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The Earth’s atmosphere is made largely of nitrogen (78%) and oxygen (21%). Most of the remaining 1% is argon. These gases are transparent to sunlight, which passes through them and heats the Earth’s surface. The warmed land and oceans in turn heat the lower atmosphere. Some of this heat is radiated back into space. If this were the whole story, the average temperature of the Earth would be -18°C (-0.4°F) instead of the 15°C (59°F) that it is. The reason for the extra warmth is that there are gases in the atmosphere that absorb energy before it is lost into space and then slowly release it back into the atmosphere. The gases that are responsible for this “greenhouse effect” are called greenhouse gases.

Since 99.9% of the atmosphere is made of nitrogen, oxygen, and argon, there is very little room left for other gases. But even tiny amounts of greenhouse gases have a large effect on climate.

There are two factors that determine how much impact a particular greenhouse gas has. First, there is its Global Warming Factor (GWF) – its ability to absorb and then release heat. The GWF is arbitrarily set to 1 for carbon dioxide (CO₂). The values for the other gases indicate their potency relative to CO₂.

The second factor is how much of the gas there is in the atmosphere. This table shows both factors for some greenhouse gases. Although CO₂ is a weaker greenhouse gas than the others, there is much more of it in the atmosphere, so it has the greatest impact.

Carbon Dioxide

When we breathe, we take oxygen from the air and release CO₂. This is part of the process of respiration, which enables plants and animals to get energy. CO₂ is also released in forest fires and from volcanoes. Humans contribute CO₂ to the atmosphere by burning wood, coal, and petroleum.

Methane

Methane is produced naturally by bacteria called methanogens, which feed on plant and animal material in environments with no oxygen. Methanogens live under stagnant water in swamps, where they produce bubbles of methane called “swamp gas” or “marsh gas.” Methanogens also live in the digestive systems of animals, where they help to break down grass and other organic matter into nutrients. Termites produce a lot of methane. Each termite produces only about half a microgram per day, but there are so many of them in the world that together they produce about 20 million tons per year.

Some agricultural activities also produce methane. Rice is usually grown in flooded fields. The stagnant water covering soil encourages methanogenesis like in a swamp.

Domestic cattle produce more methane than wild animals. One cow produces about 50 l (13 gal) of methane per day, and the world's commercial cattle and sheep produce about 100 million tons per year.

Nitrous Oxide

N₂O is a gas released by ammonium nitrate, which is widely used as a fertilizer to increase crop yield.

Chlorofluorocarbons

CFCs are compounds of chlorine, fluorine, hydrogen, and carbon. They do not occur naturally. CFCs were first synthesized in 1892, but there was no known use for them at the time. They later proved useful as propellants in aerosol cans and as refrigerants. But there turned out to be a problem. When released into the atmosphere, CFCs migrate up to the stratosphere, where they break down ozone molecules that are present there. Ozone is a form of oxygen in which three oxygen atoms are combined into an O3 molecule. Normally free oxygen forms O₂ molecules.

The ozone layer reduces the penetration of ultraviolet radiation. This radiation is harmful to humans, causing skin cancers and cataracts, and may also interfere with the growth and reproduction of other organisms. Because of this, the use of CFCs was banned by international treaty in 1987. Although CFCs are extremely potent greenhouse gases, they exist in only very small quantities in the atmosphere. And now that they are banned, that presence will be further reduced.