Carbon Cycle Explained
The Carbon Cycle. A key biogeochemical pathway for all living beings and climate change.

The Carbon Cycle: How Does It Work?

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What Is The Carbon Cycle?

The “carbon cycle” is one of the most important biogeochemical cycles, operating within Planet Earth. It’s a circular pathway which allows carbon atoms to be continually used and re-used, in countless chemical reactions – such as, photosynthesis and respiration – that help to sustain life.

Carbon moves through Earth’s main components – the air (atmosphere), the rivers and oceans (hydrosphere), the ice, snow and permafrost (cryosphere), the soil (pedosphere), living creatures (biosphere) and the rocks (lithosphere) – changing its form from gas to liquid to solid, as needed.

Since the Earth and its atmosphere form a closed environment, the actual amount of carbon in the system remains constant, although its form and location within the system may change. Another oddity is the fact that the carbon cycle is actually two separate pathways – one fast, one slow – which are differentiated by the length of time that the carbon remains unavailable for use.

In the fast carbon cycle, the carbon moves between plants and the atmosphere over a period not much longer than a human lifespan. By contrast, the slow carbon cycle – in which carbon moves from the atmosphere to rock and back to atmosphere – might take many millions of years to complete.

Ever since the Industrial Revolution, the carbon cycle as a whole has gradually become more and more unbalanced, due to an ever-growing increase of carbon entering it. This ongoing increase is due exclusively to our extraction of vast amounts of carbon-rich fossil fuels (coal, peat, oil, natural gas), from deep down within the rock, which are then burned to create power and heat for industrial and domestic purposes. 1 2

As we now know, this combustion of fossil fuel energy emits such a large amount of greenhouse gas emissions that it overloads Earth’s climate system, and causes a rise in Earth’s temperature – a process we now refer to as climate change or global warming.

Simple Carbon Cycle Diagram
How the Carbon Cycle Works. Image: © UCAR: Center for Science Education

Why Is Carbon Important?

Scientifically speaking, the “carbon cycle” is a complex system of chemical, biological, geological and physical processes. It is only one of six biogeochemical cycles (the others being the oxygen, nitrogen, hydrogen, phosphorus, and sulfur cycles) involving elements that are vital for life.

Carbon itself is the basic building block of life. All living organisms contain carbon because it is able to create multiple, stable bonds with other molecules, forming long chain molecules with multiple properties. It provides the stable structure necessary for compounds like nucleotides, amino acids, sugars, lipids and Deoxyribonucleic acid (DNA).

Without carbon, none of these materials could form these complex molecules and thus contribute to the chemistry of life. As well as being a key constituent of biological compounds, carbon is a major ingredient of many minerals, such as limestone.

Why is the carbon cycle called a biogeochemical cycle?

Biogeochemical cycles are called “biogeochemical” because the cyclical pathways they follow move through both the living “biosphere” (from “bio-” meaning life) and non-living “geosphere” (from “geo-” meaning earth) compartments of Earth. In terms of a local ecosystem, these cycles traverse biotic and abiotic parts of the ecosystem. Take carbon, for example. This moves through living things (plants, animals, humans) as well as non-living things (rocks, sand, minerals, and the like).

How Does The Carbon Cycle Work?

Carbon Cycle Diagram
The Carbon Cycle – showing movement of carbon between land, atmosphere and ocean in billion tons per year. Red numbers are human contributions, yellow are natural events and white are stored carbon. Image: © Earthobservatory.nasa.gov

The carbon cycle actually consists of several differing cycles, or circular scenarios, in which the element is used and re-cycled. Here are a few examples: 

Atmosphere To Land By Plant Photosynthesis

Carbon starts off as carbon dioxide (CO2) in the air. Here, it is absorbed by plants who use it to create energy for themselves. They do this using a chemical process called photosynthesis. During this process, plants combine CO2 (from the air) with water (from their roots) in the presence of sunlight, to form a store of energy (carbohydrates). Oxygen (O2) is a by-product that is released into the atmosphere. 3 4

Because they are able to create energy from abiotic (non-living) sources, plants are known as primary producers. Other highly successful primary producers are oceanic phytoplankton, who photosynthesize in the sunlit shallows of the ocean. Primary producers are the foundation of the food web on land and sea.

