Unfortunately, this balance has been disrupted in recent years. In fact, current atmospheric CO2 concentrations are estimated to be as much 40% higher than pre-industrial levels. While there have been fluctuations in the past, the increased burning of fossil fuels, rate of deforestation, and changing land use since the Industrial Revolution has resulted in a rapidly shifting CO2/O2 balance as more CO2 is released into the atmosphere. Since approximately 30% of the CO2 in the atmosphere is absorbed by the ocean, changes in the atmosphere lead to changes in the ocean. The world's oceans are becoming more acidic as a result of this increased influx of CO2; negatively impacting and ultimately threatening an abundance of marine life. The chemistry associated with ocean acidification - a decrease in the pH of the ocean over a long period of time due to rising CO2 levels - is outlined in the infographic below.
The effects of ocean acidification are many. Chemical reactions resulting in increased levels of hydrogen ions, and therefore increased acidity, occur when CO2 is absorbed by seawater. This in turn leads to decreased numbers of available carbonate ions; the building blocks for calcium carbonate structures. The decrease in pH is significant to calcifying species like coral reefs, oysters and other shelled marine organisms such as shrimp which use carbonate ions to build and maintain their skeletons and shells. The tendency of carbonate ions to combine chemically with the excess hydrogen ions in the sea water results in the formation of bicarbonate ions. While this is good in one way; it removes some hydrogen ions from the water and thereby mitigates some of the increased acidity produced by dissolved carbon dioxide. It is bad in another; when the water surrounding calcifying organisms is low with respect to carbonate concentration, the usually insoluble calcium carbonate that makes up their shells starts to dissolve.
This close-up image is evidence that pteropods from the natural environment along the U.S. West Coast are showing signs that their shells are dissolving. http://oceanservice.noaa.gov/facts/acidification.html
The rate at which the CO2 levels are rising, along with the associated decrease in carbonate availability and related mineral dissolution, is occurring so rapidly that it is difficult for marine organisms to adequately adapt on an evolutionary scale. The impact on the populations of some of the more vulnerable species such as pteropods; small calcifying (or shelled) organisms that live as zooplankton in the water column and are an important prey species for many fish, is becoming increasingly evident. Changes in ocean chemistry thus ultimately put the entire marine food web at risk.