Carbon-Oxygen Cycle

The carbon-oxygen cycle is the process of cycling oxygen and carbon through Earth’s environment. The cycle includes phenomena, such as photosynthesis, which makes oxygen by producing sugar from carbon dioxide, and respiration, which uses oxygen to break sugar down into carbon dioxide. The cycle is critical for the homeostasis of the environment.

The Carbon-Oxygen Cycle

The carbon-oxygen cycle involves the flow of carbon and oxygen through the environment. Carbon is recycled between carbon dioxide and carbon compounds in life-forms. The cycle includes the transfer mechanisms of the carbon compounds and oxygen in the biosphere, atmosphere, hydrosphere, and pedosphere. Climatological research focuses on the rates of processing and the homeostasis of the system to determine carbon dioxide release and capture.

The carbon-oxygen cycle is the result of the storage of carbon from atmospheric (or, in the case of aquatic organisms, dissolved) carbon dioxide as organic material through photosynthesis and its release through respiration, combustion, or decomposition. All of these combine the carbon in the organic material with atmospheric or dissolved oxygen as CO₂. This CO₂ is then released into the atmosphere, where it is reabsorbed by photosynthetic organisms, such as cyanobacteria, some protists, and plants, starting the process anew.

The cycle shows how different parts of Earth’s environment and climate are interrelated. For example, analyzing the flow of material from rivers into seas allows scientists to consider the impact of biologic materials, and the release of gases into the atmosphere and the capture of carbon by plants allow scientists to see the effect of land use on climate.

By examining the environmental factors in a given area—factors, such as land use or propensity for fire—one can get an idea of how a given region will impact other areas and estimate the total amount of gas it might contribute to the atmosphere. Additionally, environmental factors provide hints as to effective methods of managing or sequestering carbon. A forest is an example of carbon sequestration.

Based on the carbon-oxygen cycle, it is known that trees take in a certain amount of carbon in time, that the trees will produce living space for some number of animals, and that those animals will produce a certain amount of carbon dioxide throughout their lives and when they die and decompose. Also factored is whether a forest is prone to fires. By looking at such factors, one can get an idea of how much carbon dioxide a forest can remove from the atmosphere, for how long, and if there is anything humans can do to make that forest more efficient.

However, most of the oxygen actually released into Earth’s atmosphere occurs because of oceanic plankton, but rainforests absorb huge amounts of carbon. Northern forests are another major site of oxygen turnover.

Processes

Photosynthesis is a set of chemical reactions occurring in plants, some bacteria, and some protists. Photosynthesis uses energy from the Sun to turn carbon dioxide and water into sugar and oxygen.

The process itself is the interaction of several sets of reactions: the light-dependent reactions and the Calvin or dark cycle. The light-dependent reactions separate the hydrogen from the oxygen in water and are thus the oxygen-producing portion of the cycle; they prepare reactants needed for the other cycles. The Calvin or dark cycle turns the carbon dioxide and light cycle products into sugar. The light cycle runs directly on captured photons, whereas the Calvin cycle does not depend directly upon sunlight.

Along with rates of release from combustion, which is the conversion of organic material to carbon dioxide and water vapor, also needing to be factored in are rates of respiration and decomposition. Respiration is how organisms get energy from organic compounds, such as sugars. Plants and animals use aerobic respiration, which evolved after oxygen became prevalent in the atmosphere. Aerobic respiration occurs in special organelles of eukaryotic cells called ribosomes. Here, sugars are broken down into carbon dioxide and water in a process known as the Krebs or citric acid cycle.

Decomposition releases various greenhouse gases, as decomposing organisms are often excellent habitats for anaerobic life-forms. Anaerobic respiration releases carbon dioxide, hydrogen sulfide, and methane. Thus, the total impact of the disruption of an environment, such as the burning of a forest, is not only in the combustion of the trees but also in the drop in photosynthesis and decomposition of any detritus.

History of the Cycle

Understanding of the carbon-oxygen cycle has existed since the eighteenth century after the discoveries of Antoine Lavoisier, who found that respiration was a process similar to burning, and Joseph Priestley, who outlined the idea of the relationship between blood and air and who also discovered oxygen. However, scientists discovered the full mechanics of the carbon-oxygen cycle and its implications in subsequent centuries.

The workings of the chemical reactions of photosynthesis were discovered in the 1940s, and research continues into the role of quantum physics in the light-harvesting stage of photosynthesis. With the growing awareness of climate change, more work is being done to examine the cycle.

In the twenty-first century, research techniques often take samples from the area in question, be they soil samples to estimate bacterial carbon production or air samples to measure carbon levels at a given site. These samples are then analyzed together to show how each part affects the collective carbon-oxygen balance for that area. The carbon data are often mixed with data on other climatological factors, such as albedo, humidity, and temperature, to build a picture of the effects on climate.

