Radiative forcing is defined as the difference between the radiative energy received by the Earth and energy radiated back to space. The measure is taken at the top of the troposphere (lower atmosphere).
It is a measure of how the energy balance of the Earth-atmosphere system is influenced when factors that affect climate (such as greenhouse gases, solar energy, ozone and aerosols) are altered. This radiative balance controls the Earth's surface temperature. The term forcing is used to indicate that Earth's radiative balance is being pushed away from its normal state.
The amount of energy entering the Earth's atmosphere is determined by the energy input from the sun. Over time, the radiation that enters the Earth's atmosphere is balanced by the same amount of energy being radiated back out to space. This is known as the Earth's energy balance. It is this balance that controls the Earth's surface temperature.
Various processes in the Earth's atmosphere exert a control on these inputs and outputs of radiation. An example of one such factor is the presence of aerosols. Aerosols scatter incoming solar radiation, reflecting some of it back out to space. This is an example of negative radiative forcing and has a cooling effect on the earth's atmosphere.
An example of a controlling factor on outgoing radiation, is the presence of greenhouse gases in the atmosphere. These gases act to prevent some of the outgoing radiation from escaping. This is positive radiative forcing and has a warming effect on the earth's atmosphere.
Clearly, if there is a change in the concentration of aerosols, or greenhouse gases, these processes will be enhanced or reduced. Radiative forcing is usually quantified as the rate of energy change per unit area of the globe as measured at the top of the atmosphere, and is expressed in units of watts per square metre (W/m2).
When radiative forcing is positive, the energy of the Earth-atmosphere system will ultimately increase, leading to a warming of the system. In contrast, for a negative radiative forcing, the energy will ultimately decrease, leading to a cooling of the system. The term forcing is used to indicate that Earth's radiative balance is being pushed away from its normal state.
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- IPCC Fifth Assessment Report, Working Group 1 report The Physical Science Basis , Chapter 2