HEAT ABSORPTION IN THE ATMOSPHERE
If the temperatures of Venus and the Earth were determined only by their distances from the sun, their average temperatures would be 100oC and -18oC, respectively. In the atmospheres of both these planets, there exist molecules which absorb heat. Since much of the solar energy is retained around the planet instead of radiating into space, the planets are warmer than they otherwise would be. Venus, whose atmosphere is 96% CO2, has an average temp of 450oC. The earth's atmosphere possesses water vapor and CO2 which absorb heat and its average temp is 15oC.

These gases allow light to penetrate through the atmosphere but, in absorbing heat, they raise earth's temperature. Because of this, these gases have been compared to the panes of a greenhouse roof and thus are often called "greenhouse gases". About eighty-four percent of the sun's energy is absorbed by molecules of the atmosphere rather than radiating directly into space-this is the "Greenhouse Effect".

Why do some molecules absorb heat? Each molecule has a specific shape as the atoms' electrons try to maximize the distance between each other. These atoms can vibrate in their positions if packets of electromagnetic energy can be absorbed and each chemical bond is prepared to absorb energy of only certain wavelengths. (The forms of electromagnetic energy, which include heat, radio waves, light, UV light, and X-rays, differ in the length of their waves.) If infrared energy (heat) passes through molecules of carbon dioxide, certain wavelengths of this infrared energy are absorbed rather than transmitted (see the chart below). It is observed that carbon dioxide (CO2) can absorb infrared wavelengths of 4 and 15 microns. In other words, carbon dioxide absorbs heat (American Chemical Society, 1994).

Other molecules that absorb infrared such as methane (CH4), N2O, and chlorofluorocarbons (CFCs) are also greenhouse gases. Molecules such as oxygen (O2) and nitrogen (N2) don't absorb heat and are not greenhouse gases.
What would happen if the amount of carbon dioxide in the atmosphere were to change? The concentration of carbon dioxide (CO2) in the atmosphere has not always been constant throughout earth's history, nor has the average temperature of the planet. The levels of CO2 in the early atmosphere were about 1000x what they were today. This higher concentration of CO2 was important for maintaining a temperature for the beginning of life since the sun, as a young star, produced less energy (25-30% less). These levels gradually receded as carbon dioxide dissolved in oceans and formed calcium carbonate which can then be incorporated into corals, sea shells, and eventually limestone. The ocean holds 20 times the amount of carbon as contained in all life on land, the terrestrial biomass. (There are an estimated 38,100 gigatons of carbon contained in the deep ocean and 1,020 in the surface ocean.)


Early in life's history, the activity of living things began to change the amount of carbon in the atmosphere. Cyanobacteria and later algae performed photosynthesis and incorporated atmospheric carbon into organic matter. Living things are made of molecules of carbon and their activity can affect the amount of carbon dioxide in the air.
CHEMICAL REACTIONS AND PHOTOSYNTHESIS
In chemical reactions, chemical bonds between atoms are broken and atoms are rearranged to form different chemical substances. Atoms are never lost in chemical reactions-there are the same number of atoms in both the reactants and the products.
6 CO2 + 6 H2O + energy -----> C6H12O6 + 6O2
In the above reaction, going from left to right, carbon dioxide and water can be converted to glucose (a sugar) and oxygen gas. There are just as many carbon, oxygen, and hydrogen atoms in the reactants on the left as there are in the products on the right. The above reaction depicts the process of photosynthesis: plants take carbon dioxide from the air and water from their roots to create biomolecules such as sugar. In the process, oxygen is released as a waste. In the 17th century, a researcher named Van Helmont put a 5 pound tree in 200 lb. of soil. After 5 years, the tree weighed 170 lb. and the soil had lost only a few ounces. "It's the water!" he exclaimed, being half right. This observation refuted the idea of Aristotle that all of a plant's matter came from the soil which had lasted almost 2000 years.
Glucose molecules (which, when joined to gather to make long chains, make up cellulose and starch) make up most of a plant's weight. Glucose is made of C, H, and O atoms (carbon, hydrogen, and oxygen); these atoms can be gotten from the rearranging of water and carbon dioxide molecules. Carbon fixation is the incorporation of inorganic C (from carbon dioxide) into organic molecules.

Of course, the sugars and other biomolecules of plants are important to animals as well. Whether it is a butterfly drinking nectar, a deer eating leaves, or a human eating French fries, animals can use these biomolecules as an energy source and as a source of the biomolecules needed for growth. For example, when people eat French fries (plant starch makes up about half of the American diet), our cells break down the glucose molecules in a set of reactions which accomplishes the inverse of what occurred in photosynthesis.
C6H12O6 + 6O2----------------->6 CO2 + 6 H2O + energy
Glucose and oxygen react to form carbon dioxide and water in a process known as cellular respiration. The carbon dioxide which you are exhaling right now came from the food you ate, which ultimately was produced by photosynthesis in plants. The energy released in the process is what we use to move, keep ourselves warm, and think. What happens to the biomolecules of a plant if an animal doesn't eat them? Typically, decomposers-organisms such as bacteria and fungi-perform cellular respiration to break this dead matter into carbon dioxide and water (as in the mushrooms decomposing the dead tree stump).

Carbon is continually recycled through what is known as the carbon cycle. Plants convert carbon dioxide into organic matter. When animals eat plants and perform respiration to obtain energy or when decomposers break down dead plants or animals, the carbon in organic matter is converted to carbon dioxide once again.