Carbon Cycle Definition
The carbon cycle is the cycle by which the element carbon moves through our Earth’s various systems. It is a fascinating and complex process because living things, atmospheric changes, ocean chemistry, and geologic activity are all part of this cycle.
Any planet or other living environment must be a closed system; that is, materials necessary for life are not created or destroyed. For life to be sustainable, vital materials such as carbon, oxygen, water, and nitrogen must be consumed and re-released to the environment at roughly equal rates. If rates of consumption exceed rates of return, the consequences can be dire.
Carbon is an essential element for life as we know it because of its ability to form multiple, stable bonds with other molecules – and particularly with other carbon molecules. This is why nucleotides, amino acids, sugars, and lipids all depend on carbon backbones: carbon provides a stable structure that allows the chemistry of life to happen. Without carbon, none of these molecules could exist.
Our type of life is referred to by scientists as “carbon-based” by scientists. Some scientists speculate that life on other planets could be based on a different chemistry, but no evidence of non-carbon-based life has yet been found.
Originally, carbon was created through nuclear fusion in the hearts of stars. When the Earth was formed, it was formed from some gases and minerals that contained carbon – which happened to be the ideal ingredient to form biological molecules. In time, living things began to take carbon from the atmosphere and even from rocks and use it to build living materials such as sugars, proteins, and lipids.
Early photosynthesizers such as cyanobacteria transformed Earth’s atmosphere by turning huge amounts of carbon dioxide gas into molecular oxygen. This was not their goal – but rather a happy side effect of these first life forms removing the “C” from CO2. This left O2 in the atmosphere – and countless molecules of carbon serving as part of the machinery for life.
In time, other forms of life discovered that the new O2 in the atmosphere could be used to power a highly efficient method of liberating the energy stored in carbon-based organic molecules like sugars. Using this process of cellular respiration, animals and other oxygen-breathers started turning O2 back into CO2 – effectively spitting out the carbon atoms once contained in sugars, proteins, and lipids after extracting all of their energy.
This happy balance of plants turning carbon dioxide into living matter while animals reduce it back into gases for the plants to consume has existed for billions of years. In the process, new steps became incorporated – such as the formation of fossil fuels, which occurs when organic matter such as dead plants and animals become trapped underground by geologic processes.
The graphic below illustrates some common ways in which carbon moves through the ecosystem:
Carbon Cycle (click to play animated GIF)
Many scientists have raised concerns that recent human activity may be throwing off the carbon balance of the Earth. By burning huge amounts of fossil fuels and cutting down roughly half of the Earth’s forests, humans have significantly decreased the Earth’s ability to take carbon out of the atmosphere and turn it into living matter, while simultaneously releasing billions of years worth of carbon that used to be living matter into the atmosphere.
The most acute risk from these practices is that fossil fuels will run out, since they are being consumed much faster than they are created. But a risk which might be much more devastating is that of changing Earth’s atmospheric composition – since carbon dioxide is a greenhouse gas, continuing to release carbon dioxide while cutting down trees could result in dangerous changes to the Earth’s climate.
Function of Carbon Cycle
The carbon cycle, under normal circumstances, works to ensure the stability of variables such as the Earth’s atmosphere, the acidity of the ocean, and the availability of carbon for use by living things. Each of these components is of crucial importance to the health of all living things – especially humans.
Carbon dioxide in the atmosphere acts as a greenhouse gas, trapping sunlight and heat near the Earth’s surface. This isn’t always a bad thing – some carbon dioxide in the atmosphere is good for keeping the Earth warm.
But Earth has experienced catastrophic warming cycles in the past, such as the Permian extinction.
No one is sure what caused the Permian extinction – it is thought to have been due to a drastic increase in the atmosphere’s carbon dioxide levels, which may have been caused by an asteroid impact, volcanic activity, or even massive forest fires. Whatever the cause, during this warming episode, temperatures rose drastically, much of the Earth became desert, and over 90% of all species living at that time went extinct.
This is a good example of what can happen if our planet’s carbon cycle experiences a big change.
