Many human activities result in large emissions of carbon dioxide (CO₂) and other so-called greenhouse gases into the atmosphere. Especially important in this regard are the use of fossil fuels such as coal, oil, and natural gas to generate electricity, to heat spaces, and as a fuel for vehicles. In addition, the disturbance of forests results in large emissions of CO₂ into the atmosphere. This can be caused by the conversion of forests into agricultural or urbanized land uses, and also by the harvesting of timber.

During the past several centuries, human activities have resulted in large emissions of CO₂ into the atmosphere, causing substantial increases in the concentrations of that gas. Prior to 1850 the atmospheric concentration of CO₂ was about 280 ppm, while in 1997 it was about 360 ppm. Other greenhouse gases (these are also known as radiatively active gases) have...