Pictured left: Per capita anthropogenic greenhouse gas emissions by country for the year 2000 including land-use change.
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. In the Solar System, the atmospheres of Venus, Mars, and Titan also contain gases that cause greenhouse effects. Greenhouse gases greatly affect the temperature of the Earth; without them, Earth's surface would be on average about 33 °C (59 °F) colder than at present.
Since the beginning of the Industrial Revolution, the burning of fossil fuels has contributed to the increase in carbon dioxide in the atmosphere from 280ppm to 390ppm, despite the uptake of a large portion of the emissions through various natural "sinks" involved in the carbon cycle. Anthropogenic (human-sourced) carbon dioxide (CO2 ) emissions come from combustion of carbonaceous fuels, principally wood, coal, oil, and natural gas.
Each gases' contribution to the greenhouse effect is affected by the characteristics of the gas, its abundance, and any indirect effects it may cause. For example, on a molecule-for-molecule basis the direct radiative effects of methane is about a 72 times stronger greenhouse gas than carbon dioxide over a 20 year time frame, but it is present in much smaller concentrations so that its total direct radiative effect is smaller. On the other hand, in addition to its direct radiative impact methane has a large indirect radiative effect because it contributes to ozone formation.