 There are two common meanings of the term "greenhouse effect". There is a "natural" greenhouse effect that keeps the Earth's climate warm and habitable.  There is also the "man-made" greenhouse effect, which is the enhancement of Earth's natural greenhouse effect by the addition of greenhouse gases from the burning of fossil fuels (mainly petroleum, coal, and natural gas). Greenhouse Effect

 In order to understand how the greenhouse effect operates, we need to understand "infrared radiation".  Greenhouse gases reduce the rate at which the Earth's surface loses infrared radiation to outer space. Because one way to increase the temperature of anything is to reduce its rate of energy loss to its surroundings, this makes the Earth's surface and lower atmosphere warmer than they would otherwise be.  You can think of greenhouse gases as sort of a "blanket" for infrared radiation -- they keep the Earth's surface and lower layers of the atmosphere warmer and the upper layers colder, than if the greenhouse gases were not there.

 About 80-90% of the Earth's natural greenhouse effect is due to water vapor and clouds.  Most of the rest is due to carbon dioxide, methane, and a few other minor gases. While the remaining gases in the atmosphere (e.g. nitrogen, oxygen) also absorb and emit a small amount of infrared radiation, their radiate effect on temperature is so weak that they can be neglected.  While methane is a much more potent greenhouse gas than carbon dioxide, there is far less of it in the atmosphere.

 Infrared (IR) radiation is just as important to the Earth's weather and climate as sunlight is. This is because, for all of the sunlight that the Earth absorbs, an equal amount of IR radiation must travel from the Earth back to outer space.  If this was not the case, there would be global warming or global cooling. Thus, just as the temperature of a pot of water on the stove is tied to any imbalance between the rate at which the pot receives energy from the stove versus the rate at which the pot loses energy to its surroundings, global warming or global cooling is caused by an imbalance between absorbed sunlight and emitted infrared radiation. What is infrared radiation (heat radiation)?

 Everything emits infrared radiation, although some solid materials are not totally efficient emitters (their infrared emissivity is less than 1.0) and most gases are mostly transparent to infrared radiation.  But even though everything emits IR energy, we usually do not notice it because it is often weak, and we can't see infrared light like you can sunlight. It takes a very hot object for us to feel IR energy, such as a hot stove, a fire, or a brick wall heated by the sun.

 Greenhouse gases in the atmosphere, especially water vapor, "trap" (absorb and emit) some of this infrared radiation, and keep the earth habitably warm. Clouds also produce their own greenhouse effect.  The reason why the air cools so quickly on a clear, dry evening is because the lack of humidity and clouds allows large amounts of IR radiation to escape rapidly to outer space as it is emitted upward by the ground and other surfaces.

 Since there is considerable misunderstanding and misconceptions regarding the greenhouse effect, it is useful to list a few of the things the greenhouse effect is not:  1) The greenhouse effect does not operate like a greenhouse that plants are grown in. Plant greenhouses stay warm because they are enclosed, preventing warm air from escaping. In the open atmosphere, warm air that builds up at the surface rises ("convects") and mixes with air from higher altitudes, limiting warming near the surface. What the Greenhouse Effect Isn't

 2) The greenhouse effect does not require solar radiation (sunlight) to operate. The greenhouse effect would still exist if there was no sun, and the climate system was instead warmed from below by geothermal energy. 3) The greenhouse effect cannot be demonstrated with a jar or other enclosure because there is too little greenhouse gas involved. Thousands of feet of atmospheric depth are required for the greenhouse effect to have a measurable effect on temperature.

 The greenhouse effect is entirely due to the fact that the atmosphere absorbs and emits infrared energy, combined with a heat source to warm the bottom of the atmosphere (in our case, the Sun) and the cold depths of outer space above the top of the atmosphere.  The greenhouse gases (and clouds) reduce the ability of the Earth's surface to cool, thus raising its temperature above what it would be without those greenhouse gases.

 The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in the atmosphere warm a planet's lower atmosphere and surface.  It was proposed by Joseph Fourier in 1824, discovered in 1860 by John Tyndall and was first investigated quantitatively by Svante Arrhenius in 1896.Joseph FourierJohn Tyndall Svante Arrhenius History of Greenhouse gases

 By their percentage contribution to the greenhouse effect on Earth the four major gases are  water vapor, 36–70% water vapor  carbon dioxide, 9–26% carbon dioxide  methane, 4–9% methane  ozone, 3–7% ozone  The major non-gas contributor to the Earth's greenhouse effect, clouds, also absorb and emit infrared radiation and thus have an effect on radiative properties of the atmosphere.clouds Contribution of greenhouse gases

 The "greenhouse effect" of the atmosphere is named by analogy to greenhouses which get warmer in sunlight, but the mechanism by which the atmosphere retains heat is different.  A greenhouse works primarily by preventing absorbed heat from leaving the structure through convection, i.e. sensible heat transport. The greenhouse effect heats the earth because greenhouse gases absorb outgoing radiative energy and re-emit some of it back towards earth.

 A greenhouse is built of any material that passes sunlight, usually glass, or plastic. It mainly heats up because the Sun warms the ground inside, which then warms the air in the greenhouse.  The air continues to heat because it is confined within the greenhouse, unlike the environment outside the greenhouse where warm air near the surface rises and mixes with cooler air aloft. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature will drop considerably.

Real greenhouses Fig: A modern Greenhouse in RHS WisleyGreenhouseRHS Wisley

 It has also been demonstrated experimentally (R. W. Wood, 1909) that a "greenhouse" with a cover of rock salt (which is transparent to infrared) heats up an enclosure similarly to one with a glass cover. Thus greenhouses work primarily by preventing convective cooling.R. W. Wood