Carbon Dioxide in Earth's Atmosphere - Sources of Carbon Dioxide

Sources of Carbon Dioxide

Natural sources of atmospheric carbon dioxide include volcanic outgassing, the combustion of organic matter, wildfires and the respiration processes of living aerobic organisms; man-made sources of carbon dioxide include the burning of fossil fuels for heating, power generation and transport, as well as some industrial processes such as cement making. It is also produced by various microorganisms from fermentation and cellular respiration. Plants convert carbon dioxide to carbohydrates during a process called photosynthesis. They gain the energy needed for this reaction through the absorption of sunlight by pigments such as chlorophyll. The resulting gas, oxygen, is released into the atmosphere by plants, which is subsequently used for respiration by heterotrophic organisms and other plants, forming a cycle.

Most sources of CO₂ emissions are natural, and are balanced to various degrees by natural CO₂ sinks. For example, the natural decay of organic material in forests and grasslands and the action of forest fires results in the release of about 439 gigatonnes of carbon dioxide every year, while new growth entirely counteracts this effect, absorbing 450 gigatonnes per year. Although the initial carbon dioxide in the atmosphere of the young Earth was produced by volcanic activity, modern volcanic activity releases only 130 to 230 megatonnes of carbon dioxide each year, which is less than 1% of the amount released by human activities (at approximately 29,000 megatonnes). These natural sources are nearly balanced by natural sinks, physical and biological processes which remove carbon dioxide from the atmosphere. For example, some is directly removed from the atmosphere by land plants for photosynthesis and it is soluble in water forming carbonic acid. There is a large natural flux of CO₂ into and out of the biosphere and oceans. In the pre-industrial era these fluxes were largely in balance. Currently about 57% of human-emitted CO₂ is removed by the biosphere and oceans. The ratio of the increase in atmospheric CO₂ to emitted CO₂ is known as the airborne fraction (Keeling et al., 1995); this varies for short-term averages but is typically about 45% over longer (5 year) periods. Estimated carbon in global terrestrial vegetation increased from approximately 740 billion tons in 1910 to 780 billion tons in 1990.