by John Arthur Harrison, Ph.D.
Carbon is the fourth most abundant element in the universe, and is absolutely essential to life on earth. In fact, carbon constitutes the very definition of life, as its presence or absence helps define whether a molecule is considered to be organic or inorganic. Every organism on Earth needs carbon either for structure, energy, or, as in the case of humans, for both. Discounting water, you are about half carbon. Additionally, carbon is found in forms as diverse as the gas carbon dioxide (CO2), and in solids like limestone (CaCO3), wood, plastic, diamonds, and graphite.
The movement of carbon, in its many forms, between the atmosphere, oceans, biosphere, and geosphere is described by the carbon cycle (Figure 1). This cycle consists of several storage pools of carbon (black text) and the processes by which the various pools exchange carbon (purple arrows and numbers). If more carbon enters a pool than leaves it, that pool is considered a net carbon sink. If more carbon leaves a pool than enters it, that pool is considered net carbon source.
Figure 1: A cartoon of the global carbon cycle. Pools (in black) are gigatons (1Gt = 1x109 Tons) of carbon, and fluxes (in purple) are Gt carbon per year. Illustration courtesy NASA Earth Science Enterprise.
The global carbon cycle, one of the major biogeochemical cycles, can be divided into geological and biological components. The geological carbon cycle operates on a time scale of millions of years, whereas the biological carbon cycle operates on a time scale of days to thousands of years.
The geological component of the carbon cycle is where it interacts with the rock cycle in the processes of weathering and dissolution, precipitation of minerals, burial and subduction, and volcanism (see our The Rock Cycle module for information). In the atmosphere, carbonic acid forms by a reaction with atmospheric carbon dioxide (CO2) and water. As this weakly acidic water reaches the earth as rain, it reacts with minerals at the earth’s surface, slowly dissolving them into their component ions through the process of chemical weathering. These component ions are carried in surface waters like streams and rivers eventually to the ocean, where they precipitate out as minerals like calcite (CaCO3). Through continued deposition and burial, this calcite sediment forms the rock called limestone.
This cycle continues as seafloor spreading pushes the seafloor under continental margins in the process of subduction. As seafloor carbon is pushed deeper into the earth by tectonic forces, it heats up, eventually melts, and can rise back up to the surface, where it is released as CO2 and returned to the atmosphere. This return to the atmosphere can occur violently through volcanic eruptions, or more gradually in seeps, vents, and CO2-rich hotsprings. Tectonic uplift can also expose previously buried limestone. One example of this occurs in the Himalayas where some of the world’s highest peaks are formed of material that was once at the bottom of the ocean. Weathering, subduction, and volcanism control atmospheric carbon dioxide concentrations over time periods of hundreds of millions of years.
Biology plays an important role in the movement of carbon between land, ocean, and atmosphere through the processes of photosynthesis and respiration. Virtually all multicellular life on Earth depends on the production of sugars from sunlight and carbon dioxide (photosynthesis) and the metabolic breakdown (respiration) of those sugars to produce the energy needed for movement, growth, and reproduction. Plants take in carbon dioxide (CO2) from the atmosphere during photosynthesis, and release CO2 back into the atmosphere during respiration through the following chemical reactions:
C6H12O6 (organic matter) + 6O2 6CO2 + 6 H2O + energy
energy (sunlight) + 6CO2 + H2O C6H12O6 + 6O2
Through photosynthesis, green plants use solar energy to turn atmospheric carbon dioxide into carbohydrates (sugars). Plants and animals use these carbohydrates (and other products derived from them) through a process called respiration, the reverse of photosynthesis. Respiration releases the energy contained in sugars for use in metabolism and changes carbohydrate “fuel” back into carbon dioxide, which is in turn released back to the atmosphere. The amount of carbon taken up by photosynthesis and released back to the atmosphere by respiration each year is about 1,000 times greater than the amount of carbon that moves through the geological cycle on an annual basis.
On land, the major exchange of carbon with the atmosphere results from photosynthesis and respiration. During daytime in the growing season, leaves absorb sunlight and take up carbon dioxide from the atmosphere. At the same time plants, animals, and soil microbes consume the carbon in organic matter and return carbon dioxide to the atmosphere. Photosynthesis stops at night when the sun cannot provide the driving energy for the reaction, though respiration continues. This kind of imbalance between these two processes is reflected in seasonal changes in the atmospheric CO2 concentrations. During winter in the northern hemisphere, photosynthesis ceases when many plants lose their leaves, but respiration continues. This condition leads to an increase in atmospheric CO2 concentrations during the northern hemisphere winter. With the onset of spring, however, photosynthesis resumes and atmospheric CO2 concentrations are reduced. This cycle is reflected in the monthly means (the light blue line) of atmospheric carbon dioxide concentrations shown in Figure 2.
Figure 2: The “Keeling curve,” a long-term record of atmospheric CO2 concentration measured at the Mauna Loa Observatory (Keeling et al.). Although the annual oscillations represent natural, seasonal variations, the long-term increase means that concentrations are higher than they have been in 400,000 years (see text and Figure 3). Graphic courtesy of NASA’s Earth Observatory.