Another important variable effected by the carbon cycle is the acidity of the ocean. Carbon dioxide can react with ocean water to form carbonic acid. This has been an important stabilizing force of of the carbon cycle over the years, since the chemical equilibrium between carbon dioxide and carbonic acid means that the ocean can absorb or release carbon dioxide as atmospheric levels rise and fall.
However, as you might guess, increasing ocean acidity can mean trouble for sea life – and this might eventually pose a problem for other parts of the carbon system. Many forms of sea life that have shells, for example, can take carbon out of the water to create the calcium carbonate that they make their shells out of. If these species suffer, the ocean may lose some of its ability to remove carbon from the atmosphere.
Lastly, of course, there is the role of living things in the carbon cycle. The activity of plants and animals has been one of the major forces affecting changes to the carbon cycle in the past several billion years. Photosynthesizers have changed Earth’s atmosphere and climate drastically by taking huge amounts of carbon out of the atmosphere and turning that carbon into cellular materials.
Those activities created free oxygen and the ozone layer, and generally set the stage for the evolution of animals that obtain their energy by breaking down the organic materials created by photosynthesizers and extracting the energy that the photosynthesizers used to make those molecules.
With one particular species of animals – humans – having drastically reduced the amount of plant life available to turn carbon into organic material, the future of Earth’s carbon cycle is uncertain.
All such cycles in closed systems eventually correct themselves – but sometimes this happens through drastic population reduction of the offending species through starvation.
Examples of the Carbon Cycle
The carbon cycle consists of many parallel systems which can either absorb or release carbon. Together, these systems work to keep Earth’s carbon cycle – and subsequently its climate and biosphere – relatively stable.
Here are some examples of parts of Earth’s ecosystems that can absorb carbon, turn carbon into living matter, or release carbon back into the atmosphere.
Atmosphere
One major repository of carbon is the carbon dioxide in the Earth’s atmosphere. Carbon forms a stable, gaseous molecule in combination with two atoms of oxygen.
In nature, this gas is released by volcanic activity, and by the respiration of animals who affix carbon molecules from the food they eat to molecules of oxygen before exhaling it.
Humans also release carbon dioxide into the atmosphere by burning organic matter such as wood and fossil fuels.
Carbon dioxide can be removed from the atmosphere by plants, which take the atmospheric carbon and turn it into sugars, proteins, lipids, and other essential molecules for life.
It can also be removed from the atmosphere by absorption into the ocean, whose water molecules can bond with carbon dioxide to form carbonic acid.
In recent years, scientists have raised concerns that by cutting down about half of Earth’s forests, humans may be decreasing the Earth’s ability to remove carbon dioxide from the atmosphere at the same time they’re adding new sources by burning wood and fossil fuels.
Many organizations that hope to fight man-made climate change now plant trees which can remove carbon dioxide from the atmosphere, as well as advocating for alternative energy sources and less burning of fossil fuels such as gasoline, oil, and coal.
Lithosphere
The Earth’s crust – called the “lithosphere” from the Greek word “litho” for “stone” and “sphere” for globe – can also release carbon dioxide into Earth’s atmosphere. This gas can be created by chemical reactions in the Earth’s crust and mantel.
Volcanic activity can result in natural releases of carbon dioxide. Some scientists believe that widespread volcanic activity may be to blame for the warming of the Earth that caused the Permian extinction.
While the Earth’s crust can add carbon to the atmosphere, it can also remove it. Movements of the Earth’s crust can bury carbon-containing chemicals such as dead plants and animals deep underground, where their carbon cannot escape back into the atmosphere.
Over millions of years, these underground reservoirs of organic matter liquefy and become coal, oil, and gasoline. In recent years, humans have begun releasing much of this sequestered carbon back into the atmosphere by burning these materials to power cars, power plants, and other human equipment.
Biosphere
Among living things, some remove carbon from the atmosphere, while others release it back. The most noticeable participants in this system are plants and animals.
Plants remove carbon from the atmosphere. They don’t do this as a charitable act; atmospheric carbon is actually the “food” which plants use to make sugars, proteins, lipids, and other essential molecules for life.
