Question

How are solar forcing and anthropogenic forcing related to one another, relatively, over time and where we are now?
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Answer 1

Anthropogenic forcing is one of two parts of radiative forcing in the classification used to describe disturbances in the Earth’s energy budget considering humans as a primary factor in the Earth’s climate system. The radiative forcing (in units of W/m²) is the net downward radiative flux at the surface or 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 economic activities. Social-economic activities cause changes in the amount of atmospheric radiatively active gases; in the number of gaseous precursors of atmospheric aerosols and atmospheric ozone (O₃), and 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 differences in their emissions. Emissions of their gaseous precursors define changes in O₃ and atmospheric aerosols. 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 economic activities. The different radiatively active gas is an atmospheric water vapor, which has both direct (via irrigation and land use) and indirect (via the change in cloud cover) influences from human economic activities. The phrase greenhouse gases combine 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 (often in mass per unit of human economic activities), and a quantity of a particular social-economical events per unit time. Rigorous anthropogenic forcing estimation is difficult, as it carries uncertainties from every step of its calculation. Each level 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 economic activities are classified in primary, secondary, and tertiary industries. Primary (agriculture, forestry, and mining) and secondary (construction and manufacturing) industries are the primary direct emitters and controllers of land use. In tertiary sectors, transportation, electricity, and gas suppliers are the main air polluters. Population, wealth, leadership, and technology are factors that define the specification of economical human 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 order 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 level.

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 most significant warming contribution to changes in anthropogenic forcing from pre-industrial times. The input from the anthropogenic aerosol and land use are very uncertain in magnitude and patterns. Except for black carbon, most of the aerosols have a cooling effect. The indirect impact of the aerosols is more extensive than its direct influence. The contribution from airplane contrails is small.