Radiative forcing

Radiative forcing  ΔF is defined by the IPCC (Intergovernmental Panel on Climate Change) as the change in net,  downward minus upward, irradiance (expressed in Watts per square metre, W/m2) at the tropopause (boundary between the  troposphere and the stratosphere). Radiative forcing is due to a change in an external driver of climate change, such as, for example, a change in the concentration of carbon dioxide or the output of the Sun. Radiative forcing is computed with all tropospheric properties held fixed at their unperturbed values, and after allowing for stratospheric temperatures, if perturbed, to readjust to radiative-dynamical equilibrium. Radiative forcing is called instantaneous if no change in stratospheric temperature is accounted for.

The IPCC defines radiative forcing relative to the year 1750 (the approximate start of the Industrial Revolution) as standard. A positive forcing (more incoming energy) tends to warm the Earth, while a negative forcing (more outgoing energy) tends to cool it. A positive change in net irradiation can be caused, for instance, by more intense insolation (radiation by the Sun), or by a carbondioxide increase in the atmosphere.

Reference
Annex I User guide and access to more detailed information