Radiative forcing
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The generalised concept of radiative forcing in climate science is any change in the radiation (heat) entering the climate system or changes in radiatively active gases. It also has a more specific technical definition - see "ipcc usage" section.
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Radiation balance
Most of Earth's energy which affects weather comes from the Sun. The planet and its atmosphere absorb and reflect some of the energy, with that which is absorbed tending to produce warming. An amount of heat is radiated back to space, tending to cool the planet. The balance between absorbed and radiated energy determines the average temperature. The planet is warmer than it would be in the absence of the atmosphere: see greenhouse effect for details.
Radiative forcing
The radiation balance can be altered by factors such as intensity of solar energy, reflection by clouds or gases, absorption by various gases or surfaces, and emission of heat by various materials. Any such alteration is a radiative forcing, and new balances will be reached. In the real world this continually happens in various areas, such as where sunlight is striking, depth and density of atmospheric areas with various amounts of gases, clouds, and aerosols, and where seasons alter the ground cover.
A positive forcing tends to warm while a negative forcing tends to cooling.
IPCC usage
The term “radiative forcing” has been employed in the IPCC Assessments with a specific technical meaning to denote an externally imposed perturbation in the radiative energy budget of the Earth’s climate system, which may lead to changes in climate parameters [1] (http://www.grida.no/climate/ipcc_tar/wg1/212.htm). The exact definition used is:
- The radiative forcing of the surface-troposphere system due to the perturbation in or the introduction of an agent (say, a change in greenhouse gas concentrations) is the change in net (down minus up) irradiance (solar plus long-wave; in Wm-2) at the tropopause AFTER allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values. [2] (http://www.grida.no/climate/ipcc_tar/wg1/214.htm#611)
In the context of climate change, the term forcing is restricted to changes in the radiation balance of the surface-troposphere system imposed by external factors, with no changes in stratospheric dynamics, without any surface and tropospheric feedbacks in operation (i.e., no secondary effects induced because of changes in tropospheric motions or its thermodynamic state), and with no dynamically-induced changes in the amount and distribution of atmospheric water (vapour, liquid, and solid forms).
Related measures
Note that radiative forcing is intended as a useful mechanism to compare and explain different (CO2; land-use changes) perturbations to the climate system. Other possible tools can be constructed for the same purpose: for example Shine et al, An alternative to radiative forcing for estimating the relative importance of climate change mechanisms, GEOPHYSICAL RESEARCH LETTERS, 2003 say "...recent experiments indicate that for changes in absorbing aerosols and ozone, the predictive ability of radiative forcing is much worse... we propose an alternative, the "adjusted troposphere and stratosphere forcing". We present GCM calculations showing that it is a significantly more reliable predictor of this GCM's surface temperature change than radiative forcing. It is a candidate to supplement radiative forcing as a metric for comparing different mechanisms...". In this quote, the word "predictive" may be confusing: it refers to the ability of the tool to help explain the response, not to the ability of GCMs to forecast climate change.