Atmospheric gases that absorb infrared radiation and re-emit it back to the Earth’s surface, causing the greenhouse effect. On Earth, the most important greenhouse gases are water vapour, methane, carbon dioxide and nitrous oxide. The water vapour content in the atmosphere is quite variable and water vapour has the strongest greenhouse effect. It is produced by the respiration of all living organisms and evaporation from water surfaces. The second most potent greenhouse gas is methane, which is relatively low in the atmosphere. It is produced by a number of metabolic processes of animals and especially bacteria. Carbon dioxide is produced by respiration, volcanic eruptions and the burning of biomass or fossil fuels and makes up only about 0.04% of the atmosphere. Tectonic processes, changes in the biosphere or human activity can affect the composition and concentration of greenhouse gases.