Greenhouse gases in the atmosphere
Main article: Greenhouse effect
The greenhouse effect was discovered by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896. It is the process by which absorption and emission of infrared radiation by atmospheric gases warms a planet''s atmosphere and surface.
Existence of the greenhouse effect as such is not disputed. Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F), without which Earth would be uninhabitable. The debate centers on how the strength of the greenhouse effect is changed when human activity increases the atmospheric concentrations of some greenhouse gases.
On Earth, the major greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%. Some other naturally occurring gases contribute very small fractions of the greenhouse effect; one of these, nitrous oxide (N2O), is increasing in concentration owing to human activity such as agriculture. The atmospheric concentrations of CO2 and methane have increased by 31% and 149% respectively above pre-industrial levels since 1750. These levels are considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 20 million years ago.
 Fossil fuel burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. Most of the rest is due to land-use change, in particular deforestation.
The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume. Future CO2 levels are expected to rise due to ongoing burning of fossil fuels and land-use change. The rate of rise will depend on uncertain economic, sociological, technological, and natural developments, but may be ultimately limited by the availability of fossil fuels. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100. Fossil fuel reserves are sufficient to reach this level and continue emissions past 2100, if coal, tar sands or methane clathrates are extensively used.
Positive (reinforcing) feedback effects such as the expected release of methane from the melting of permafrost peat bogs in Siberia (possibly up to 70,000 million tonnes) may lead to significant additional sources of greenhouse gas emissions not included in climate models cited by the IPCC.