carbon Dioxide Information Analysis Center. Projected atmospheric CO2 data represent the IPCC IS92a “business as usual” case. Click to enlarge.
As CO2 reacts with seawater it forms carbonic acid, causing a reduction in seawater pH. Seawater is naturally ‘buffered’ against these pH changes, but the buffering process consumes carbonate ions. Carbonate ion is an essential ingredient in the creation of calcium carbonate shells and skeletons produced by a large number of marine organisms (e.g., corals, marine plankton, coralline algae, and shellfish).
Carbonic acid is formed as a result of dissolving CO2 in water. In seawater, some of the acid (H+) is consumed by reacting with carbonate ion (CO32-) to form bicarbonate ion (HCO3-). Thus, ocean acidification not only reduces surface ocean pH, but also reduces the availability of carbonate ion.
The term Ocean Acidification is new, but the concept has been recognized for more than three decades. Trends toward lower pH can now be clearly seen in sustained observations recorded at ocean time series stations and during repeated geochemical surveys. In coming decades, ocean acidification could affect some of the most fundamental biological and geochemical processes of the sea.
The figure shows the changes in surface ocean (50 m) chemistry at the Hawaiian Ocean Time-Series over the last 20 years. The top panel shows a decline in carbonate ion concentration, the middle panel shows a trend of decreasing pH, and the bottom graph shows a concomitant CO2 increase in the water and the atmosphere.
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