Greenhouse gases are those gases in the atmosphere that have an influence on the earth's energy balance. They cause the so-called greenhouse effect. The best known greenhouse gases, carbon dioxide (CO₂), methane and nitrous oxide, can be found naturally in low concentrations in the atmosphere. However, the proportion has increased significantly since the beginning of the last century due to various man-made sources. In addition to these trace gases, which can only be found in very low concentrations in the atmosphere, water vapour is probably the most important greenhouse gas. However, it only plays a major role in relation to the natural greenhouse effect, since the absorption capacity of water vapour in the air is directly related to temperature. Water vapour therefore has little influence on man-made climate change.