Greenhouse effect occurs when trapping of heat by certain gases in the atmosphere of a planet, such as the earth, causing temperatures at the planet's surface and in the lower atmosphere to be higher than they otherwise would be. In the case of the earth, its atmosphere is largely transparent to visible-wavelength solar radiation, which reaches the earth's surface. Some of this radiation is reflected from the surface, and the rest is absorbed. Much of the absorbed radiation is reemitted at infrared wavelengths and is in turn largely absorbed by certain gases, dubbed greenhouse gases, present in the atmosphere. These gases reradiate the thermal radiation back toward the earth—hence the heating effect.
The greenhouse effect refers to the change in the steady state temperature of a planet or moon by the presence of an atmosphere containing gas that absorbs and emits infrared radiation. Greenhouse gasses are gases in an atmosphere that absorb and emit radiation within the thermal infrared range. In our solar system, the atmospheres of Venus, Mars and Titan also contain gases that cause greenhouse effects.Greenhouse gases, mainly water vapor, are essential to helping determine the temperature of the Earth; without them this planet would likely be so cold as to be uninhabitable. Although many factors such as the sun and the water cycle are responsible for the Earth's weather and energy balance, if all else was held equal and stable, the planet's average temperature should be considerably lower without greenhouse gases.
Human activities have an impact upon the levels of greenhouse gases in the atmosphere, which has other effects upon the system, with their own possible repercussions. The most recent assessment report compiled by the IPCC observed that "changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system", and concluded that...