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The carbon cycle
All life on Earth is based on the chemistry of carbon. All living organisms are composed of similar combinations of carbon-containing molecules and water. Largely because of its involvement in living processes, carbon continuously cycles through the different components of the Earth – the biosphere, the atmosphere, the ocean and even the outer part of the solid Earth – the lithosphere. This global cycling of carbon in and out of living, dead and inorganic reservoirs is called a biogeochemical cycle. There are six main reservoirs of carbon, which in order of size are: - The deep ocean
- Deep-sea sediments
- Soil (including organic debris)
- The upper ocean
- The atmosphere
- Terrestrial organisms
The carbon cycle showing the six main global reservoirs of carbon. The boxed numbers are the sizes of the reservoirs in 1012 kg.The terrestrial carbon cycle is dominated by the processes of photosynthesis and respiration and involves three major reservoirs – the atmosphere, plant biomass and soil. Animals are an insignificant carbon reservoir when compared with plant biomass (only about 0.01% of it). Without human influence, transfers between these reservoirs roughly balance each other – for example, plants absorb carbon as they grow, but release it as they decay. The global ‘carbon budget’ is the balance of the exchanges (incomes and losses) of carbon between the carbon reservoirs or between one specific loop (for example, atmosphere–biosphere) of the carbon cycle. The examination of the carbon budget of a particular reservoir can provide information about whether it is functioning as a source or sink for carbon dioxide. The global carbon cycle involves interlinking cycles over three time-scales. The terrestrial carbon cycle is driven by biological processes and acts over a short time-scales of months and years to decades; the marine carbon cycle involves chemical, biological and physical components and acts over an intermediate time-scale of up to hundreds of thousands of years; the geological carbon cycle involving rocks and sediments acts over time-scales of hundreds of millions of years. The amount of time spent in each of these reservoirs by carbon has enormous implications for its potential to affect climate.
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