Carbon dioxide, methane and ozone are the major greenhouse gases that are effected by human activities. While water vapor accounts for the majority of the greenhouse effect, it is a naturally occurring gas. Other man influenced gases, such as nitrous oxide and the fluorocarbons play a lessor role. The effect of each of the gases is difficult to quantify as their effect is not additive, and often overlap or interact and vary with atmospheric conditions. Because of it's production as a byproduct of the combustion of fossil fuels, man activities are directly responsible for the increased levels of carbon dioxide. But methane is also a potent greenhouse gas whose atmospheric concentrations are directly influenced by man.

Both carbon dioxide and methane are naturally occurring gases, constantly circulating through the ecosystem. In addition to their disturbution throughout the atmosphere, these gases interact with the soil, vegetation, rocks, and water where they may be absorbed as a gas, or undergo chemical reactions to form new carbon-based compounds. Once again it is difficult to precisely quantify the flux of these gases due to their world-wide distribution and variability.

Figure 1. illustrates the annual movement of tens of billions of carbon dioxide between the atmosphere, water, and soil which are the major reservoirs, or "sinks" of CO_{2 -} and under other conditions, a "source" for carbon dioxide.

Figure 1. The Carbon Cycle (NASA Earth Observatory)

In addition to the atmosphere, the oceans act as a major sink for carbon dioxide with cold water absorbing more CO₂ than warm water. Just as a warming carbonated beverage releases bubbles of CO₂, so will warming ocean temperatures. Absorbed CO₂ removed from the ocean (and other waters) by the photosynthesis of algae and other aquatic vegetation, and then returned to the system as the plants are consumed by animals. Dissolved carbon dioxide also chemically reacts with water to form carbonic acid (CO₂ + H₂0 = H₂CO₃) which acidifies the ecosystem, and is thought to be responsible for coral bleaching; and, presents a major threat to other marine life.

In terrestrial ecosystems, carbon is the "backbone" of biological cycles and systems, converted to sugars by the photosynthesis of plants, and either respired by plants and animals, or temporarily stored as wood or other cellulose compounds.

Man has added carbon dioxide to the atmosphere with the combustion of fossil fuels, open fires, deforestation, and the production of cement from limestone (CaCO₃) being major sources. Figure 1. is by no means complete, nor is our knowledge of carbon sinks and sources. For example, fires are burning in coal deposits around the world, some have been started by man, others were started by natural forces. These exceedingly difficult to extinguish fires⁽¹⁾, may prove to be one major sources of atmospheric carbon dioxide.

1.) Bhattacharya, Shaoni. Feb.14, 2003. Wild coal fires are a 'global catastrophe'. New Scientist.