The Photosynthesis Formula: 

Carbon dioxide + water + sunlight -> carbohydrate + oxygen
CO2 + H2O + sunlight -> CH2O + O2

The carbon taken in by plants via photosynthesis is released back into the atmosphere in one of four ways:

  1. Respiration (breathing and the chemical processes involved in breathing). Plants exhale CO2 when they respire, although they exhale only about half the CO2 than they take in during photosynthesis. Animals also exhale CO2 when breathing, as do microorganisms. 5
  2. Decomposition or decay. When plants die and decay their remains are broken down by various microorganisms, mostly fungi or bacteria. These decomposers typically release enzymes to break down the decaying material, after which they ingest the nutrients in the material. In the process, they either produce carbon dioxide or methane as waste. If there is oxygen available, the microbes make carbon dioxide. If there is no oxygen available, they make methane.
  3. Transfer of carbon to another organism. When a rabbit eats grass, carbon from the grass becomes part of the fats and proteins in the rabbit. The process continues if the rabbit it eaten by a fox. No matter who eats what, at some point the surviving animal dies and its remains are broken down and absorbed by decomposers and other heterotrophs, called detritivores, which (as in (1) above) releases CO2 into the atmosphere. The same decay and breakdown process happens to waste material from animals.
  4. Forest fires ignited by lightning are another return pathway for the movement of carbon back into the atmosphere. Unfortunately, the effect of wood burning on climate change is significant, whether occuring through natural mechanisms or human action.

Atmosphere To Land By Rainfall 

Instead of being used by a plant, atmospheric carbon dioxide might be dissolved by rainfall (CO2 is a very soluble gas) and deposited on land. Here, one of two things may happen to it. It may be drunk by a rabbit, or consumed and exhaled by soil biota, as in (1) above. Or, it may be absorbed into the soil where, after years of compaction and cementation, it turns into sedimentary rock.

It will remain trapped in the rock until, after millions more years, geological forces melt it and expel it into the atmosphere through volcanic activity. (Note: At present, volcanoes discharge on average between 130 and 380 million tons of carbon dioxide into the atmosphere, each year.)

In certain cases, when dead trees and other plant matter build up in buried layers faster than they can fully decay, these layers of organic carbon eventually – over millions of years – become transformed into deposits of coal or other fossil fuels like petroleum or natural gas.

Atmosphere To Ocean

Atmospheric carbon dioxide might be dissolved by rainfall and fall directly into the sea. Or, it might fall on land first, where, if the rainfall is heavy, it may be washed into drains or streams and then into rivers, eventually arriving at the ocean. In either case, once the CO2 water reaches the ocean, two basic outcomes are possible.

It may be photosynthesized in sunlit waters by tiny marine organisms called phytoplankton. Many of these organisms are consumed by other sea creatures, in which case the carbon passes up the food chain. (But see: Marine Microbes Drive the Aquatic Food Web for an explanation of the microbial loop and viral shunt, both designed to keep the carbon for the benefit of microorganisms.)

If the plankton are not consumed but simply die, the CO2 generally passes into the deep ocean and dissolves into the sea-water as the plankton decay. A small proportion then becomes buried in sediment on the ocean floor. 6

Here, during the course of millions of years, chemical and physical processes turn these sediments into sedimentary rock, like limestone. (You know the rest.) However, most of the dissolved CO2 eventually returns to the surface via ocean currents and passes into the atmosphere, although this process can take centuries, even millennia.

Instead of being absorbed by phytoplankton, the CO2 may be captured by photosynthesizing mangroves, seagrasses or other sea plants. This CO2 – known as “blue carbon” – is then stored underwater in their roots and soil. So unlike carbon captured by terrestrial plants, blue carbon remains in the soil even if the mangroves die.

If it is not photosynthesized and simply remains dissolved in the ocean, most of it becomes part of the continuous exchange of CO2 between the ocean and the atmosphere. The process is largely controlled by the difference in CO2 concentration between the sea and the air above it.

For example, when the CO2 pressure in the air is higher than that of the water below, CO2 diffuses across the air-sea boundary into the sea water. Conversely, if the CO2 pressure in the water is higher than that of the air above, CO2 diffuses across the air-sea boundary into the atmosphere.

The process is also affected by the difference in relative temperatures between air and sea. Thus, If the sea temperature were to rise (everything else being the same) then the kinetic energy of the CO2 molecules in the seawater will rise and more CO2 molecules will leave the ocean than would enter the ocean. This will continue until the CO2 pressure in the air increased to the point that it balanced the CO2 pressure in the water. Conversely, if the sea temperature fell (everything else being the same), more CO2 molecules will enter the water than would leave it.

The impact of temperature on the ocean-atmosphere exchange of CO2, is illustrated by the fact that most CO2 is absorbed by polar waters, whereas most CO2 is emitted by tropical seas. This explains how thermohaline circulation currents absorb huge amounts of carbon dioxide in the extreme northern Atlantic, before downwelling into the depths taking the CO2 with them.

