Greenhouse gas

Greenhouse gas reflects radiation that the Earth emits, preventing it from escaping into space. This occurrence is known as the "greenhouse effect".

Many greenhouse gases are natural, with water vapor being the most common and responsible for most of the greenhouse effect on Earth. Other significant greenhouse gases include carbon dioxide, methane, nitrous oxide, chlorofluorocarbon (CFCs) and ozone. 

Without greenhouse gases, life as we know it on earth would likely not be possible as heat is essential for sustaining life. Natural emissions of greenhouse gases vary, and events like massive volcanic eruptions such as those which created Siberian Traps around a quarter billion years ago, might have released enough gases to contribute to the Permian–Triassic extinction which caused to destructive effects on life on earth. 

However, humans are introducing additional greenhouse gases to the atmosphere, leading to an enhanced greenhouse effect and increase in the planet's average temperature. The most significant greenhouse gas that contributed by humans is carbon dioxide, currently composing approximately 0.04% of the atmosphere. Human activities like burning fossil fuel such as oil, coal and natural gas for transportation, energy production and industrial processes are the main sources of carbon dioxide emission.  The major contributor to these emissions is the industrial sector.   The Food and Agriculture Organization has reported that emissions associated with livestock, contribute to 7.1 gigatons (GT) of carbon dioxide equivalent (CO2-eq) per year, accounting for 14.5% of all human-caused greenhouse emissions. This exceeds the 13% that contributed by global transportation (including cars and planes) each year.

Water vapor is the most abundant greenhouse gas, and it responds to climate changes. When the atmosphere is warm, it holds more water vapor, which increase the probability of cloud formation and precipitation. 

In addition to burning fossil fuels, human activities also reduce the earth's capacity to absorb carbon dioxide from the atmosphere by deforestation. Methane emission is also increase due to the activities like raising cattle and other farm animals, including as geese, turkeys, pigs, chickens, and sheep. Scientists have shown that producing 1 kg of beef generated more CO2 emissions than a three-hour drive with all household lights on.Meat production 'beefs up emissions' 19 July 2007 The Guardian Further, human activity introduces water vapor to the atmosphere through increased evaporation from cooling towers in thermal cycle power plants and creation of artificial lakes which contributes to the  global warming.

 References 

 Other websites 
 The Climate Change Guide easy-to-understand information on the Greenhouse gases