The carbon cycle involves the exchange of carbon atoms between living things, the earth, oceans, and atmosphere. The atmosphere contains mostly carbon dioxide, which is drained by photosynthesis and cold ocean parts. Carbon is replenished by respiration, burning fossil fuels, and volcanic eruptions. Human activity has unbalanced the cycle, causing rising carbon dioxide levels. The biosphere contains the most condensed carbon pool, and soil and deep oceans also hold significant amounts. Human industry releases 5.5 gigatons of fossil fuel emissions annually, with efforts to limit them. Remaining coal, oil, and gas reserves are limited.
The carbon cycle is the means by which carbon atoms are exchanged between living things, the earth, the oceans and the skies; respectively biosphere, geosphere, hydrosphere and atmosphere. There are about 1,000,000 gigatons of carbon on Earth, most of which is locked up in sedimentary rocks and never reaches the surface. On the surface, carbon is continually engaged in a dynamic exchange of consumption and production. This active exchange is called the carbon cycle.
The atmosphere contains about 750 gigatons of carbon, mostly in the form of carbon dioxide (CO2). At the end of the carbon cycle, carbon is continuously drained by 1) the process of photosynthesis, which uses carbon dioxide to create carbohydrates, and 2) the colder parts of the oceans, which are capable of absorbing carbon dioxide . At the end of the carbon cycle production, atmospheric carbon is continuously replenished by the following processes: respiration of plants and animals and their decay; the burning of fossil fuels; limestone reactions; the release of carbon dioxide from warm parts of the ocean; and volcanic eruptions. Human burning of fossil fuels has slightly unbalanced the global carbon cycle or balance, resulting in an atmosphere with rising levels of carbon dioxide.
The biosphere contains about 600 gigatons of carbon, which is the most condensed carbon pool on Earth. Through the carbon cycle, carbon circulates in and out of the biosphere as plants and animals breathe, excrete, die, and photosynthesise. Soil in the form of organic matter, which plants and animals become when they die, contains about 1,500 gigatons of carbon. Some of this carbon sinks deep into the Earth in sedimentary rocks, never to emerge again. A part reaches the oceans and dissolves. The deep oceans contain a significant amount of carbon, about 38,000 gigatons.
About 5.5 gigatons of fossil fuel emissions are released into the atmosphere each year by human industry, creating a notional high-heavy carbon cycle. International coalitions have been formed to limit these emissions, with moderate success so far. The remaining coal seams amount to about 3,000 gigatons of carbon, and the remaining oil and gas reserves make up about 300.
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