These changes in solar energy are the result of periodic variation in the Earth s orbit and the tilt of the Earth s axis. Changes in the concentrations of heat-trapping gases, known as the greenhouse gases , also appear to have played an important role. Scientific studies have shown a very close relationship between global temperature changes and atmospheric concentrations of these gases during the past 160,000 years. Most greenhouse gases occur naturally. However, modern industry and lifestyles have led to new sources of greenhouse gases, as well as to the emission of entirely new greenhouse gases.
Water vapour comes from natural respiration, transpiration, and evaporation.
The amount of water vapour stored in the atmosphere increases as the Earth s surface temperature rises. Carbon dioxide comes from the decay of materials, respiration of plant and animal life, and the natural and human-induced combustion of materials and fuels. It is removed from the atmosphere through photosynthesis and ocean absorption. Of the greenhouse gases, carbon dioxide is second only to water vapour in volume. Although there is less methane than carbon dioxide in the atmosphere, methane is a more effective heat-trapping greenhouse gas.