The Global Carbon Cycle

By Jessica Verschay

1,000,000,000,000 of the atoms in your body were once part of Plato. Some others are from Cleopatra, while still more may have been part of George Washington. In fact, the very air you are breathing contains carbon atoms that were once exhaled by virtually everyone in history! The same carbon atoms that moved through Charles the Great, Leonardo Da Vinci, Mary Queen of Scots, and even Jesus Christ are a part of you! With every breath you breathe, you are connected physically to history. Centuries from now, the air you are exhaling at this very moment will be a part of forests, prairies, oceans, and even future generations. However, as Francois Morel writes in Principles and Applications of Aquatic Chemistry, “Behind the vertigo of multiple reincarnations and the comforting thought of all the great people each of us has been lies the profound and fundamental principle of conservation of mass” (p1). The law of conservation of mass states that atoms are neither created nor destroyed throughout all of their physical and chemical transformations. This accounts for the fact that all people who have ever lived are a part of each other. Specifically, carbon atoms are transformed and transferred over the entire earth, including the air, water, and all organisms in the global carbon cycle. Since this cycle is such an integral part of human life and the world around us, it is important to understand how it works, including the chemistry involved in it. Without the proper functioning of the carbon cycle, every aspect of life would be different. It is important to understand how it works in order to understand the danger of it not working.

The global carbon cycle is defined as the cycle of carbon usage by which energy flows through the earth’s ecosystem, or the exchange of carbon between living organisms and the nonliving environment. This cycle has two main aspects: (1) Distribution of the element among the various reservoirs and (2) The flux, or transfer, of the element between reservoirs. Carbon is stored in our planet in the following major sinks: (1) as organic molecules in living and dead organisms found in the biosphere; (2) as the gas carbon dioxide in the atmosphere; (3) as organic matter in soils; (4) in the lithosphere as fossil fuels and sedimentary rock deposits such as limestone (different forms, all composed of carbonate minerals) and dolomite (CaMg(CO₃)₂); (5) in the oceans as dissolved atmospheric carbon dioxide and as calcium carbonate shells in marine organisms and fossils. The distribution of carbon among the reservoirs is of great concern because an imbalance can have severe consequences. For example, the atmospheric carbon dioxide traps energy radiating from the earth’s surface. The recent carbon dioxide increases in this reservoir are causing the so-called greenhouse effect, which may lead to serious consequences for the entire planet.

The total global carbon pool is about 49,000 metric gigatons ( I metric gigaton equals 1x10⁹ metric tons). The oceans account for the majority (71%) of the world’s carbon, mostly in the form of bicarbonate and carbonate ions. Fossil carbon makes up about 22%, while dead organic material and phytoplankton add 3%. Terrestrial ecosystems account for 3% more, and the remaining 1% is held in the atmosphere. In a year, about 25% of the atmospheric carbon pool is exchanged with the terrestrial biosphere and the oceans. The concentration of carbon in the atmosphere is the most important, and the most varying. The balance of processes that consume or produce carbon dioxide determines this concentration. These processes include photosynthesis, respiration, decomposition, and air-water exchange. Unfortunately, humans are upsetting nature’s delicate balance of these cycles by activities such as the burning of fossil fuels and deforestation.

The two main exchanges within the global carbon cycle are between the ecosystem and the atmosphere, and between the hydrosphere and the atmosphere. The basic cycle in the ecosystem begins when photosynthesizing plants convert carbon dioxide from the atmosphere or water into carbon-based sugar molecules. These molecules are then chemically modified and used by the organisms. The carbon is either returned to the atmosphere or water by aerobic respiration or is passed on to herbivores that eat the plants and thereby use the organic compounds (compounds which contain carbon). More of this carbon is given off by aerobic respiration and the rest is passed on to the carnivores that eat the herbivores. Eventually, all of the carbon compounds are broken down by decomposition and the carbon is released as carbon dioxide to be used again by plants.

