Over the past century or so the global (land and sea) temperature has increased by 0.6 ± 0.2 °C]. The effects of global warming are increasingly visible.
At the same time, atmospheric carbon dioxide has increased from around 280 parts per million (by volume) in 1800 to around 315 in 1958 and 367 in 2000, a 31% increase over 200 years. Other greenhouse gas emissions have also increased.
Future carbon dioxide levels are expected to continue rising due to ongoing fossil fuel usage, though the actual trajectory will depend on uncertain economic, sociological, technological, and natural developments.
The IPCC SRES gives a wide range of future carbon dioxide scenarios, ranging from 540 to 970 parts per million by 2100.
Coal-burning power plants, vehicle exhausts, factory smokestacks, and other waste vents of the human environment contribute about 22 billion tons of carbon dioxide and other greenhouse gases into the earth's atmosphere each year. Animal agriculture, manure, natural gas, rice paddies, landfills, coal, and other sources contribute about 250 million tons of methane each year and about half of human emissions have remained in the atmosphere.
The atmospheric concentrations of carbon dioxide and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750.
This is 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 carbon dioxide values this high were last attained 40 million years ago. About three-quarters of the anthropogenic emissions of carbon dioxide to the atmosphere during the past 20 years is due to fossil fuel burning.
The rest is predominantly due to land-use change, especially
deforestation