Atmosphere To Rock To Ocean

If rainfall containing the dissolved CO2 lands on exposed rock (particularly silicates, or carbonate rocks like limestone or dolomite), it causes microscopic chemical weathering of the rock surface, before being washed away by rainfall run-off to the ocean.

In the case of silicate weathering, the chemical equation is as follows:

2CO2 (two molecules of carbon dioxide) + H2O (water) + CaSiO3 (calcium silicate rock) -> Ca2 (calcium) + 2HCO3 (bicarbonate) + SiO2 ( dissolved silica)

ocean-atmosphere exchange
The exchange of gases and particles between the ocean and the atmosphere. SOLAS research is aimed at better understanding the role that these exchanges play in ocean biogeochemistry, atmospheric chemistry and climate. © The Surface Ocean – Lower Atmosphere Study (SOLAS)

In the ocean, marine calcifying organisms use the calcium and bicarbonate to build shells and skeletons (a mechanism called carbonate precipitation). One molecule of CO2 is released back into the atmosphere. The chemical equation for this is as follows:

Ca2 (calcium) + 2HCO3 (bicarbonate) -> CaCO3 (calcium carbonate) + H2O (water) + CO2 (released into air)

When the marine organisms die, their shells and skeletal remains sink into deep water where they end up as sediments on the ocean floor. Here, after millions of years, chemical and physical processes turn these sediments into sedimentary rock, in a process known as lithification. (You know the rest).

Why Is Rock Weathering Important?

Rock weathering is the best example of how the “fast” carbon cycle interacts with the “slow” carbon cycle. The fast cycle basically comprises the photosynthesis/respiration process, which circulates carbon between plants and the atmosphere over a period not much longer than a human lifespan.

The slow cycle, which involves the movement of carbon between the atmosphere and lithosphere, operates over the course of tens of millions of years, or longer. The function of the slow cycle is to store large amounts of carbon to help maintain a correct carbon balance in the atmosphere. But it may be more responsive than we think. It seems that, the more CO2 in the atmosphere, the faster the chemical weathering process becomes. 7

How Do Cows Contribute To The Carbon Cycle?

All animals – including cows – exhale carbon dioxide when they breathe. But cows are special. They are ruminants and – like all cattle, sheep, goats, deer and other ruminants – have several different stomach compartments, which allow them to absorb nutrients from tough plant-based matter by assiduously chewing it and allowing it to ferment, prior to digestion. Fermentation occurs as a result of action by microorganisms, in the cows’ special stomach (rumen). The point is, this fermentation or decaying process takes place in the absence of oxygen, and therefore produces a different carbon-rich gas, called methane (CH4). When cows burp or pass wind, they release this digestive methane into the atmosphere. So, cows contribute to the carbon cycle in two ways: they release CO2 by exhaling, and methane by passing wind. See also: Why Are Methane Levels Rising?

The carbon cycle is inextricably linked with other biogeochemical cycles, notably the water cycle, because carbon dioxide is continuously being dissolved and extracted from the atmosphere by rainfall, and flushed into streams, rivers and lakes, eventually reaching the ocean.

The water cycle, also called the hydrological cycle, describes how water moves above, on, or just below the surface of the Earth. As it circulates around the hydrosphere it changes state, from liquid to gas (water vapor) or solid (ice) and back.

From the ocean – which contains roughly 96.5 percent of all water on the planet – water evaporates into water vapor under the heat of the sun. The moist air rises but then cools down as it rises, condensing back into liquid water as a result. Condensation leads to the formation of clouds. As the liquid droplets in the clouds get bigger, they fall as rain, sleet or snow. This is called precipitation.

If rainfall is very heavy, the excess surface water may be washed into drains or streams and then into rivers, as described above, or it may drain away more slowly, percolating into underground streams and ultimately reaching the ocean. Otherwise it may be absorbed into the soil. From the soil it may be sucked upwards into trees or plants by the action of their roots, and later released into the atmosphere via transpiration through the stomata on the underside of their leaves.

Trees and the Carbon Cycle

As we have seen, trees and other plants serve as a natural carbon capture and storage system, absorbing CO2 via photosynthesis, of which only about half is returned to the atmosphere by respiration. The remainder is stored in their leaves, shoots and stems/trunks. In total, scientists estimate that plants absorb at least one quarter of all human CO2 emissions. 8 9

What’s more, it is well-documented that as CO2 in the atmosphere increases, so does plant growth and photosynthesis, which should increase the CO2-capturing ability of trees even further. See: Is More CO2 Good for Plants?

