There are several greenhouse gases responsible for global warming, and humans emit them in many different ways.
The main reason why there are greenhouse gases is because of the combustion of fossil fuels coming from cars, factories, and electricity production.
Carbon dioxide is the gas most responsible for warming.
There are other gases that contribute to global warming, including; Methane released from landfills and agriculture (coming from the digestive systems of grazing animals), nitrous oxide that come from fertilizers, gases that are used for refrigeration, industrial processes, and the loss of forests that would store possibly carbon dioxide.
Methane creates a lot of greenhouse gases, it is ranked second after carbon dioxide. Methane is produced when organic matter is broken down by bacteria under oxygen starved conditions. This takes place in the intestines of herbivorous animals. There is also an increase in the amount of concenrated livestock prdouction, this makes the levels of methane released in the atmosphere increase.
Markham, D. (2012). Global Warming Effects and Causes: A Top 10 List. PlanetSave 2012
Retrieved from http://planetsave.com/2009/06/07/global-warming-effects-and-causes-a-top-10-list
Retrieved from http://planetsave.com/2009/06/07/global-warming-effects-and-causes-a-top-10-list