The amount of carbon dioxide released into the atmosphere as a result of human activity increases year by year. About half of this goes into the atmosphere, enhancing the greenhouse effect, whilst a quarter is sequestered by land-based vegetation and the other quarter by oceans . The price is a change in sea water chemistry, a process known as ocean acidification.
The ocean has a high capacity for uptake of carbon dioxide yet a slow mixing time in relation to the rate of human impact. Once the gas has been absorbed into the water, chemical reaction processes yield carbonate ions, through which a host of marine organisms from crustaceans and molluscs to corals and urchins are able to form biogenic calcium carbonate and thus build their shells and skeletons. Ocean acidification involves shifts in the chemical balance of carbonates, which is causing a decline in such carbonate concentrations and lower availabilty for these creatures; a rise in hydrogen ions, meanwhile, is causing the water's pH level to drop (acidification).
Oceanic chemical change has now been documented beyond doubt by time-series studies, repeated surveys, and estimates of anthropogenic carbon concentrations. The greatest impact is generally on the water's surface layer due to its exposure to and contact with air. Penetration of carbon into deep waters is highly variable but can be explained on the basis of global ocean circulation and mixing processes.
Surface layer, seasonal and regional variations in temperature, salinity, and alkalinity have always influenced seawater acidity and continue to do so. Biological primary production and respiration involve the removal of carbon dioxide from the atmosphere and the consequential pH variations. This natural seasonal variability can amount to more than 0.1 pH unit. Over a longer time scale the anthropogenic effect slowly shifts the baseline of ocean pH.
Differentiating between the drivers that affect ocean acidity – natural and anthropogenic – as well as the extent of their influence is one of the important points we will be examining in Theme Session H.
© Celeste Leroux ; Alaska Sea Grant