Atmospheric Composition and the Greenhouse Effect

The carbon-oxygen cycle affects climate because it affects the Earth’s atmosphere. Ignoring water vapor, which is variable, it is known that the standard dry atmosphere is 78.09 percent nitrogen, 20.95 percent oxygen, 0.93 percent argon, and 0.039 percent carbon dioxide; the rest of the atmosphere comprises various trace gases. Water vapor composes, on average, about 1 percent of the atmosphere.

The carbon-oxygen cycle plays a vital role in maintaining homeostasis in the atmosphere. The cycle “cycles” air and carbon to maintain equilibrium. The gases in the atmosphere and their relative amounts have an essential impact on climate. Some gases absorb infrared rays reflected or emitted from the Earth’s surface and send them back to Earth, thus trapping the energy and making Earth hotter. This process is known as the greenhouse effect.

Gases that are best at emitting in the infrared range are known as greenhouse gases and include water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Water vapor and carbon dioxide make up the largest contribution to the effect, with methane and ozone playing a smaller role. Many of these gases are produced in industrial reactions, such as combustion. Combustion from cars and factories and other human activities (especially since the start of the Industrial Revolution) have been major factors in anthropogenic climate change. Here, the carbon-oxygen cycle comes into play. There is a maximum rate at which a particular environment can cycle the carbon dioxide into oxygen, meaning that excess carbon dioxide builds up in the atmosphere, increasing the greenhouse effect.

By studying the components of the cycle (that is, the plants and animals that are part of the process), scientists can find the effects of increased temperature on plants and animals. The environmental impact (increased environmental stress) reduces the capacity to handle the increased carbon dioxide. This creates a positive feedback loop that further increases the temperature. Increased global temperature could cause increased cloud cover, which would increase albedo. Increased temperature also could spawn algal blooms in the oceans, which would absorb carbon dioxide and thus reach a new equilibrium. Much environmental damage will occur before such an event happens.

The greenhouse effect is not wholly negative, however. It is needed to some degree for life to exist on this planet. Without greenhouse gases, the planet would be significantly colder than it is now. Additionally, Earth’s atmospheric composition has varied throughout its history. The Earth’s early atmosphere was different from today’s atmosphere. It was comprised mostly of water vapor, carbon dioxide, hydrogen sulfide, and ammonia. In geologic time, it did not take long for the oceans to form from rainwater. All the greenhouse gases of the time kept the average Earth temperature reasonably high.

Once photosynthetic life evolved about 3.5 billion years ago, that life began to break down the methane and carbon dioxide in the atmosphere, and the level of oxygen began to increase. This resulted in what is called the oxygen catastrophe, in which many of the organisms at the time became extinct, leaving those that adapted to the new situation by developing aerobic respiration. This development significantly impacted later life, as aerobic respiration became far more efficient.

The oxygen catastrophe also may have caused the Huronian glaciation event, which occurred most likely due to greenhouse gas depletion by photosynthetic life-forms. The event was one of the longest glaciation periods in Earth's history (300 million years). It and the later Snowball Earth periods that likely caused the evolution of multicellular life and the Cambrian explosion were ended only by the buildup of greenhouse gases from volcanic eruptions and by the weathering of rocks.

Equally, after this period, greenhouse gas levels oscillated by period; for example, during the Devonian period, carbon dioxide was higher, and oxygen was lower than at contemporary levels. In the Cretaceous period, both carbon dioxide and oxygen levels were higher. During both periods, the average temperature was higher than it is today.

A dramatic example of how the carbon-oxygen cycle figures into climate would be the Permian-Triassic mass extinction event (250 million years ago), in which a shutdown of the carbon dioxide cycle may have been responsible for the greatest mass extinction in Earth’s history. While the exact causes remain unknown, the mass extinction is thought to have begun with the eruption of the Siberian Traps, a massive set of volcanoes that released carbon dioxide and debris into the atmosphere.

The largest eruptions in known history probably caused a small period of global cooling, which stressed many environments. As a result, many trees died before they could process the carbon dioxide added to the atmosphere. Evidence seems to show that global warming set in around the peak of the extinction, and it is thought that the oceans became saturated with carbon dioxide to the point in which there was a degassing event. The seas released their carbon in huge clouds that swept over the land, suffocating all nearby creatures. Massive erosion suggests that the climate became arid, and the interiors of continents were more or less lifeless. While another event on this scale is unlikely, it provides a strong example of the phenomena that can arise from the disruption of the carbon-oxygen cycle.

Greenhouse Gas Emissions

More than 35 billion tons of carbon dioxide is released each year into the atmosphere. Compounding that are ongoing releases of other gases, such as methane, which has twenty-five times the effect of carbon dioxide (as a greenhouse gas) in a one-hundred-year period.