Plants use the energy of sunlight, harvested through photosynthesis, to build these organic compounds out of carbon dioxide and other trace elements. Indeed, the term “photosynthesis” comes from the Greek words “photo” for “light” and “synthesis” for “to put together.”
Here lies arguably the most important part of the carbon cycle: all life is made of carbon. Without plants or other organisms that could turn inorganic carbon compounds like carbon dioxide into organic compounds, life could not exist.
Indeed, none of the building materials for our cells, from our DNA to our cell membranes, could exist without this ability of photosynthesizers to turn carbon dioxide into life!
In a gracefully balanced set of chemical reactions, animals eat plants (and other animals), and take these synthesized molecules apart again. Animals get their fuel from the chemical energy plants have stored in the bonds between carbon atoms and other atoms during photosynthesis.
In order to do that, animal cells dissemble complex molecules such as sugars, fats, and proteins all the way down to single-carbon units – molecules of carbon dioxide, which are produced by reacting carbon-containing food molecules with oxygen from the air.
In this way, most of the carbon eaten by animals ends up back where it started before it was absorbed by a plant – as part of a carbon dioxide molecule in the atmosphere.
Carbon in plants and animals that is not consumed by other animals can be broken down by other living things until it becomes carbon dioxide again, or can be sequestered deep in the Earth as fossil fuels.
Oceans
The Earth’s oceans have the ability to both absorb and release carbon dioxide. When carbon dioxide from the atmosphere comes into contact with ocean water, it can react with the water molecules to form carbonic acid – a dissolved liquid form of carbon.
Like most chemical reactions, the rate of this reaction is determined by the equilibrium between the products and the reactants.
When there is more carbonic acid in the ocean compared to carbon dioxide in the atmosphere, some carbonic acid may be released into the atmosphere as carbon dioxide.
On the other hand, when there is more carbon dioxide in the atmosphere, more carbon dioxide will be converted to carbonic acid, and ocean acidity levels will rise.
Some scientists have raised concerns that acidity is rising in some parts of the ocean, possibly as a result of increased carbon dioxide in the atmosphere due to human activity.
Although these changes in ocean acidity may sound small by human standards, many types of sea life depend on chemical reactions that need a highly specific acidity level to survive.
As a result, there is concern that increasing ocean acidity due to carbonic acid may contribute to the die-offs of some marine ecosystems, and even to extinctions of marine species.
Quiz
1. Which of the following is NOT a vital component of the carbon cycle?
A. Photosynthesizers take carbon from the atmosphere and turn it into sugars, proteins, lipids, and other vital materials for life.
B. Oxygen-breathers break down organic materials into energy and carbon dioxide, which they release back into the atmosphere.
C. Geologic activity releases carbon in the form of volcanic gases.
D. The ocean absorbs carbon in the form of carbonic acid or calcium carbonate.
E. None of the above.
2. Which of the following is NOT true of carbon levels in Earth’s atmosphere?
A. The composition of Earth’s atmosphere has changed drastically over time.
B. Drastic changes to the temperature of Earth’s surface have happened as a result of changes in the atmosphere’s carbon levels.
C. Mass extinctions have occurred as a result of changes to the atmosphere’s carbon levels.
D. It is impossible to significantly change the carbon levels in Earth’s atmosphere.
3. Which of the following would NOT be a possible outcome if the carbon cycle were severely disrupted?
A. Severe global cooling as a result of fewer greenhouse gases in the Earth’s atmosphere.
B. Severe global warming as a result of more greenhouse gases in the Earth’s atmosphere.
C. Drastic changes to ocean ecosystems due to changing ocean acidity.
D. Drastic changes to land ecosystems as a result of changing temperatures and weather patterns.
E. None of the above.
References
- Falkowski, P. (2000). The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System. Science, 290(5490), 291-296. doi:10.1126/science.290.5490.291
- Houghton, J. T. (2001). Climate change 2001: the scientific basis: contribution of Working Group I to the third assessment report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press.
- Hoffman, W. B. (n.d.). Permian Extinction Article, Mass Extinction Information, Park Tourism Facts. Retrieved May 10, 2017, from http://science.nationalgeographic.com/science/prehistoric-world/permian-extinction
Carbon Cycle
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