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In climate science, radiative forcing is (loosely) defined as the change in net irradiance at the tropopause. "Net irradiance" is the difference between the incoming radiation energy and the outgoing radiation energy in a given climate system and is thus measured in Watts per square meter. The change is computed based on "unperturbed" values, as defined by the Intergovernmental Panel on Climate Change (IPCC) as the measured difference relative to the year 1750, the defined starting point of the industrial era. A positive forcing (more incoming energy) tends to warm the system, while a negative forcing (more outgoing energy) tends to cool it. Possible sources of radiative forcing are changes in insolation (incident solar radiation), or the effects of variations in the amount of radiatively active gases and aerosols present. Because the IPCC regularly assesses the radiative forcing, it also has a more specific technical definition - see "IPCC usage" section.

Radiation balance

The vast majority of the energy which affects Earth's weather comes from the Sun. The planet and its atmosphere absorb and reflect some of the energy, while long-wave energy is radiated back into space. Conservation of Energy requires that the energy entering the Earth from the Sun and the energy radiated away from the Earth be equal. Any short term excess is always accompanied by an equal and opposite short term deficit and visa versa. The radiative forcing from greenhouse gases and clouds establish a temperature gradient between the Earth's surface and space such that the equivalent temperature of the Earth as seen from space is about 255K. This is the temperature that the Earth must present to space in order to be in equilibrium with the incident energy from the Sun minus the reflected energy: see Stefan-Boltzmann law. The temperature gradient makes the planet's surface warmer than it would be in the absence of an atmosphere containing greenhouse gases and clouds: see greenhouse effect.

The radiation balance can be altered by factors such as changes in the intensity of the incident solar energy, changes to the albedo and the emission of heat from various sources. Any such alteration causes a new balance to be reached. In the real world this happens continuously as sunlight hits the surface, clouds and aerosols form and seasons alter the ground cover. Radiative forcing from greenhouse gases and clouds is different and doesn't change the energy balance, but changes the profile of the temperature gradient in the atmosphere which may include changing the surface temperature. The energy re-radiated via greenhouse gas absorption is a delayed form of incident solar energy that has already forced the system. If this is counted as input energy to the system, it would be counted twice and violate Conservation of Energy.

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 . 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) 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.

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).

Radiative forcing can be used to estimate a subsequent change in equilibrium surface temperature ΔT_s change arising from that radiative forcing via the equation:

${\displaystyle \Delta T_{s}=~\lambda ~\Delta F}$

where λ is the climate sensitivity, usually with units in K/(W/m), and ΔF is the radiative forcing. A typical value is 0.8 K/(W/m), which gives a warming of 3K for doubling of CO₂.

For an average surface temperature of 288K, the Stefan-Boltzmann law can be used to calculate how much extra power is required to increase the temperature by 1K and it's about 5.4 W/m. This would require a value of λ of about 0.185. Values greater than this imply that the net forcing requires some sort of amplification factor to increase the temperature beyond what the additional power density can do by itself. Considering only the consequences of the Stefan-Boltzmann law, doubling CO₂ would result in only a 0.7K rise.

Examples of radiative forcing calculations

Radiative forcing (often measured in watts per square meter) can be estimated in different ways for different components. For the case of a change in solar irradiance, the radiative forcing is the change in the solar constant divided by 4 and multiplied by 0.7 to take into account the geometry of the sphere and the amount of reflected sunlight. For a greenhouse gas, such as carbon dioxide, radiative transfer codes that examine each spectral line for atmospheric conditions can be used to calculate the change ΔF as a function of changing concentration. These calculations can often be simplified into an algebraic formulation that is specific to that gas.

For instance, the simplified first-order approximation expression for carbon dioxide is:

${\displaystyle \Delta F=5.35\times \ln {C \over C_{0}}~\mathrm {W} ~\mathrm {m} ^{-2}}$

where C is the CO₂ concentration in parts per million by volume and C₀ is the reference concentration. The relationship between carbon dioxide and radiative forcing is logarithmic so that increased concentrations have a progressively smaller warming effect. The constant is empirical and must be scaled for different values of C₀. For a constant of 5.35, C₀ is restricted to the CO₂ concentration at the beginning of the Industrial Revolution.

Formulas for other greenhouse gases such as methane, N2O or CFCs and the reference value of C₀ are given in the IPCC reports .

Not all forms of radiative forcing represent energy entering the climate system. The energy of the radiative forcing from greenhouse gases is a form of delayed solar irradiance. Considering the radiative forcing from both solar irradiance and greenhouse gases as input power will violate Conservation of Energy by counting the same energy twice.

Related measures

Radiative forcing is intended as a useful way to compare different causes of perturbations in a climate system. Other possible tools can be constructed for the same purpose: for example Shine et al 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.

References

1. Myhre et al., New estimates of radiative forcing due to well mixed greenhouse gases, Geophysical Research Letters, Vol 25, No. 14, pp 2715-2718, 1998
2. Shine et al, An alternative to radiative forcing for estimating the relative importance of climate change mechanisms, Geophysical Research Letters, Vol 30, No. 20, 2047, doi:10.1029/2003GL018141, 2003

• IPCC glossary

See also

• Anthropogenic heat
• Emission standard
• Sulfate
• Earth's energy budget

External links

• United States National Research Council (2005), Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties, Board on Atmospheric Sciences and Climate

• Climate forcing
• Atmospheric radiation