Radiative forcing

Definition

Let $N$ be the net downward radiative flux at the top of the atmosphere, in $\mathrm{W/m^2}$ . It is absorbed solar minus outgoing infrared radiation, averaged over the globe and over a year. Consider an imposed change of an external factor of the climate system. Examples are the concentration of a greenhouse gas, the amount of aerosols or the solar output. The radiative forcing $\Delta F$ of the change is the resulting change of $N$ , with the global mean surface temperature held at its prior value.

Variants differ in what else is held fixed:

the instantaneous forcing holds the whole state of the atmosphere fixed;
the stratospheric-adjusted forcing lets the temperature of the stratosphere adjust to radiative equilibrium, and measures the flux at the tropopause;
the effective radiative forcing lets the atmosphere and land adjust, as computed in a climate model, with the sea surface temperature held fixed.

Remarks

Example: a widely used fit for carbon dioxide at concentration $c$ , relative to a reference $c_0$ , is $\Delta F = 5.35 \ln(c / c_0) \ \mathrm{W/m^2}$ . For a doubling, $c / c_0 = 2$ , it gives $\Delta F = 5.35 \ln 2 \approx 3.7 \ \mathrm{W/m^2}$ . The logarithm reflects the saturation of the strongest absorption bands of carbon dioxide.

Non-example: the extra infrared emission of a planet that has already warmed is not a forcing but part of the response, since the definition holds the surface temperature fixed.

A positive forcing makes $N > 0$ , so the climate system gains energy and warms until $N$ returns to $0$ .

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Climate model concept
Radiative forcing definition

Tag	`EWJNRCTB9Z`
URI	`m2://EWJNRCTB9Z`
Kind	definition
Writing	W0 stub
MSC	86A08, 86A10
Defines	Radiative forcing
Uses	Climate model
Changed	3 November 2026

Radiative forcing

Definition

Remarks

Reading path

Further reading

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