Changes in the concentration of greenhouse gases within the atmosphere lead to changes in radiative fluxes within the atmosphere and at its boundaries. This paper describes an experiment within the Radiative Forcing Model Intercomparision Project that uses benchmark calculations made with line-by-line models to identify parameterization error in this quantity. The instantaneous forcing to which the world has been subject is computed using a set of 100 profiles, selected from a re-analysis of present-day conditions, that represent the global annual mean forcing with sampling errors of less than 0.01 \si{\watt\per\square\meter}. Agreement in estimates of forcing among six contributing line-by-line models is excellent, with standard deviations typically less than 0.025 \si{\watt\per\square\meter}, suggesting that parameterization error will be readily resolved. The impact of clouds on this forcing is estimated using diagnostic calculations across a range of climate models, while adjustments due to stratospheric temperature re-equilibration are estimated assuming fixed dynamical heating.