Direct greenhouse gas emissions

Between the period 1970 to 2004, greenhouse gas emissions (measured in CO₂-equivalent) increased at an average rate of 1.6% per year, with CO₂ emissions from the use of fossil fuels growing at a rate of 1.9% per year.Total anthropogenic emissions at the end of 2009 were estimated at 49.5 gigatonnes CO₂-equivalent. These emissions include CO₂ from fossil fuel use and from land use, as well as emissions of methane, nitrous oxide and other greenhouse gases covered by the Kyoto Protocol.

Greenhouse gases

Greenhouse gases are those that absorb and emit infrared radiation in the wavelength range emitted by Earth. In order, the most abundant greenhouse gases in Earth's atmosphere are:

• Water vapor (H
  2O)
• Carbon dioxide (CO
  2)
• Methane (CH
  4)
• Nitrous oxide (N
  2O)
• Ozone (O
  3)
• Chlorofluorocarbons (CFCs)
• Hydrofluorocarbons (incl. HCFCs and HFCs)

Atmospheric concentrations are determined by the balance between sources (emissions of the gas from human activities and natural systems) and sinks (the removal of the gas from the atmosphere by conversion to a different chemical compound or absorption by bodies of water).The proportion of an emission remaining in the atmosphere after a specified time is the "airborne fraction" (AF). The annual airborne fraction is the ratio of the atmospheric increase in a given year to that year's total emissions. As of 2006 the annual airborne fraction for CO₂ was about 0.45. The annual airborne fraction increased at a rate of 0.25 ± 0.21% per year over the period 1959–2006.

At present, the primary source of CO₂ emissions is the burning of coal, natural gas, and petroleum for electricity and heat.