Greenhouse effect

The detailed causes of the recent warming remain an active field of research, but the scientific consensus is that the increase in atmospheric greenhouse gases due to human activity caused most of the warming observed since the start of the industrial era. This attribution is clearest for the most recent 50 years, for which the most detailed data are available.

The greenhouse effect was theorized 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 warm a planet's lower 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. On Earth, the major greenhouse gases are water vapor, which causes about 36–70 percent of the greenhouse effect (not including clouds); carbon dioxide (CO₂), which causes 9–26 percent; methane (CH₄), which causes 4–9 percent; and ozone, which causes 3–7 percent. The issue is how the strength of the greenhouse effect changes when human activity increases the atmospheric concentrations of some greenhouse gases.

Human activity since the industrial revolution has increased the concentration of various greenhouse gases, leading to increased radiative forcing from CO₂, methane, tropospheric ozone, CFCs and nitrous oxide. Molecule for molecule, methane is a more effective greenhouse gas than carbon dioxide, but its concentration is much smaller so that its total radiative forcing is only about a fourth of that from carbon dioxide. Some other naturally occurring gases contribute 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 CH₄ have increased by 31% and 149% respectively since the beginning of the industrial revolution in the mid-1700s. 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 approximately 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 385 parts per million (ppm) by volume. Human activities have caused the atmospheric concentrations of carbon dioxide and methane to be higher today than at any point during the last 650,000 years . 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.

Inasmuch as the greenhouse effect is due to human activity, it is a forcing effect that is separate from forcing due to climate variability.