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Black carbon (BC) is a a very fine form of particulate matter based on the element carbon that is produced by incomplete combustion processes, and often is referred to commonly as soot. As of 2009, it began to be considered a major contributing factor to global warming by the scientific community, where it can block infrared heat emissions from the Earth's atmosphere back into space. Estimates are that black carbon is responsible for anywhere from 10% to 40% of all radiative blocking, though it only has a short lifetime of a few weeks in the upper atmosphere as opposed to a greenhouse gas like carbon dioxide that can persist for up to a century in the atmosphere before it is chemically broken down. The major human activities that produce black carbon aerosols are the burning of wood and vegetation, the use of coal for energy production, and the running of diesel-powered engines. In the United States, for instance, as of 2011, 90% of all BC emissions are produced in the transportation sector by diesel-powered trucks.
The element carbon forms bonds with many other elements in nature, and black carbon itself can take on several different forms. Unlike stable compounds such as black diamond carbon or black carbon steel, however, atmospheric black carbon is a form of the element that is usually weakly bound to organic molecules based on the source material from which it originated. Where it contains a micro-crystalline structure that is similar to graphite, it can absorb visible light rays as well as longer wavelength infrared light known as heat radiation, both of which normally escape into space from the upper atmosphere and prevent the planet from overheating.
Recent research in the 21st century has revealed that black carbon emissions are the second largest element contributing to global warming after that of carbon dioxide gas. They are also seen as a source of pollution that is easily reduced due to the material's short-term duration in the atmosphere. Improved efficiencies in heating equipment or cooking technology in developing nations as well as air pollution controls on coal-fired power plants and diesel engines could rapidly reduce BC pollution rates on a widespread scale.
Diesel-powered vehicles and industrial plants can be retrofitted with scrubbers that can prevent up to 70% of black carbon emissions over a life for the particulate trap of eight years. Replacing cooking stoves in nations such as India and China where coal or other biomass burning is widely done, with liquefied petroleum gas (LPG) fired cook stoves, is also seen as a practical and economical way to reduce black carbon emissions. Since soot is a major contributing factor to respiratory ailments, this would also improve the health of those who are affected by air pollution from BC stoves.
Studies as of 2009 in the Arctic and Antarctic and as of 2010 in the Himalayan mountain range have revealed that black carbon is a major contributing factor to the melting of glaciers. This is despite the fact that emissions of BC primarily originate in tropical regions, with east Asian nations being a major producer of the pollutant by volume. A significant factor in black carbon emission rates is that of deforestation in tropical rainforest regions. Reducing deforestation is seen as a more easily mitigated pollution source than previous attempts to reduce global warming gas production levels such as that of carbon dioxide and nitrogen oxide, which are inevitable byproducts of worldwide industry and transportation networks.