The largest sources of anthropogenic emissions are deforestation and the impact of industrial technology, including factories, cars, and cement making (which releases carbon dioxide). However, agriculture also releases greenhouse gases; for example, cows release 40 percent of anthropogenic methane.

Humans have always released carbon dioxide into the environment. Various Indigenous North American cultures burned forests annually to encourage habitats for large game animals. Also, much of Europe has been deforested for agricultural space and shipbuilding. However, the emissions from industrial and postindustrial technology are on a grander scale. Equally important is that the carbon being released now has not been part of the carbon-oxygen cycle since it was stored as coal or oil. In many cases, carbon dioxide released now was stored more than 250 million years ago in ancient swamps. Thus, with the combination of environmental disruption and greenhouse gas emission, modern activities are more akin to a sustained volcanic eruption than any other Earth event.

As the system moves further from homeostasis, the compensatory mechanisms, such as increased plant growth, are overloaded or not given a chance to function (as with heavy deforestation). This forms positive feedback, pushing the environment further from equilibrium. However, something as simple as planting additional trees can stall the harmful trend. An important point is that the carbon in a living tree has not been expelled; it remains stored as long as the tree lives. Once the tree dies, the carbon dioxide is released.

Carbon Sequestration

Carbon sequestration involves storing carbon in a stable state to negate carbon’s harmful environmental effects. As discussed, trees serve as reservoirs for carbon.

Many efforts focus on manipulating the carbon-oxygen cycle to increase the amount of carbon stored. The means of sequestering carbon range from genetically modifying plants to employing a more efficient form of photosynthesis to timing forest harvesting and replanting to ensure peak carbon storage, as in the Canadian lumber industry. Other forms of carbon sequestration under consideration include injecting carbon into coal seams, sinking carbon to the bottom of the ocean through algae growth, producing certain carbon-absorbing compounds, and conducting controlled burns that increase the health of an ecosystem, as in the Australian savanna. With proper manipulation of the carbon-oxygen cycle through methods, such as carbon sequestration, many of the worst effects of climate change can be mitigated.

Carbon sequestration methods, such as agricultural and ocean-based approaches, are biological. Scientists can also use geological techniques, such as employing saline aquifers or relying more heavily on graphene production for modern devices. However, researchers increasingly rely on emerging technologies in the fight against global climate change. Carbon Capture and Storage (CCS) captures carbon dioxide produced by industry, compresses it, and stores it in geological formations. Renewable energy sources are also used to power electrochemical cells that capture carbon dioxide. 

Principal Terms

biosphere: life on Earth and the area it inhabits

carbon sequestration: the process of storing carbon in a stable state to negate carbon’s effects on climate

combustion: reactions by which oxygen and organic materials become carbon dioxide and water

decomposition: the process by which organic matter is broken down into its most basic components by microorganisms

greenhouse effect: the process by which heat is trapped on Earth

homeostasis: property of a system to maintain a certain internal state, such as temperature

hydrosphere: water on Earth and its area

pedosphere: the soil

photosynthesis: a set of chemical reactions that use solar power to make sugars and oxygen from carbon dioxide and water

respiration: the process by which organisms break down organic material for energy

Bibliography

Baltz, James. “6 Ways to Remove Carbon Pollution from the Atmosphere.” World Resources Institute, 17 Mar. 2023, www.wri.org/insights/6-ways-remove-carbon-pollution-sky. Accessed 23 July 2024.

Bashkin, V. N., and Robert W. Howarth. Modern Biogeochemistry. New York: Kluwer Academic, 2002.

“Carbon Capture Technology and How it Works.” National Grid, 26 Mar. 2024, www.nationalgrid.com/stories/energy-explained/carbon-capture-technology-and-how-it-works. Accessed 23 July 2024.

Kirby, Richard R. Ocean Drifters: A Secret World Beneath the Waves. Winchester, England: Studio Cactus, 2010.

Kondratyev, K. Y., V. F. Krapivin, and Costas A. Varotsos. Global Carbon Cycle and Climate Change.

Le´vêque, C. Ecology from Ecosystem to Biosphere. Enfield, N.H.: Science, 2003.

“A New Way to Capture and Recycle Carbon Dioxide from Industrial Emissions.” American Chemical Society, 30 Aug. 2023, www.acs.org/pressroom/presspacs/2023/august/new-way-to-capture-and-recycle-carbon-dioxide-from-industrial-emissions.html. Accessed 23 July 2024.

Vallero, Daniel A. Environmental Biotechnology: A Biosystems Approach. Amsterdam: Academic, 2010.

Walker, Sharon, and David McMahon. Biochemistry Demystified. New York: McGraw-Hill, 2008.