The Carbon Cycle
Carbon is found in all Earth's systems: Carbon is an element. It's found in both organic stores (living things) and inorganic stores (e.g. rocks, gases and fossil fuels).
Carbon is transferred between different stores: The carbon cycle is the process by which carbon is stored and transferred. The carbon cycle is a closed system - there are inputs ad outputs of energy, but the amount of carbon in the system remains the same. However, some carbon is locked away (sequestered) in long-term stores, e.g. rock and fossil fuels deep underground. If these are released bu e.g. burning fossil fuels, they are effectively inputs.
Natural processes can change the carbon cycle:
Humans can also impact the carbon cycle: Since the industrial revolution, the impact that humans have on the carbon cycle has increased hugely. We're currently causing carbon flows from the lithosphere and biosphere to the atmosphere to happen much faster than they would naturally.
Carbon flows happen over different time and spatial scales: Fast carbon flows quickly transfer carbon between sources. It only takes a matter of minutes, hours or days. Photosynthesis, respiration, combustion and decomposition are examples of fast carbon flows. Sequestration is a slow carbon flow. It takes millions of years form carbon to be sequestrated in sedimentary rocks. The carbon flow taking place also depend on spatial scale. For example, at a plant scale, respiration and photosynthesis are the main floes. At an ecosystem scale, carbon flows such as combustion and decomposition also occur. At a continental scale, all of the carbon flows including sequestration occur.
The carbon cycle affects the Atmosphere, Land and Oceans: The carbon cycle is fundamental to life on Earth. When there is a change to the carbon cycle, it can have a significant impacts on the atmosphere, land and oceans.
Carbon stores change in size over time because of carbon flows:
The carbon budget is the balance between carbon inputs and outputs: The carbon budget is the difference between the inputs of carbon into a subsystem and outputs of carbon from it. The balance of the inputs and outputs of a subsystem determines whether it acts as a carbon source (releases more carbon than absorbed) or a carbon sink (absorbs more carbon than releases).
Land: The carbon cycle allows plants to grow - if there was no carbon in the atmosphere, plants could not photosynthesise. If there was no decomposition, dead plants would remain where they fell and their nutrients would never be recycled. Changes in the carbon cycle can reduce the amount of carbon stored in the land. E.g. permafrost melting releases carbon previously stored into the atmosphere. An increase in global temperatures could also increase the frequency of wildfires.
Atmosphere and Climate: The carbon cycle affects the amount of gases containing carbon in the atmosphere. These are greenhouse gases - they trap some of the Sun's energy, keeping some of the heat in and keeping the planet warm. As the concentrations of greenhouse gases in the atmosphere increase, temperatures are expected to rise. This is global warming. Changes in temperature across the globe will affect other aspects of the climate.
Oceans: As part of the carbon cycle, carbon dioxide is dissolved directly into the oceans from the atmosphere. CO2 in oceans is used by organisms such as phytoplankton and seaweed during photosynthesis and by other marine organisms to form calcium carbonate shells and skeletons. Increased levels of CO2 in the atmosphere can increase the acidity of the oceans because the oceans initially absorb more CO2. This can have adverse effects on marine life. Global warming can also affect oceans, such as high temperatures do not allow organisms to survive. Warmer water is also less able to absorb CO2, so as temperatures rise the amount of CO2 that could potentially be dissolved in the sea decreases.
Combustion transfers carbon stored in living, dead or decomposed biomass to the atmosphere by burning. Wildfires cause carbon flow.
Decomposition transfers carbon from dead biomass to the atmosphere and the soil. After death, bacteria and fungi break organisms down. CO2 and methane are released. Some carbon is transferred to the soil in the form of humus.
Respiration transfers carbon from living organisms to the atmosphere. Plants and animals break down glucose for energy, releasing carbon dioxide and methane in the process.
Chemical weathering transfers carbon from the atmosphere to the hydrosphere and biosphere. Atmospheric carbon reacts with water vapour to form acid rain. When this acid rain falls onto rocks, a chemical reaction occurs which dissolves the rocks. The molecules resulting from this reaction may be washed into the sea. Here, they react with CO2 dissolved in the water to form calcium carbonate, which is used by sea creatures.
Carbon form the atmosphere can be sequestered (captured and held) in sedimentary rocks or as fossil fuels. Rocks and fossil fuels form over millions of years when dead animal and plant material in the ocean falls to the floor and is compacted. Carbon in fossil fuels is sequestered until we burn them - combustion.
Photosynthesis transfers carbon stored in the atmosphere to biomass. Plants and phytoplankton use energy from the Sun to change carbon dioxide and water into glucose and oxygen. This enables plants to grow. Carbon is passed through the food chain and released through respiration and decomposition.
CO2 is directly dissolved from the atmosphere into the ocean. It is also transferred to the oceans when it is taken up by organisms that live in them, e.g. plankton. Carbon is also transferred from the ocean to the atmosphere when carbon-rich water from deep in the oceans rises to the surface and release CO2.
Hydrosphere: CO2 is dissolved in rivers, lakes and oceans. The oceans are the second-largest carbon store on Earth, containing 0.04%. The majority of carbon is found deep in the ocean in the form of dissolved inorganic carbon. A small amount is found at the ocean surface where is is exchanged with the atmosphere.
Cryosphere: Contains less than 0.01% of Earth's carbon. Most of the carbon in the cryosphere is in the soil in areas of permafrost (permanently frozen ground) when decomposing plants and animals have frozen into the ground.
Biosphere: Carbon is stored in the tissues of living organisms. It is transferred to the soil when living organisms die and decay. The biosphere contains 0.004% of the Earth's total carbon.
Atmosphere: Carbon is stored as carbon dioxide and in smaller quantities as methane. The atmosphere contains about 0.001% of the Earth's carbon.
Lithosphere: Over 99.9% of the carbon on Earth is stored in sedimentary rocks such as limestone. About 0.004% of the carbon on Earth is stored in fossil fuels, such as coal and oil.
Wildfires rapidly transfer large quantities of carbon from biomass. Loss of vegetation decrease photosynthesis, so less carbon is removed from the atmosphere. In the longer term, however, fires can encourage the growth of new plants, which take in carbon from the atmosphere for photosynthesis. Depending on the amount and type of regrowth, fires can have a neutral effect on the amount of atmospheric carbon.
Carbon stored within the Earth in magma is released during volcanic eruptions. The majority enters the atmosphere as CO2. Recent volcanic eruptions have released much less CO2 than human activities. However, there is the potential for a very large eruption to disrupt the carbon cycle significantly.
Hydrocarbon extraction and use: Extracting and burning of fossil fuels releases CO2 into the atmosphere. Without human intervention, the carbon would remain sequestered in the lithosphere for thousands or millions of years to come.
Farming practices: Animals release CO2 and methane when they respire and digest food. Ploughing can release CO2 stored in soil. Growing rice in rice paddies releases a lot of methane. Carbon emissions from farming practices have increased. Mechanisation of farming has also increased CO2 emissions.
Deforestation: Forests may be cleared for agriculture, logging, or to make way for developments. Clearance reduces the size of the carbon store and, if the cleared forest is burned, there is a rapid flow of carbon from the biosphere to the atmosphere.
Land use changes: Vegetation is removed to make way for buildings - this reduces carbon storage in the biosphere. Concrete production releases lots of CO2 and lots of concrete is used when urban areas expand.