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Radiative Forcing
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Major sources of radiative forcing
Source: IPCC Working Group I Fourth Assessment Report Summary for Policymakers. Author: Leland McInnes. Permission: GNU Free Documentation License.Radiative forcing is the measure of the effects of external forces on the Earth's radiation balance.1 Radiation from the sun reaches the Earth in the form of shortwave radiation (ultraviolet and visible light). The Earth absorbs the radiation and re-emits it in the form of longwave radiation (heat). Various factors can effect the balance of this process, either by changing the amount of energy reaching the Earth or by changing the amount of energy escaping back into space. Changes in radiative forcing result in changes in the Earth's effective temperature. The effects can be calculated using the following equation:
?T = ? ?F
where ?T is the change in effective temperature, ? = the climate sensitivity factor, generally estimated at about 0.5 K/ (Wm-2), and ?F is the change in radiative forcing.
Some important sources of radiative forcing are greenhouse gases, black carbon, aerosols, and albedo.
Greenhouse gases
Greenhouse gases are gases that prevent the longwave radiation emitted by the Earth from leaving the atmosphere, effectively warming the planet. As the concentration of a greenhouse gas increases, so does its radiative forcing. The forcing due to different greenhouse gases is dependent on the molecular properties and atmospheric concentrations of the gas. For example, the change in radiative forcing due to changes in the concentration of carbon dioxide is calculated by:
?F = ? ln (C/C0)
where ?F is the change in radiative forcing, C/C0 is the ratio of the final concentration of CO2 to the intitial concentration of CO2, and ? is a constant generally calculated as being between 5 and 6 W/m2.23
Aerosols
Unlike greenhouse gases, aerosols, usually sulfate, have a negative radiative forcing, lowering the effective temperature of the Earth. Aerosols effect the atmosphere in two key ways. First, they act to block incoming solar radiation. As radiation from the sun reaches the Earth's atmosphere, some of it is reflected back into space by sulfate aerosols, effectively lowering the Earth's temperature. Secondly, aerosols can seed cloud formation, as water vapor condenses on their surfaces. Most clouds also act to block incoming radiation, lowering the Earth's temperature further.
Albedo
Albedo is a measure of the reflectivity of the Earth. As solar radiation reaches the surface of the earth, a percentage of it is reflected back into space as shortwave radiation, rather than being absorbed and re-emitted as heat. Different types of terrain impact the Earth's albedo differently. Ice and snow for example are white, and therefore have very high albedos, reflecting most of the radiation from the sun back into space. Oceans on the other hand are relatively dark and so absorb most incoming radiation, warming the Earth.
The Earth's albedo is currently 0.3 (30% of the radiation reaching the Earth is reflected back into space). However, seemingly small changes in the Earth's albedo can have a profound forcing effect. An example of how Earth's albedo is changing is the melting of the Arctic ice cap. As the ice melts it is replaced by the darker ocean water, lowering the Earth's albedo and warming the planet.
Footnotes
1. "Radiative Forcing" IPCC Third Assessment Report. 2001. Retrieved April 19, 2010.
2. "Carbon Dioxide" IPCC Third Assessment Report. 2001. Retrieved April 19, 2010.
3. Myhre, et al. "New estimates of radiative forcing due to well mixed gases" Geophysical Research Letters, vol 25. 1998.
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