Carbon cycle:
The process in which carbon travels from the atmosphere into organisms and the Earth and then back into the atmosphere is called carbon cycle. Carbon is an essential element for all life and carbon cycle is the process that tells us how carbon moves from one factor to the other.

The carbon cycle
The carbon cycle is most easily studied as two interconnected subcycles:
• One dealing with rapid carbon exchange among living organisms
• One dealing with long-term cycling of carbon through geologic processes
Although we will look at them separately, it's important to realize these cycles are linked. For instance, the same pools of atmospheric and oceanic 2CO2 that are utilized by organisms are also fed and depleted by geological processes.
As a brief overview, carbon exists in the air largely as carbon dioxide—CO2 gas, which dissolves in water and reacts with water molecules to produce bicarbonate HCO3. Photosynthesis by land plants, bacteria, and algae converts carbon dioxide or bicarbonate into organic molecules. Organic molecules made by photosynthesizers are passed through food chains, and cellular respiration converts the organic carbon back into carbon dioxide gas.
Longterm storage of organic carbon occurs when matter from living organisms is buried deep underground or sinks to the bottom of the ocean and forms sedimentary rock. Volcanic activity and, more recently, human burning of fossil fuels bring this stored carbon back into the carbon cycle. Although the formation of fossil fuels happens on a slow, geologic timescale, human release of the carbon they contain—as \text{CO}_2CO2start text, C, O, end text, start subscript, 2, end subscript—is on a very fast timescale.
The biological carbon cycle
Carbon enters all food webs, both terrestrial and aquatic, through autotrophs, or self-feeders. Almost all of these autotrophs are photosynthesizers, such as plants or algae.
Autotrophs capture carbon dioxide from the air or bicarbonate ions from the water and use them to make organic compounds such as glucose. Heterotrophs, or other-feeders, such as humans, consume the organic molecules, and the organic carbon is passed through food chains and webs.
How does carbon cycle back to the atmosphere or ocean? To release the energy stored in carbon-containing molecules, such as sugars, autotrophs and heterotrophs break these molecules down in a process called cellular respiration. In this process, the carbons of the molecule are released as carbon dioxide. Decomposers also release organic compounds and carbon dioxide when they break down dead organisms and waste products.
Carbon can cycle quickly through this biological pathway, especially in aquatic ecosystems. Overall, an estimated 1,000 to 100,000 million metric tons of carbon move through the biological pathway each year. For context, a metric ton is about the weight of an elephant or a small car!
The geological carbon cycle
The geological pathway of the carbon cycle takes much longer than the biological pathway described above. In fact, it usually takes millions of years for carbon to cycle through the geological pathway. Carbon may be stored for long periods of time in the atmosphere, bodies of liquid water—mostly oceans— ocean sediment, soil, rocks, fossil fuels, and Earth’s interior.
The level of carbon dioxide in the atmosphere is influenced by the reservoir of carbon in the oceans and vice versa. Carbon dioxide from the atmosphere dissolves in water and reacts with water molecules in the following reactions:
CO2+H2O⇌H2CO3⇌HCO3−+H+⇌CO32−+2H+
The carbonate released in this process combines with Ca2+ ions to make calcium carbonate 3CaCO3 a key component of the shells of marine organisms. When the organisms die, their remains may sink and eventually become part of the sediment on the ocean floor. Over geologic time, the sediment turns into limestone, which is the largest carbon reservoir on Earth.
On land, carbon is stored in soil as organic carbon from the decomposition of living organisms or as inorganic carbon from weathering of terrestrial rock and minerals. Deeper under the ground are fossil fuels such as oil, coal, and natural gas, which are the remains of plants decomposed under anaerobic—oxygen-free—conditions. Fossil fuels take millions of years to form. When humans burn them, carbon is released into the atmosphere as carbon dioxide.
Another way for carbon to enter the atmosphere is by the eruption of volcanoes. Carbon-containing sediments in the ocean floor are taken deep within the Earth in a process called subduction, in which one tectonic plate moves under another. This process forms carbon dioxide, which can be released into the atmosphere by volcanic eruptions or hydrothermal vents.
Human impacts on the carbon cycle
Global demand for Earth’s limited fossil fuel reserves has risen since the beginning of the Industrial Revolution. Fossil fuels are considered a nonrenewable resource because they are being used up much faster than they can be produced by geological processes.
When fossil fuels are burned, carbon dioxide is released into the air. Increasing use of fossil fuels has led to elevated levels of atmospheric CO2 Deforestation—the cutting-down of forests—is also a major contributor to increasing CO2 levels. Trees and other parts of a forest ecosystem sequester carbon, and much of the carbon is released as CO2 if the forest is cleared.

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