The Earth has an atmosphere of the proper depth and chemical composition. About 30% of incoming energy from the sun  is reflected back to space while the rest reaches the earth, resulting in warming the air, oceans, and land, and maintaining an average surface temperature of about 15 ºC.

The chemical composition of the atmosphere is also responsible for nurturing life on our planet. Most of it is nitrogen (78%); about 21% is oxygen, which all animals need to survive; and only a small percentage (0.036%) is made up of carbon dioxide which plants require for photosynthesis. 

The atmosphere carries out the critical function  of maintaining life-sustaining conditions on Earth, in the following way: each day, energy from the sun is absorbed by the land, seas, mountains, etc. If all this energy were to be absorbed completely, the earth would gradually become hotter and hotter. But actually, the earth both absorbs and, simultaneously releases it in the form of infra red waves (which cannot be seen by our eyes but can be felt as heat, for example the heat that you can feel with your hands over a heated car engine). All this rising heat is not lost to space, but is partly absorbed by some gases present in very small (or trace) quantities in the atmosphere, called greenhouse gases (GHGs). Greenhouse gases (for example, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), water vapour), re-emit some of this heat to the earth\'s surface. If they did not perform this useful function, most of the heat energy would escape, leaving the earth cold (about -18 ºC) and unfit to support life.

However, ever since the Industrial Revolution began about 150 years ago, man-made activities have added significant quantities of GHGs to the atmosphere. The atmospheric concentrations of carbon dioxide (CO2), methane (CH4), and nitrous oxide  (N2O) have grown by about 31%, 151% and 17%, respectively, between 1750 and 2000 (Intergovernmental Panel on Climate Change, IPCC 2001).

As the GHGs  are transparent to incoming solar radiation, but opaque to outgoing longwave radiation, an increase in the levels of GHGs could lead to greater warming, which, in turn, could have an impact on the world\'s climate, leading to the phenomenon known as climate change. Indeed, scientists have observed that over the 20th century, the mean global surface temperature increased by 0.6°C (IPCC 2001). They also observed that since 1860 (the year temperature began to be recorded systematically using a thermometer), the 1990\'s have been the warmest decade.

Important greenhouse gases are: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFC), perfluorocarbons (PFC), and sulfur hexafluoride (SF6). Water vapor is also an important greenhouse gas, but since humans do not generally have a direct affect on water vapor concentration in the atmosphere, it is not included in this  paper. Because each greenhouse gas traps different amounts of heat and stays in the atmosphere for different lengths of time, studies use measures of global warming potential (GWP) to compare between gases. Carbon dioxide is used as the benchmark, so all other gases are measured in carbon dioxide equivalence (CO2e)2.