Greenhouse gases cause the Greenhouse effect, which is a naturally occurring phenomenon, responsible for the heating of the earth's surface as well as atmosphere. Major greenhouse gases are carbon dioxide and methane. Greenhouse gases allow short wavelength radiations (like UV radiations) to come in but do not allow long-wave radiations to go out, so they absorb long-wave (infrared) radiations from the earth and emit it again towards the earth. This downward flux of radiation is called "greenhouse flux". This cycle continues till there are no long-wave radiations left to emit from the earth's surface. If there were no greenhouse effect, the Earth’s average surface temperature would be a very chilly -18°C( 0°F) instead of the comfortable 15°C ( 59°F) that it is today.