In order to understand the carbon cycle, it is imperative to understand the chemistry involved in the processes of photosynthesis and respiration. The overall chemical equation for photosynthesis is 6CO₂ + 12H₂O + energy (from solar radiation) à C₆H₁₂O₆ + 6O₂ +6H₂O. This may appear to be simple, but the actual chemistry is much more complex. At the beginning of photosynthesis, photons, or rays of sunlight, strike the chlorophyll molecule, causing them to absorb light energy. As a response to this addition of energy (signifying an endothermic reaction), their electrons jump to a higher energy orbit. These high-energy electrons are then, with the help of a chlorophyll-enzyme combination, picked up by carrier molecules. As the electrons are removed, they are replaced by electrons from water with the aid of a water-splitting enzyme. The electrons riding on the carrier molecules are attracted to the positive hydrogen ion (opposite charges attract), forming a hydrogen atom. These hydrogen atoms then move through a series of channels of the ATP (adenosine triphosphate) producing enzyme, ultimately producing ATP molecules, which the plant uses for energy. The unused electrons, which didn’t yet bond with hydrogen ions, are placed on NADP, another carrier molecule. These enter the Calvin Cycle, in which five different enzymes work to produce the much-needed sugars. In summary, photosynthesis takes energy, water, and carbon dioxide from the surrounding air, water, and sunlight to produce glucose molecules (needed for cellular respiration) and oxygen (which other organisms, such as humans, breathe).

Photosynthesis reduces more than 10% of the carbon dioxide in the air into carbohydrates single-handedly. However, as the carbon dioxide levels in the atmosphere continue to rise dramatically, there is justifiable concern that the levels will not be able to be reduced by photosynthesis. This could change the entire carbon cycle by destroying the equilibrium between the production and destruction of carbon. As Le Chatelier’s Principle states, when a stress (in this case, increased carbon levels) is applied to a system in dynamic equilibrium (the carbon cycle), the equilibrium tends to adjust to minimize the effect of the stress. Any “adjustments” in the carbon cycle will have serious, far-reaching consequences.

The overall equation for respiration is the opposite of photosynthesis. It is C₆H₁₂O₆ + 6O₂ + 6 H₂O ---> 6CO₂ + 12H₂O + ATP (energy). Respiration is the complex process by which oxygen is taken from the air and transported to cells for the oxidation (a reaction in which an atom, ion, or molecule loses an electron) of organic molecules. The products of oxidation, carbon dioxide and water, are then returned to the environment. In other words, respiration is the series of biochemical oxidations in which organic molecules, such as carbohydrates, amino acids, and fatty acids (eventual products of photosynthesis) are converted to carbon dioxide and water. Respiration is an extremely complex process, involving both glycolysis and the Krebs cycle. Glycolysis, the anaerobic portion of respiration, has a net gain of 2 ATP molecules and 2 NADH (electron carrier) molecules. The next step, which begins the aerobic portion of respiration, is the oxidation of pyruvic acid (produced in glycolysis). One of the three carbons of pyruvate splits apart and becomes a part of CO₂. This is known as decarboxylation, where a carbon is attached to oxygen. It leaves behind acetic acid, which couples with a carrier molecule called coenzyme A to form acetyl coenzyme A (acetyl CoA), which is necessary for respiration to continue. The Krebs cycle then begins with 4-carbon oxaloacetate and combines with Acetyl CoA to cycle through one complete turn. After the Acetyl CoA is oxidized to CO₂ and H₂O, the electrons drive proton pumps that generate the ATP, which is the goal of respiration. Respiration releases carbon back into the atmosphere and also passes some on to the carnivores that feed on the herbivores. Eventually, all of the carbon compounds are broken down by decomposition, and the carbon dioxide is released as CO₂ to be used again by plants; the cycle starts all over again.

