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Carbon cycle

Carbon cycle - sustaining life on earth

The element carbon can exist in both a gaseous form like carbon dioxide or in liquid or solid form by combining with other elements to form compounds. Such compounds may be in the form of petrol or diesel, living tissue or as inert compounds like carbonates present in many rocks constituting the earth's crust. The process by which carbon is cycled between various forms is called the carbon cycle and this cycle has been implicated in many of the changes in the earth's climate over geological time scales.

carbon cycle

Tiny structures inside green leaves called chloroplasts use the energy of sunlight to break apart the molecules of carbon dioxide (CO2). Plants use the carbon to form organic compounds which in turn are used to build living tissue comprising leaves, flowers, bark and wood while the release of oxygen helps humans, animals and insects to breathe. Carbon dioxide is also absorbed by the oceans where it is used by phytoplankton, the organism at the start of the oceanic food chain, to form their skeletons.

The combustion of fossil fuels transforms the element carbon from a solid or liquid form to carbon dioxide which being lighter than air, then congregates in the upper atmosphere. There it becomes a greenhouse gas as it has the ability to trap some of sun’s rays reflected back into space at a longer wavelength.

If more carbon dioxide is absorbed than given off, then the carbon dioxide level in the atmosphere will fall and this has been associated with the onset of previous ice ages in which global temperatures have fallen. The current ice caps of Greenland and Antarctica and Alpine glaciers are remnants of the last ice age. Conversely, if more carbon dioxide is given off than absorbed, then the carbon dioxide concentration in the atmosphere will rise and there is clear evidence over geological time that this has resulted in periods of global warming.

Reference: Life on Earth by T Flannery (Allen Lane, London) 2011



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