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Carbon Dioxide: Overview
Compounds that contain the element carbon are referred to as "organic." These compounds are very important. They are present in all living things. The carbon element is continually moving among the earth's lithosphere, hydrosphere, biosphere, and atmosphere in various forms--as carbon dioxide (CO2), sugars or carbohydrates (CnH2nOn), and and calcium carbonate (CaCO3), to name just a few. The movement of carbon among the earth's spheres, as diagrammed below, is known as the carbon cycle.

Image of a diagram which shows the carbon cycle.  This image links to a more detailed image.The boxes in the diagram indicate carbon sources (places of origin) and carbon sinks (places of storage). The arrows indicate the movement of the element among the earth's spheres. (Click on image at left for a larger view.)

Green plants play a very important role in the carbon cycle. They absorb carbon dioxide (CO2) from the atmosphere and produce carbon-containing sugars. This process is called photosynthesis. There are two main steps in photosynthesis. First, plants trap the sun's light energy in a compound called chlorophyll. This energy is converted to a chemical form called adenosine triphosphate (ATP). In the second step, plants use the energy from ATP to produce sugar (C6H1206). The process of photosynthesis requires water (H2O). It also produces water, as well as oxygen (O2). The net chemical reaction for the process of photosynthesis is:
 6 CO2 + 6 H2O + sunlight ------> C6H12O6 + 6 O2.

Animals eat plants to obtain the energy trapped during photosynthesis. As the animals' bodies break down the carbohydrates in the plant tissue, CO2 is released to the atmosphere. This process is called respiration. The net chemical reaction for the process of respiration is the exact opposite of photosynthesis: 6 O2 + C6H12O6 ------> 6 H2O + 6 CO2. Plants, too, respire as they break down the organic molecules in themselves in order to release the stored energy. Plants and animals also release CO2 to the atmosphere when they decompose or decay. The chemical reaction for this process is the same as that for respiration.

When dead plants and animals slowly decay under high pressure and high temperatures, they may form pools of energy known as fossil fuels. Fossil fuels include coal, oil, and natural gas. People burn fossil fuels, as well as fresh vegetation, to release the energy stored in them. The energy is used for heat, operating automobiles, etc. The chemical process of burning fuel--known as combustion--is the same as respiration and decomposition: 
 6 O2 + C6H12O6 ------> 6 H2O + 6 CO2

Since the Industrial Revolution, humans have burned increasingly greater amounts of fossil fuels in order to produce more energy. As the practice of burning fossil fuels grows, so does the amount of carbon dioxide emitted to the atmosphere. Historical data from ice cores and modern data collected from the Mauna Loa observatory in Hawaii support this. The graph below depicts a 30% increase in atmospheric concentrations of carbon dioxide (CO2) since 1860.

Image of a graph showing atmospheric concentrations of carbon dioxide from 1855 to 1996.

Image: Atmospheric concentrations of carbon dioxide from 1855 to 1996. Figure adapted from an image courtesy of the Whitehouse Initiative on Global Climate Change.

There is growing evidence that increases in atmospheric concentrations of CO2 may increase the rate of global climate change due to the greenhouse effect. This is because CO2 contributes to 55% of the greenhouse effect. Increases in atmospheric concentrations of CO2 may also have great impacts on plant growth by affecting rates of photosynthesis.

 

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