What is the Carbon Cycle?

The carbon cycle is the process through which carbon is cycled through the air, ground, oceans, plants and animals. Included in the cycle are various sinks (or stores) of carbon and processes by which the carbon is exchanged from one sink to another. In scientific terms, every molecule of carbon that is here now was present millions of years ago-a basic law of nature meaning that matter can be changed, but not created or destroyed.

Carbon is an element and a basic building block of life. At the simplest level, plants absorb carbon dioxide from the air during photosynthesis. The carbon molecules are used to make sugars and starches, which in turn feed the growth of cell walls. Plants release the oxygen part of the CO₂ molecule back into the air, and that is what we breathe. Carbon remains in the plant or tree, even when it's made into furniture or lumber, and is released only when the wood rots or is burned.

A typical 2,500 square foot wood-frame home has 30 metric tonnes of carbon stored in its structure, the equivalent of driving an average passenger car for five years, or using about 12,200 litres of gasoline.

The other major exchange of CO₂ occurs between the oceans and the atmosphere. As in a forest, the process of photosynthesis is also used by marine plants (including microscopic phytoplankton), which absorb dissolved CO₂ in the oceans, use the carbon for growth and release the oxygen into the water, which the fish then "breathe."

The carbon cycle is obviously very complex, and each process has an impact on the others. However, it is clear that two important exchange points for carbon have been heavily impacted by humans. First, carbon stored as coal, oil and natural gas is being burned by industry and automobiles and released into the atmosphere in large amounts. Close to eight billion tonnes of carbon are emitted through human-related processes every year, most via fossil fuel combustion.

Second, land plants that absorb CO₂ have been reduced as forests continue to be cleared for agriculture-mainly in the developing world. As a result, less CO₂ is being absorbed. Approximately 20 per cent of the increase in CO₂ levels is attributed to the loss of forest cover or deforestation in developing regions.

The net impact is an upset in the natural balance, which is causing a change in the Earth's climate.