Fluctuation dissipation theorem in a general circulation model

We use the Fluctuation Dissipation Theorem (FDT) as formulated by Leith [ 1975] and Bell [ 1980] to asses the climatic sensitivity of the Whole Atmosphere Community Climate Model (WACCM). The lagged covariance matrix is evaluated from the "natural'' (unperturbed) variability of the model. Perturbation in both the carbon dioxide content and the solar constant are used to obtain the response function from which the sensitivity matrix is calculated for different level and at the surface of the model. We show that even if the perturbed model does not reach equilibrium the FDT gives reliable values for the climate sensitivity for both the solar radiative and the carbon dioxide forcing.