Air

The atmosphere and the gases that comprise it are responsible for storing and circulating energy in the form of heat. The atmosphere absorbs this heat when it arrives from the sun and also when it has been reflected or radiated from the surface of the Earth. Most of the atmosphere is made up of nitrogen (N2, 78%) and oxygen (O2, 21%). Neither of these is a greenhouse gas, however, because they do not trap heat. That means that on average only 1% of the atmosphere is made up of greenhouse gases. The three most important of these are water vapor (H20v, 1-3%), carbon dioxide (CO2, ~.035%), and methane (CH4, ~.00018%). Clouds also play an important role in the climate by reflecting energy incoming from the sun back into space, or absorbing and retaining in the climate system energy that is reflected from Earth.