The accuracy of radiation parameterizations is not well-known under one widely-used perturbation: concentrations of carbon dioxide quadrupled from preindustrial concentration, which has been used to provide strong forcing to determine the climate sensitivity of coupled atmosphere-ocean models.
We examine the behavior of the radiation parameterizations of many climate models participating in the CMIP ensemble under 4xCO2 conditions. The parameterizations are compared to reference line-by-line models thought to be absolutely accurate to within 1 W/m2. We examine four diverse base atmospheres from the present-day drawn from the Continual Intercomparison of Radiation Codes for which the accuracy of each parameterization is already known. Fluxes are recomputed for each base case assuming only that CO2 concentrations have quadrupled. We investigate the accuracy of the parameterizations' forcing estimate relative to the reference model examining, among other things, the degree to which accuracy in present-day flux indicates accuracy in forcing from increased CO2, and the dependence of forcing accuracy on temperature and humidity structure.