The term greenhouse effect is used to describe the warming effect that certain gases have on the temperature of the earth's atmosphere under normal conditions. Once it strikes and warms the earth's surface, longwave radiation is given off and goes back into the atmosphere. While some of this longwave radiation or heat escapes into space, most of it is absorbed or held by carbon dioxide and other gases that exist in small quantities in the atmosphere. Human population growth and related industrial expansion, have led to greater air pollution and a change in the composition of the earth's atmosphere. Some pollutants enhance the natural greenhouse effect, resulting in increased global atmospheric temperatures.
Water vapour is the main greenhouse gas.
Human activities are not known to have had a significant influence on the atmospheric concentration of water vapour. This has occurred for two reasons. First, plants take up CO2 through the process of photosynthesis. The destruction of vegetation, as occurs in deforestation, reduces the amount of CO2 that is removed from the atmosphere. Second, when forests are cleared, and burnt or left to rot, CO2 is released.