Greenhouse Effect:

The heating that occurs when gases such as carbon dioxide trap heat escaping from the Earth and radiate it back to the surface; so-called because the gases are transparent to sunlight but not to heat and thus act like the glass in a greenhouse.
OR
The overall warming of the earth's lower atmosphere primarily due to carbon dioxide and water vapour which permit the sun's rays to heat the earth, but then restrict some heat-energy from escaping back into space.

Greenhouse Gases:
i) Carbon dioxide CO2
ii) Methane
iii) Water Vapour
iv) Chloro Fluoro-Carbon (CFC)
v) Nitrous oxide

Mechanism OF Greenhouse Effects:
• Our Earth receives most of its energy, called radiation, from the Sun.
• This energy is electromagnetic radiation in the form of Visible light, with small amounts of Infrared (IR) and Ultraviolet (UV).
• The incoming Visible solar energy has a very short wavelength and passes right through the atmosphere.
• The Earth's surface absorbs the solar energy and releases it back to the atmosphere as Infrared (IR) radiation, some of which goes right back into space.
• But some of the IR radiation emitted by the Earth is absorbed by greenhouse gases in the atmosphere and sent back towards the Earth's surface.
• That warms the Earth's surface. Three main gases in our atmosphere that contribute to the greenhouse effect are carbon dioxide, methane, and water.
• These gases absorb the infrared radiation emitted by the Earth and re-radiate the energy as heat back towards the Earth, causing a warming known as the greenhouse effect.
• The warming due to greenhouse gases is expected to increase as humans add more greenhouse gases to the atmosphere.

Sources Of Greenhouse Gases:
Carbon dioxide (CO2) is a colourless gas that's a by-product of the combustion of organic matter. Today human activities are pumping huge amounts of CO2 into the atmosphere, resulting in an overall increase in carbon dioxide concentrations.

Methane occurs naturally when organic material decomposes. Man-made processes produce methane in several ways:
• By extracting it from coal
• From large herds of livestock (i.e., digestive gases)
• From the bacteria in rice paddies
• Decomposition of garbage in landfills

Importance Of Greenhouse Effect:
Following is the importance of greenhouse effect.
i) Without greenhouse effect, the Earth would not be warm enough for humans to live.
ii) Without greenhouse gases, heat would escape back into space and Earth’s average temperature would be about 600 F colder.