Anthropogenic climate change

Long-term alteration in the Earth's climate caused by human activities. The scientific community reached a consensus in the late 20th century that such activity has caused an increase in the average temperature at the surface of the Earth and will continue to do so into the future unless some human activities are reduced or their effects are in some way offset.

The main cause of ACC is the increase in carbon dioxide levels in the atmosphere originating from the gaseous emissions of manufacturing and transport. Carbon dioxide is described as a ‘greenhouse gas’ (by a partial analogy with the warming effect of greenhouses); the second most important greenhouse gas put into the atmosphere by human activity is methane, produced by cattle-farming, landfill, and other activities.

The possibility that naturally originating carbon dioxide could have influenced climate in the past – principally in initiating or ending ice ages – was suggested by the British physicist John Tyndall (1820–1893) in 1859. Later, the Swedish physicist Svante Arrhenius (1859–1927) studied the possibility that human-produced carbon dioxide could warm the Earth in the future. He calculated that a future doubling of the amount of atmospheric carbon dioxide from the pre-industrial level would raise the Earth's temperature by 5–6°C/9–11°F. Atmospheric levels of carbon dioxide have risen from 280 ppm (parts per million) before the Industrial Revolution to 383 ppm by early 2007. There has also been a temperature rise. For the period 1850–2005 alone, there was an increase in temperature of 0.76°C/1.37°F.

There are numerous complexities in the behaviour of the climatic system that introduce uncertainties; for example, rising temperature would probably increase cloud cover, which would tend to cool the surface. However, in its regular series of reports, the Intergovernmental Panel on Climate Change (IPCC) has recorded a growing consensus that the observed warming is caused by carbon dioxide increases due to human activity (rather than to variations in solar radiation or cosmic radiation), and that the Earth faces rising temperatures. In the IPCC's 2007 report a number of scenarios were evaluated, varying according to the degree to which industrial emissions are reined in. The lowest predicted an end-of-century temperature rise of 1.1–2.9°C/2.0–5.2°F, the highest predicted a rise of 2.4–6.4°C/4.3–11.5°F.