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Radiative forcing
The change in average net radiation at the top of the troposphere (lower atmosphere), which is a measure of how the energy balance of the Earth–atmosphere system is influenced when factors that affect climate (e.g. greenhouse gases, solar energy, ozone, aerosols are altered. The word radiative arises because these factors change the balance between incoming shortwave solar radiation and outgoing longwave radiation within the Earth’s atmosphere. 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.
In detail
The amount of energy entering the Earth’s atmosphere is determined by the energy input from the sun. Over time, this radiation entering is balanced by the same amount of energy being radiated back out to space, termed the Earth’s energy balance, and it is this balance which controls the Earth’s surface temperature. There are various processes operative in the Earth’s atmosphere that exert a control on these inputs and outputs, however. An example of a controlling factor on the incoming shortwave radiation is the presence of aerosols which scatter incoming radiation, resulting in some of this incoming source being reflected back out to space. An example of a controlling factor on outgoing longwave radiation, is the presence of greenhouse gases in the atmosphere (the Greenhouse Effect), which act to prevent some of the long wave radiation from escaping.
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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