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What Regulates Gas Composition of the Atmosphere?

Earth has enough gravity to hold onto an atmosphere. The moon does not, and Mars barely does. The Earth's atmosphere is not static. There are several factors -- some geological, some biological and some astronomical -- that regulate Earth's atmosphere. An example of what can happen when the atmosphere gets out of balance can be seen on Venus: 900-degree surface temperature and a permanent overcast that rains sulfuric acid.

Oceans
- The oceans and the atmosphere work together to modify and stabilize each other. They are in many ways mirror images of each other -- for example, the jet streams in the atmosphere mirror the oceanic rivers like the Gulf Stream and the Japanese current. They stabilize each other's temperatures and there is also a constant moisture exchange. A great deal of oxygen leaves the ocean and goes into the atmosphere all the time. The oceans constantly evaporate and therefore are the major contributor to cloud formation. Clouds protect both the Earth and the atmosphere from the damaging rays of the sun.
Life
- Plant and animals have a relationship that goes through the atmosphere. Plants take in carbon dioxide and release oxygen -- these gases are inputs and outputs for the photosynthesis process. Animals take in oxygen and produce carbon dioxide -- inputs and outputs for the metabolism process. Plants evolved first and between the time that plants evolved and animals appeared, the atmosphere was increasingly rich in oxygen. The balance between plants and animals controls the oxygen-to-carbon dioxide ratio in the atmosphere.
Forests
- Currently oxygen is slightly of 20% of the atmosphere and carbon dioxide is less than 10%. As a result of deforestation, the ratio of oxygen to carbon dioxide is changing. Forests and oceans are the main contributors to the oxygen level of the atmosphere. Coal- and gasoline-based technology are making matters worse by converting oxygen to carbon dioxide as well. In California there is already legislation controlling forest management -- with the specific goal of atmospheric balance.
Greenhouse Gases
- Greenhouse gases are those that make the atmosphere act like a greenhouse -- sunlight enters the atmosphere, and the heat is captive inside. Some of these gases -- like ozone -- are helpful because they prevent dangerous ultraviolet from reaching the earth. Some of these gases are completely natural, such as water vapor, and they produce clouds which also protect the earth (but can trap heat as well). The most dangerous of the greenhouse gases (carbon dioxide, nitrous oxide, methane and fluorocarbons) collect in a layer in the upper atmosphere and also cause the breakdown of ozone.

Nature