By Magali Devic

Ocean Oceans cover two-thirds of the planet and represent the largest active carbon sink. This is why they naturally play a vital role in the Earth's carbon cycle. Currently, 48% of the carbon emitted to the atmosphere by fossil fuel burning is sequestered into the ocean. Carbon chemistry of seawater acts as a buffer, enabling the oceans to hold 50 times more carbon dioxide (CO2) than the atmosphere does.

The exchange of carbon between the important reservoirs of the biosphere, atmosphere and oceans is known as thecarbon cycle.

Carbon reservoirs

Gigatons C / year
Prof. David Archer, Professor of Geophysical Science, University of Chicago

In the oceans, carbon dioxide exchange is largely controlled by sea surface temperatures, circulating currents, and by the biological processes of photosynthesis and respiration. There is a higher capacity to hold a gas with a lower temperature than with a higher temperature, which means that more carbon dioxide can dissolve in cold water than in warm. These cold dense waters sinking at high latitudes are rich in carbon and act to move large quantities of carbon from the surface to deep waters. This mechanism is known as the "solubility pump" or physical mixing. Cold, downward moving currents such as those that occur over the North Atlantic absorb carbon dioxide and transfer it to the deep ocean.

Upward moving currents such as those in the tropics bring carbon dioxide up from depths andrelease it to the atmosphere.

Another key biological process occurs in the ocean carbon cycle when carbon dioxide from the atmosphere dissolves in the ocean; it undergoes rapid chemical reactions and only a small fraction remains as carbon dioxide. The carbon dioxide and the associated chemical forms are collectively known as dissolved inorganic carbon or DIC. The presence of organisms like plankton (microscopic plants and animals) or calcites transports gases and nutrients from the ocean surface to the deep; this movement is also known as a "biological pump."

The biological pump, in essence, removes carbon dioxide from the surface water of the ocean, changing it into living matter and distributing it to the deeper water layers, where it is out of contact with the atmosphere.

Eventually when plankton decays, the CO2 is then released into the water; most of it becomes absorbed in the sea-water. Although a small but possibly significant percentage of the sinking organic material becomes buried in the ocean sediment, most of the dissolved carbon dioxide eventually returned to the surface. This sedimentary process takes place at an extremely low rate measured in hundreds to thousands of years, meaning that the carbon that is released by human activities will not become geologically sequestered again for many thousands of years.

Significance of These New Results

The rate of ocean uptake of CO2 depends on how much CO2 is emitted, on the temperature of the ocean waters, and on the rate that excess carbon dioxide is absorbed by plants and soil and is transported down into the ocean depths by plankton. Primary evidence for recent large-scale changes in the global carbon cycle stems from the continuous time series of atmospheric carbon dioxide measurements obtained from 1958 to the present at the top of the extinct volcano Mauna Loa in Hawaii (Keeling et al., 1976).

Atmospheric CO2

Source:American Meteorological Society

Prior to the Industrial Revolution, the annual uptake and release of carbon dioxide by the land and the ocean had been on average just about balanced. However, ice core results indicate that the atmospheric concentration of carbon dioxide is now higher than experienced on Earth for at least the last 750,000 years and geological evidence suggests that the time is likely many millions of years and is expected to continue to rise, leading to significant temperature increases by the end of this century.

If the oceans did not soak up any CO2,atmospheric CO2 levels would be much higher than the current level of 380 parts per million by volume (ppmv) - probably around 500-600 ppmv. One of the greatest concerns is that the ability of oceans to absorb CO2 will decrease. According to Pr. Andrew Watson, "The speed and size of the change show that we cannot take for granted the ocean sink for the carbon dioxide." Environmental analyst Roger Harrabin similarly commented on BBC that "the ocean might become 'saturated' with our emissions."

The critical amount of anthropogenic (human-induced) emissions already present in the atmosphere (mainly originating from combustion of fossil fuels, cement production, agriculture and deforestation, etc,) driving global warming will also have a significant effect on the chemistry and biology of the oceans.

The uptake of anthropogenic CO2 by the ocean changes the chemistry of the oceans and can potentially have significant impacts on the biological systems in the upper oceans. In June 2005, The Royal Society (the United Kingdom's National Academy of Science) released a report analyzing theimpact of increasing atmospheric carbon dioxide on ocean acidification. Surface oceans have an average pH globally of about 8.2 units. Carbon emissions in the atmosphere have lowered the ocean pH, increasing the acidity of the ocean by 30 percent in the last 100 years, according to the National Oceanic and Atmospheric Administration (NOAA). NOAA also projects that, by the end of the century, current levels of carbon dioxide emissions could result in the lowest levels of ocean pH in 20 million years. A balanced pH is vital in order to maintain water quality favorable to marine life and in order to keep the ocean serving as a "carbon reservoir." If the oceans become too acidic, the shells of animals such as scallops, clams, crabs, plankton and corals are immediately threatened.

Variability of marine pH

Although studies into the impacts of high concentrations of CO2 in the oceans are still in their infancy, evidence indicates that reduced ocean carbon uptake is starting to occur and that this poses a serious hazard because this is likely to speed up global warming, as occurred when this type of feedback was initiated during the early warming stages of previous interglacials On October 16th 2007, the US Senate passed a provision proposed by Senator Frank Lautenberg (D-NJ) to Protect Oceans from Acidification. The legislation, co-sponsored by Sen. Barbara Boxer (D-CA) would focus more research attention onocean acidification, which threatens marine life and the fishing industry.

Both the trends in ocean acidification and CO2 absorption will have very large implications, perhaps comparable to the potential impacts of more rapid melting of the Greenland Ice Sheet. Moreover, reduced CO2 absorption by the oceans could accelerate warming greatly, pushing the climate toward a more precipitous melting of the Greenland ice sheet.

CoralThe recent developments give heightened urgency to our having a grasp of the ocean acidification and CO2 absorption trends. Although research and resources aiming at monitoring oceans should be drastically enhanced to fully understand the various consequences that will bring about anthropogenic Co2 emissions, there is cause for great concern over the threat carbon dioxide poses for the health of our oceans.