Anthropogenic Forcing

Anthropogenic forcing (AF) is one of two parts of radiative forcing (RF) in the classification used to describe disturbances in the Earth's energy budget [Page 64]when humans are considered as an external factor to the Earth's climate system. The RF (in units of watts per meter 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 RF is used to predict surface climate response and for comparative studies of different forcings. A synonym for AF is the term human-induced forcing. The other part of RF is termed natural forcing (NF), which is a disturbance of the Earth's energy budget without direct or indirect human influences. Examples of NF are volcanic eruptions, solar variability, or changes in a space object's orbital parameters.

AF is a change in the Earth's energy balance due to human economical activities (HEAs). HEAs cause changes in the amount of atmospheric radiatively active gases (RAGs); in the amount of gaseous precursors of atmospheric aerosols (AA) and atmospheric ozone (O3); and in the Earth's system's albedo (ESA). RAGs, such as carbon dioxide (CO2), methane (CH4), nitrous dioxide (N2O), and chlorofluorocarbons (CFCs), are mixed well in the atmosphere, while O3 and AA have regional structures due to their shorter turnover (lifetime) in the atmosphere. Changes in the atmospheric concentrations of RAGs are accounted for by changes in their emissions. Changes in O3 and AA are defined by emissions of their gaseous precursors. Changes in ESA are related to changes in land use practices, reflective aerosols emissions, and cloud cover due to air pollution and climate change. It is assumed that changes in RAGs, aerosols, and ESA due to natural causes are small in comparison to changes from HEAs. The unique RAG is an atmospheric water vapor (WV), which has both direct (via irrigation and land use) and indirect (via change in cloud cover) influences from HEAs. The term greenhouse gases (GHG) combines RAGs, O3, and WV in one class. For policy applications, the total atmospheric RAGs amount is represented by an equivalent amount of CO2.

Regional and temporal AF strength can be calculated using an approach that requires an estimation of a few parameters: the RF per-unit emitted quantity (usually in watts per square meter per mass), an emission factor (usually in mass per unit of HEA), and a quantity of a particular HEA per unit time. Rigorous AF estimation is difficult, as it carries uncertainties from every step of its calculation. Each step is based on an accuracy of information provided of a particular science: social science in description of social infrastructure of a region or a country, economics for HEAs quantification in terms of emissions and land use, and atmospheric and climate sciences for RF calculations and conversion of the emissions to the atmospheric concentrations or burden.