Slide 5 of 30
Notes:
Carbon dioxide (CO2) is a waste product of most forms of life. Sugars and other organic (from living or dead organisms) compounds are oxidized (burned) to release energy, and the resulting CO2 and water (H2O) are released (respired) to the environment. In photosynthesis, plants utilize 6 molecules of CO2 , in conjunction with 6 molecules of water (H2O) and the energy from sunlight, to create one molecule of sucrose (C6H12O6) and 6 of molecular oxygen (O2). Sucrose is the primary fuel used by plants to build their tissues and power their everyday activities. Photosynthesis is thus the opposite process to respiration, and is entirely responsible for the presence of oxygen in the atmosphere, upon which we absolutely depend for our existence.
As part of living, plants also respire some of the sucrose they have created by photosynthesis. Most, however, is converted to wood or leaves and stored for a while. When the plant dies an army of insects, fungi, and bacteria consume this material, extracting energy from it and respiring the waste as carbon dioxide and water. We see this as decay. Thus, over a typical lifetime for a tree or a grass the two processes come into approximate balance, and the net effect on both atmospheric carbon dioxide and oxygen is about zero. Measurements from ice cores over the 1000 years prior to 1750 do indeed show approximate constancy.
A small fraction of the carbon cycling through plant tissues is converted to form which do not further decay in the soil. Coal and oil are examples of such residues from past ages. In addition, small plants in the ocean (phytoplankton) build skeletons of calcium carbonate (Ca(CO3)2), which may become trapped in sediments as limestone. Over hundreds of millions of years these small residuals have removed carbon from the biosphere (where things live, i.e. the land surface - ocean - atmosphere), creating a dynamic balance with inputs of carbon from the earth interior through volcanoes and the weathering of rocks. This natural balance is, however, extremely slow acting, and is not relevant to the system response over the next few hundred years to the geologically instantaneous conversion of fossil carbon into carbon dioxide.
When organic material decays in an environment without oxygen (typically in soil or sediments that are saturated with water) respiration is inhibited, and methane (CH4) is generated instead of carbon dioxide. This process yields less energy until the methane itself can be oxidized so it is not normally preferred, but it leads to many of the special properties of wetlands.