Just like carbon dioxide, natural and human processes create nitrous oxide. Human resources represent about forty-five percent of the cause. Some examples of how humans contribute to global warming are: power stations burning fossil fuel, using nitrogenous manure, burning rainforests, and the manure of human and animals. All these things combine add up to six percent of methane in the air.
Graph of released CO2 in the Atmosphere
Graph from http://www.androidworld.com/prod60.htm
Source: Dolan, Edward J. Our Poisoned Sky. Dutton: Cobblehill Books, 1991.