In order to understand the magnitude and potential impacts of global warming, it is important to discern whether gases are coming from natural or anthropogenic sources. The natural sources are the ones that are continuously being recycled through our atmosphere through biogeochemical cycles that operated for millions of years before humans. These processes include photosynthesis, respiration, volcanism, and a host of others. In this section, we are more concerned about how humans are affecting these cycles.

Many scientists feel one of the main reasons for global warming is the addition of carbon dioxide (CO2) to the atmosphere. Even though CO2 is actually only a small percentage of the overall composition of the atmosphere (about 0.035%), its physical properties make it a strong absorber of infrared energy. The concentration of CO2 has been increasing primarily due to the burning of fossil fuels and deforestation. Current computer models predict that doubling the amount of CO2 in the atmosphere will cause Earth’s average surface temperature to rise between 1 to 3.5 degrees Celsius (2 to 6 degrees Fahrenheit) over the next 100 years. This might not sound like a great increase, but Earth's climate is very sensitive to even small changes in surface temperature (see Glaciation Cycles).

Besides CO2, other gases introduced into the environment by humans are beginning to cause concern. Methane (CH4) is derived from anerobic bacteria in rice fields, cow flatulence, and sewage. Nitrous oxide is induced from fossil fuels and fertilizer. Finally, CFCs are generated by refrigerants and propellants. Water vapor is also a greenhouse gas, but there is much less that humans can do to control the amount in the atmosphere since it is so variable.

Above, the chart delineates the relative concentrations of gases produced from human activity. Note that water vapor is not included, even though it is a major greenhouse gas.