Anthropogenic Forcing

ANTHROPOGENIC FORCING IS one of two parts of radiative forcing in the classification used to describe disturbances in the Earths energybudgetwhenhumans are considered as a factor to the Earths climate system. The radiative forcing (in units of watts per m. squared) is the net downward radiative flux at the surface or at some level in the atmosphere, usually at the top of the atmosphere or at the tropopause. In atmospheric and climate sciences, the radiative forcing is used to predict surface climate response and for comparative studies of different forcings. A synonym for anthropogenic forcing is human-induced forcing. The other part of radiative forcing is natural forcing, which is a disturbance of the Earths energy budget without human direct or indirect influences. Examples of natural forcing are volcanic eruptions, solar variability, or changes in a space objects orbital parameters.

Anthropogenic forcing is a change in the Earths energy balance due to human economical activities. Human economical activities cause changes in the amount of atmospheric radiatively active gases; in the amount of gaseous precursors of atmospheric aerosols and atmospheric ozone (O3), and in the Earths systems albedo. Radiatively active gases, such as carbon dioxide (CO2), methane (CH4), nitrous dioxide (N2O), and chlorofluorocarbons (CFCs), are mixed well in the atmosphere, while O3 and atmospheric aerosols have regional structures due to their shorter turnover (lifetime) in the atmosphere. Changes in the radiatively active gases' atmospheric concentrations are accounted for by changes in the their emissions. Changes in O3 and atmospheric aerosols are defined by emissions of their gaseous precursors. Changes in Earths systems albedo are related to changes in land-use practices, reflective aerosols emissions, and changes in cloud cover due to air pollution and climate change.

It is assumed that changes in radiatively active gases, aerosols, and the Earths systems albedo due to natural causes are small in comparison to changes from human economical activities. The unique radiatively active gas is an atmospheric water vapor, which has both direct (via irrigation and land use) and indirect (via change in cloud cover) influences from human economical activities. The phrase greenhouse gases combines radiatively active gases, O3, and water vapor in one class. For policy applications, the total of atmospheric radiatively active gases is represented by an equivalent amount of CO2.

Regional and temporal anthropogenic forcing strength can be calculated using an approach that requires estimation of a few parameters: the radiative forcing per unit emitted quantity (usually in watts per square m. per mass), an emission factor (usually in mass per unit of human economical activities), and a quantity of a particular human economical activities per unit time. Rigorous anthropogenic forcing estimation is difficult, as it carries uncertainties from every step of its calculation. Each step is based on an accuracy of information provided by a particular science: social science in description of [Page 65]social infrastructure of a region or a country, economics for human economical activities quantification in terms of emissions and land use, and atmospheric and climate sciences for radiative forcing calculations and conversion of the emissions to the atmospheric concentrations.

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