The second important exchange within the carbon cycle is between two other great reservoirs: the atmosphere and the earth’s waters. Carbon dioxide from the atmosphere enters the water by diffusion across the air-water surface. If the CO₂ concentration within the water is less than that in the atmosphere, it diffuses into the water. Conversely, if the CO₂ concentration is greater in the water than in the atmosphere, CO₂ enters the atmosphere. The atmospheric partial pressure of CO₂ and the equilibrium partial pressure of CO₂ in the surface waters drive the rate of exchange with carbon. Therefore, the rate of exchange between the atmosphere and the oceans is dictated largely by physics (differences in the partial pressure of CO₂ in the surface waters, the temperatures of the ocean and atmosphere, and wind speeds). Once dissolved in the water, the carbon dioxide can remain as it is, or it can combine with water to form carbonate (CO₃^-2 from the reaction CO₂ + H₂O ¬® CO₃^-2 + 2H⁺) or bicarbonate (HCO₃^- from CO2 + H2O ¬® HCO₃^- + H⁺). Some forms of sea life, such as coral, clams, oysters, protozoa, or algae, combine bicarbonate with calcium cations to produce calcium carbonate (CaCO₃), which is used to construct shells and other body parts. When these organisms die, their shells fall to the ocean floor where they build up as carbonate-rich deposits. Over long periods of time, these deposits, along with carbonates that have precipitated out of the water, are altered to become sedimentary rocks and fossils. In addition, carbon is incorporated into the forest-vegetation biomass (living matter) and may stay out of circulation in the carbon cycle for many years. Incomplete decomposition of some of this organic matter, and also some of the terrestrial biomass, results in the accumulation of peat. Such accumulation during the Carboniferous period created many of the great stores of fossil fuels that run our lives today. The concentration of carbon in the water also changes with the processes of respiration and photosynthesis as described for the ecosystem. This, in turn, affects the rate of diffusion of CO₂ across the air-water surface.

Another carbon reservoir is the lithosphere, the solid, inorganic portion of the earth. The lithosphere stores carbon in both inorganic and organic forms. Inorganic deposits include fossil fuels such as coal, oil, and natural gas, oil shale, and carbonate-based sedimentary deposits. The organic forms are comprised of substances like litter, organic matter, and humic substances found in soils. Carbon dioxide is released from the interior of the lithosphere by volcanoes. However, the carbon stored in the lithosphere is not circulated as widely, or as frequently, as the carbon stored in some of the other reservoirs.

In addition to the natural processes involved in the carbon cycle, the activities of humans have a considerable impact on this global cycle. Our impact has been increasing dramatically ever since the start of the Industrial Revolution. Atmospheric concentrations have climbed from 275 parts per million (ppm) in the early 1700s to over 365 ppm today. It is estimated that future levels could reach 450 to 600 ppm by the year 2100. This is a source of global concern because atmospheric CO₂ acts as a shield over the earth. This shield is penetrated by the short-wave radiation from outer space while blocking the escape of long-wave radiation. The shield thickens as more carbon dioxide is added to the atmosphere and more heat is retained. This increases global temperatures, resulting in the so-called greenhouse effect. These increases have not yet been significant enough to nullify natural climatic variability. However, the projected increases in CO₂ imply that global temperatures could rise about 2° to 6° C (4° to 11° F) within the next hundred years. This increase would be sizeable enough to change global climates, disrupt the carbon cycle and ultimately affect human welfare. The major sources of atmospheric carbon dioxide due to human activities are the burning of fossil fuels and the modification of biomass found in grassland, woodland, and forested ecosystems. Fossil fuel emissions account for about 65% of the additional CO₂ currently in the atmosphere. The remaining 35% comes from deforestation and the conversion of natural ecosystems for agricultural use (MSN, Web). It has been demonstrated that natural ecosystems have the ability to handle between 20 to 100 times more carbon dioxide than agricultural land-use types. Therefore, humans have a much greater impact on nature’s cycles than we would sometimes like to admit. The steady increase of atmospheric carbon is something that must be studied and monitored before it is too late to reverse the damage we have done, are doing, and will continue to do in the future.

Carbon’s significant role in potential climate change is of both national and international concern. The United States National Oceanic and Atmospheric Administration recently announced the implementation of the Carbon Cycle Science Initiative. As Jessica Gorman writes in “Science News,” “ It aims to coordinate funding and research to determine how the element cycles through the land, water and atmosphere.” Intelligent decisions about our planet’s future must be based upon the best possible scientific knowledge on the carbon cycle’s function. On a global scale, the United Nation’s Framework Convention on Climate Change was held to discuss the vital issue of the effects of global warming, which is caused by the carbon cycle. The ultimate objective of the UN is to “achieve…stabilization of greenhouse gas concentrations in the atmosphere…within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner” (UN, Web).

The carbon cycle is crucial to life on earth. It is a complex process that is entwined with and affected by almost every aspect of life on earth. The understanding of the processes, both chemical and physical, involved in this cycle is imperative to knowing how our world works and how to handle the countless changes and problems that we will face in the years to come.

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