Shortage of Nutrients

Scientists worry, however, that the ability of trees to sequester carbon dioxide and limit the greenhouse effect, may soon be reduced. Why? Because as CO2 levels rise, trees are going to need extra phosphorus and nitrogen to balance their diet and increase their CO2 take-up. See also: 7 Effects of Climate Change on Plants.

However, increased plant growth is not due solely to higher CO2 levels, but also depends on the abundance of nutrients in the soil, especially nitrogen and phosphorus. If trees and plants can’t get enough nutrients, they will not grow more even if CO2 levels rise. Which raises the question: just how much extra CO2 will trees be able to absorb in the coming decades?

The answer, according to a 2019 study, is 12 percent. It says that, globally, trees and plants can increase their biomass by roughly 12 percent when exposed to the levels of CO2 that are being predicted for the year 2100. However, the researchers qualify this by emphasizing that their forecast is based on plant and forest cover remaining at current levels. In other words, there must be no further deforestation.

The study’s lead author Dr Cesar Terrer said: “Keeping fossil fuels in the ground is the best way to limit further warming. But stopping deforestation and preserving forests so they can grow more is our next-best solution.”
10 11 See also: Effects of Deforestation.

From Carbon Sinks to Carbon Sources

So, even though forests currently act as major CO2 sinks and may sequester more carbon as they grow, human activities – such as commercially-driven deforestation in the Amazon Rainforest – along with naturally occurring fires like the recent Australian Bushfires – can rapidly turn these precious sinks into CO2 sources. For example, Australia’s wildfires emitted more CO2 than the entire country did in the previous 2 years. 12

Contribution of Urban Trees to Carbon Cycle

Concerns have arisen in the wake of a 2009 study by researchers at Cal State into carbon sequestration by trees on the university’s 350-acre campus at Northridge. All 3,900 campus trees were inventoried by type and size, and their CO2 absorption calculated using data from the Center for Urban Forest Research. The average was 88 pounds of CO2 per tree per year. Then they compared the trees’ total CO2 sequestration to the amount of carbon emitted in the production of campus power sources (like electricity). They discovered that the trees sequestered less than one percent of the amount of carbon released during the same period. 13

Do Trees Emit Heat-trapping Hydrocarbons?

Some scientists caution against relying on forests as a solution to global warming until more sophisticated climate models are developed to provide a better understanding of (inter alia) the chemicals they emit and the effect dark green leaves have on Earth’s albedo.

A case in point is the study conducted by Professor Nadine Unger – one of the first global studies of its kind – into volatile organic compounds (VOCs) including isoprene, emitted by trees. Her results suggest that the conversion of forests to farmland throughout the industrial era may have had little impact on climate change. Although forest clearance releases carbon stored in trees, it raises Earth’s albedo (leading to cooling) and decreases emissions of VOCs that can both cool and warm. 14

What Is The Difference Between Carbon Sources, Carbon Sinks And Carbon Reservoirs?

  • Carbon sources” are processes which add carbon to the atmosphere. Examples of natural carbon sources include: plant or animal decay, when decomposers break down the dead material, discharging CO2 into the air as part of the process. Volcanoes and wildfires – like the huge Arctic fires – are other natural sources. Man-made carbon sources include: the burning of fossil fuels or deforestation causing the death and decay of trees and plant life. 15
  • Carbon sinks“, by contrast, are natural processes which remove CO2 from the atmosphere and store it. Examples of carbon sinks include: forests, whose trees and vegetation absorb CO2 from the air via photosynthesis (Tree-Planting: the Solution to Global Warming?); and oceans, in which some CO2 is absorbed by plankton during photosynthesis, and some dissolves in the water. The actual process of removing the CO2 from the atmosphere is called “carbon sequestration”.
  • The term “carbon reservoirs” is sometimes used (inaccurately) as a synonym for “carbon sinks”. A carbon reservoir is a store of accumulated carbon. A reservoir is not a giver or taker of carbon, merely a store. Examples of carbon reservoirs include: the deep oceans and the Earth’s crust – the lithosphere – especially its sedimentary rocks, like limestone, and its organic hydrocarbon deposits (fossil fuels). A carbon reservoir can become a net carbon source if (say) it is induced by global warming to emit some of its carbon into the atmosphere, but it remains a carbon reservoir – at least until it has no more left.

Where Is Earth’s Carbon Stored?

There is no clear consensus as to how much carbon is stored in the various sinks and reservoirs/pools on the planet. NOAA, NASA, the IPCC, as well as different bodies and groups of scientists, all seem to give different statistics. After prolonged research we have chosen the following figures.

