Anthropogenic Forcing

anthropogenic forcing is one of two parts of radiative forcing in the classification used to describe disturbances in the Earth's energy budget when humans are considered as a factor to the Earth's 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 Earth's energy budget without human direct or indirect influences. Examples of natural forcing are volcanic eruptions, solar variability, or changes in a space object's orbital parameters.

Anthropogenic forcing is a change in the Earth's 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 Earth's system's 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 Earth's system's 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 Earth's system's 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 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.

Regional anthropogenic forcing estimation is complex. Human economical activities are classified in primary, secondary, and tertiary industries. Primary (agriculture, forestry, and mining) and secondary (construction and manufacturing) industries are the main direct emitters and controllers of land use. In tertiary industries, the transportation, electricity, and gas suppliers are the main air polluters. Population, wealth, leadership, and technology are factors that define specification of the human economical activities by region.

A desirable goal for constructing accurate anthropogenic forcing estimates and projecting them into the future is to define a set of main pathways from a particular human economical activities to anthropogenic forcing, which takes into account all direct (emissions—forcing) and indirect (emissions—climate system—forcing) influences and resolves feedback loops in the Earth's system on the time scale much smaller than the period chosen for anthropogenic forcing estimation. When only the global or hemispheric changes in radiative forcing from the pre-industrial time to present are taken into account, calculation of anthropogenic forcing is based on assumptions of how the radiative forcing depends on the historical evolution of each radiatively active gas or precursor concentration or their emissions. For example, because CFCs have low concentrations in the atmosphere, their radiative forcing increases linearly with concentration.

These assumptions are derived from the numerical models, which accurately calculate the atmospheric distribution of the radiative fluxes for small temporal and spatial variations in each radiatively active agent. For aerosols, as they stay in the atmosphere only for a short time and have a large spatial variability in concentrations and radiative properties, estimation of their radiative forcing is based on inverse modeling, when the radiative forcing is constrained from aerosols' hemispheric asymmetry and observed temperature record, and on aerosols models built from the first principles. It is assumed that the total radiative forcing is a simple sum of its parts.

According to radiative forcing calculations, anthropogenic radiatively active gases make the largest warming contribution to changes in anthropogenic forcing from pre-industrial times. The contribution from the anthropogenic aerosol and land use is very uncertain in magnitude and patterns. Except for black carbon, most of the aerosols have a cooling effect. The indirect effect of the aerosols is larger than its direct effect. The contribution from airplane contrails is small.

sEE ALso: Albedo; Energy Balance Models; Radiative Feedbacks.

BIBLioGRAPHY. Intergovernmental Panel on Climate

Change (IPCC), Climate Change 1995: The Science of Climate Change. Contribution of Working Group I to the Second Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge University Press, 1996); IPCC, Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge U. Press, 2001).

Natalia Andronova University of Michigan

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