In the oceans, phytoplankton (microscopic marine plants that form the base of the marine food chain) use carbon to make shells of calcium carbonate (CaCO3 ). The shells settle to the bottom of the ocean when phytoplankton die and are buried in the sediments. The shells of phytoplankton and other creatures can become compressed over time as they are buried and are often eventually transformed into limestone. Additionally, under certain geological conditions, organic matter can be buried and over time form deposits of the carbon-containing fuels coal and oil. It is the non-calcium containing organic matter that is transformed into fossil fuel. Both limestone formation and fossil fuel formation are biologically controlled processes and represent long-term sinks for atmospheric CO2.
Recently, scientists have studied both short- and long-term measurements of atmospheric CO2 levels. Charles Keeling, an oceanographer at the Scripps Institute of Oceanography, is responsible for creating the longest continuous record of atmospheric CO2 concentrations, taken at the Mauna Loa observatory in Hawaii. His data (now widely known as the “Keeling curve,” shown in Figure 2) revealed that human activities are significantly altering the natural carbon cycle. Since the onset of the industrial revolution about 150 years ago, human activities such as the burning of fossil fuels and deforestation have accelerated, and both have contributed to a long-term rise in atmospheric CO2. Burning oil and coal releases carbon into the atmosphere far more rapidly than it is being removed, and this imbalance causes atmospheric carbon dioxide concentrations to increase. In addition, by clearing forests, we reduce the ability of photosynthesis to remove CO2 from the atmosphere, also resulting in a net increase. Because of these human activities, atmospheric carbon dioxide concentrations are higher today than they have been over the last half-million years or longer.
Because CO2 increases the atmosphere’s ability to hold heat, it has been called a “greenhouse gas.” Scientists believe that the increase in CO2 is already causing important changes in the global climate. Many attribute the observed 0.6 degree C increase in global average temperature over the past century mainly to increases in atmospheric CO2. Without substantive changes in global patterns of fossil fuel consumption and deforestation, warming trends are likely to continue. The best scientific estimate is that global mean temperature will increase between 1.4 and 5.8 degrees C over the next century as a result of increases in atmospheric CO2 and other greenhouse gases. This kind of increase in global temperature would cause significant rise in average sea-level (0.09-0.88 meters), exposing low-lying coastal cities or cities located by tidal rivers such as New Orleans, Portland, Washington, and Philadelphia to increasingly frequent and severe floods. Glacial retreat and species range shifts are also likely to result from global warming, and it remains to be seen whether relatively immobile species such as trees can shift their ranges fast enough to keep pace with warming.
Even without the changes in climate, however, increased concentrations of CO2 could have an important impact on patterns of plant growth worldwide. Because some species of plants respond more favorably to increases in CO2 than others, scientists believe we may see pronounced shifts in plant species as a result of increasing atmospheric CO2 concentrations, even without any change in temperature. For example, under elevated CO2 conditions, shrubs are thought to respond more favorably than certain grass species due to their slightly different photosynthetic pathway. Because of this competitive inequality, some scientists have hypothesized that grasslands will be invaded by CO2-responsive grass species or shrubby species as CO2 increases.
Figure 3: CO2 over the past 140,000 years as seen in an ice core and in the modern Mauna Loa record. The red line represents predicted concentrations. Figure courtesy of : www.uoregon.edu/~dogsci/dorsey/ geo103/CO2.Vostok.jpg
In an attempt to understand whether recently observed changes in the global carbon cycle are a new phenomenon, or have instead occurred throughout geologic history, scientists have devoted considerable effort to developing methods for understanding Earth’s past atmosphere and climate. These techniques include analysis of gas bubbles trapped in ice, tree rings, and ocean and lake floor sediments for clues about past climates and atmospheres. Together, these techniques suggest that over the past 20 million years, the Earth’s climate has oscillated between relatively warm and relatively cold conditions called interglacial and glacial periods. During interglacial periods, atmospheric CO2 concentrations were relatively high, and during glacial periods, CO2 concentrations were relatively low. We are currently in an interglacial warm period, and human activities are pushing CO2 concentrations higher than they have been for hundreds of thousands of years (Figure 3).
Understanding and mitigating negative impacts of atmospheric CO2 enrichment constitute two of the most central challenges that environmental scientists and policy makers currently face. In order to address this issue, the scientific community has formed the Intergovernmental Panel on Climate Change (IPCC), an international, interdisciplinary consortium comprised of thousands of climate experts collaborating to produce consensus reports on climate change science. Many nations have agreed to conditions specified by the Kyoto Protocol, a multilateral treaty aimed at averting the negative impacts associated with human-induced climate change. The United States, which is currently responsible for approximately one quarter of global CO2 emissions, has so far declined to participate in the Kyoto Protocol.
John Arthur Harrison, Ph.D. "The Carbon Cycle: What Goes Around Comes Around," Visionlearning Vol. EAS-2 (3), 2003.