Earth's Carbon Stores: Statistics
Note: All values are in metric tonnes. Sources: 16 17 18 19 20 21 22 23

Latest Statistics on Carbon Sinks and Reservoirs

For the latest statistics on the amount of accumulated carbon stored in the atmosphere, the terrestrial biosphere, the ocean and the lithosphere, see: Which is the Largest Carbon Reservoir?

For more about the timeline of our planet and its carbon cycle, see: History of Earth in One Year.

Further Reading

– “The global carbon cycle.” Princeton: Princeton University Press. (2010). Archer, David. ISBN 9781400837076.
A Science Plan for Carbon Cycle Research in North American Coastal Waters. Report of the Coastal CARbon Synthesis (CCARS) community workshop, August 19-21, 2014, Ocean Carbon and -Biogeochemistry Program and North American Carbon Program, 84 pp. Benway, H., et al. 2016.
-Ciais, P. et al. in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the -Intergovernmental Panel on Climate Change (eds. Stocker et al.) 1–106 (Cambridge University Press, 2013).
Recent increase in oceanic carbon uptake driven by weaker upper-ocean overturning. Nature 542, 215–218 (2017). DeVries, T., Holzer, M. & Primeau, F.
-“Low-latitude arc–continent collision as a driver for global cooling.” Oliver Jagoutz, Francis A. Macdonald, and Leigh Royden. PNAS May 3, 2016 113 (18) 4935-4940.
– Natural Variability and Anthropogenic Trends in the Ocean Carbon Sink. Annu. Rev. Marine. Sci. 9, 125–150 (2017). McKinley, G. A., Fay, A. R., Lovenduski, N. S. & Pilcher, D. J.
Climate change and the permafrost carbon feedback. Nature 520, 171–179. (2015). Schuur, E. A. G. et al.

References

  1. The Carbon Cycle.” NASA []
  2. The Carbon Cycle.” UCAR Center for Science Education []
  3. What is Photosynthesis? Smithsonian Science Education Center []
  4. Photosynthesis and Photosynthetic Cells: Nature []
  5. Implications of improved representations of plant respiration in a changing climate.” Chris Huntingford, et al;. Nature Communications, 2017; 8 (1) []
  6. Exactly how much carbon makes it to the ocean floor is unclear. For example, compare the article text with this sentence from the ACS. “Like inorganic carbon, most organic carbon ultimately ends up in sediments on the ocean floor.” Ocean Chemistry. ACS Climate Science Toolkit. Oceans, Ice, and Rocks. American Chemical Society. acs.org/ []
  7. High sensitivity of the CO2 sink by continental weathering to the future climate change.” Emilie Beaulieu, et al. Nature Climate Change, March 2012, []
  8. “Ocean-Atmosphere CO2 Exchange”. NOAA. []
  9. “Forests and climate change.” U.N. FAO. []
  10. “Nitrogen and phosphorus constrain the CO2 fertilization of global plant biomass”. Cesar Terrer, et al. Nature Climate Change, 2019. []
  11. Compare “Natural climate solutions.” Bronson W. Griscom, et al. PNAS October 31, 2017 114 (44) 11645-11650; first published October 16, 2017. []
  12. “Estimating greenhouse gas emissions from bushfires in Australia’s temperate forests: focus on 2019-20.” []
  13. “How green are Apple’s carbon-sequestering trees really?” []
  14. “How much can forests fight climate change?” []
  15. For research on how to turn urban carbon sources into carbon sinks, see: “Buildings as a global carbon sink.” Churkina, G., Organschi, A., Reyer, C.P.O. et al. Nat Sustain (2020). []
  16.  “An Introduction to the Global Carbon Cycle” (PDF). University of New Hampshire. 2009. []
  17. “The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System.” (2000) Science. 290 (5490): 291–296. Falkowski, P. et al. []
  18.  “Introduction to the Science of Climate Change.” Prof. Jordi Miralda-Escude, F 9:30. []
  19. IPCC. Third Assessment Report (2001). []
  20. “Storing carbon in soil: Why and how?” Charles W Rice, (January 2002). Geotimes. 47 (1): 14–17. []
  21. “Soil organic carbon pools in the northern circumpolar permafrost region” (PDF). Global Biogeochemical Cycles. 23 (2): GB2023. Tarnocai, C., et al; (2009). []
  22. “The carbon cycle and atmospheric carbon dioxide.” Prentice, I.C. (2001). In Houghton, J.T. (ed.). Climate change 2001: the scientific basis: contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. []
  23.  “Carbon dioxide in Earth’s atmosphere: Current Concentration.” Wikipedia. []
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