The carbon cycle

Carbon is the key element for life on Earth. Compounds of the element form the basis of plants, animals and micro-organisms. Carbon compounds in the atmosphere play a major part in ensuring that the planet is warm enough to support its rich diversity of life.

The mechanism of the carbon cycle operates on the basis that the carbon locked in plants and animals is gradually released into the atmosphere after they die and decompose. This atmospheric carbon is then taken up by plants which convert carbon dioxide (CO2) into stems, trunks, leaves, etc. through photosynthesis. The carbon then enters the food chain as the plants are eaten by animals.

There is also a geochemical component to the cycle mainly consisting of deep ocean water and rocks. The former is estimated to contain 36 billion tonnes and the latter 75 million billion tonnes of carbon. Volcanic eruptions and the weathering of rocks release this carbon at a relatively slow rate.

Under natural conditions the release of carbon into the atmosphere is balanced by the absorption of CO2 by plants. The system is in equilibrium, or would be if it were not for human interference.

The main human activity responsible for overturning the balance of the carbon cycle is the burning of fossil fuels which adds a further 6 billion tonnes of carbon to the atmosphere over and above the natural flux each year. In addition, when forests are converted to cropland the carbon in the vegetation is oxidised through burning and decomposition. Soil cultivation and erosion add further carbon dioxide to the atmosphere.

If fossil fuels are burnt and vegetation continues to be destroyed at the present rate, the CO2 in the atmosphere will treble by 2100. Even if there is decisive action on a global scale to reduce carbon emissions, atmospheric concentrations will still double by this date.

With the present fuel mix, every kilowatt hour of electricity used in the UK releases one kilogram of CO2. The burning of one hectare of forest gives off between 300 and 700 tonnes of CO2.

These are some of the factors which account for the serious imbalance within the carbon cycle which is forcing the pace of the greenhouse effect which, in turn, is pushing up global temperatures.

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