The role of carbon dioxide

Carbon dioxide is stored in the atmosphere, the land and in the oceans. The latter have an immense capacity to contain CO2, storing by far the largest amount and thus having a major controlling influence on atmospheric CO2 levels. Physical, biological and chemical factors are all involved in the uptake of CO2 by the oceans. Dissolved CO2 in its several forms is conveyed between surface and deeper levels by currents and water-mixing processes. However, phytoplankton plays the leading role in CO2 transport. Phytoplankton take up CO2 dissolved in the surface waters and use it for photosynthesis. Herbivores eat the phytoplankton, thus taking up the carbon and transporting it to deeper levels during diurnal migrations. Here it is released during respiration and defaecation or further distributed through the food web. The amount of CO2 taken up by the oceans is dependent on productivity and is therefore lowest in the tropics and highest in mid- and high latitudes. In the northern hemisphere, uptake of CO2 by the oceans increases during the spring and summer plankton blooms. In fact, in the tropics there is a net discharge of CO2 to the atmosphere throughout the year but this is more than made up for in higher latitudes. Anything that lowers the productivity of the open oceans will also lower the oceans' ability to absorb CO2. For example, increased water stratification resulting from warming of surface waters could prevent upward flow of nutrient-rich water necessary for phytoplankton production.

With the advent of our modern industrial society, the amount of CO2 directly entering the atmosphere has risen dramatically and now stands at about 350 ppm as compared with 280 ppm in the 1850s. The current rate of increase is about 1.3 ppm per year. Most of this extra CO2 comes from the burning of fossil fuels and the effects of de-forestation. In the latter, the trees are not only no longer there to photosynthesize and use up CO2 but the forests are often burnt, thus releasing more CO2. The oceans are helping to counteract and moderate this build-up of atmospheric CO2 because as atmospheric levels increase, more CO2 dissolves at the ocean-atmosphere interface.

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