Radiative forcing

From Wikipedia, the free encyclopedia

1 Radiation balance
2 IPCC usage
3 Related measures
4 See also
5 External links

As a general concept, the term radiative forcing in climate science means any change in the radiation (heat) entering or leaving the climate system. It can be due to changes in sunlight arriving, or to differing amounts of radiatively active gases. It also has a more specific technical definition - see "IPCC usage" section.

A positive forcing tends to warm the system while a negative forcing tends to cool it.

[edit] Radiation balance

Most of the energy which affects Earth's 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 into 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 and Radiation Balance for a mathematical explanation.

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 causes a new balance to be reached. In the real world this happens continuously as sunlight hits the surface, clouds and aerosols form, the concentrations of atmospheric gases vary, and seasons alter the ground cover.

[edit] IPCC usage

Changes in radiative forcings between 1750 and 2000 as estimated by the IPCC.

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]. 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]

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).

[edit] Related measures

Note that radiative forcing is intended as a useful way to compare different causes of perturbations in 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.