Carbon Cycle

All life on Earth is carbon-based, since carbon is the principal component of the organic compounds that make up protoplasm, constituting roughly 49% of the dry weight of organisms. Of the total quantity of global carbon, about 71% is found dissolved in the oceans, and this vast oceanic reservoir plays a key role in regulating the amount of carbon dioxide present in the atmosphere. A further significant quantity of carbon is locked away for very long periods in rocks and fossil fuels such as coal, oil and natural gas — these long-term storage locations are called 'sinks'.

The carbon cycle moves through several linked pathways. Carbon dioxide moves from the atmosphere into plants through photosynthesis, where it is fixed into organic carbon to make food. This organic carbon then moves from plants to animals through food chains. Carbon returns to the atmosphere from living things through respiration — every exhalation releases carbon dioxide — and decomposers contribute substantially to atmospheric CO2 by processing dead organic matter and waste on both land and in the oceans. When fossil fuels are burned to power factories, power plants and vehicles, carbon that had been stored underground is rapidly released as carbon dioxide gas; roughly 5.5 billion tons of carbon is released this way each year, of which about 3.3 billion tons remains in the atmosphere, with much of the rest dissolving into seawater or being deposited as calcium or magnesium carbonate in the shells of marine animals. Burning of wood, forest fires, and volcanic activity are additional natural sources of atmospheric CO2. The oceans and other water bodies also absorb carbon dioxide directly from the atmosphere, dissolving it into the water, with some of this carbon eventually lost to sediments and removed from active circulation. …