Estimating 2 × CO2 warming in an aquaplanet GCM using the fluctuation‐dissipation theorem

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Abstract
A statistical analysis based on the fluctuation‐dissipation theorem (FDT) is applied to estimate a model response to a small external forcing. An atmospheric GCM (the NCAR CCM3) coupled to an oceanic upper mixed layer is used to produce a long equilibrium run with 1 × CO2 concentration. Exploiting the FDT, we use the statistical properties of the internal variability of the system to approximately restore the linearized operator of the system. The modes of this operator are analyzed and compared with those calculated using the approach based on a perturbation method employed earlier by one of the authors. The sensitivity of the model to 2 × CO2 forcing calculated using the FDT approach compares well with the results obtained in an actual 2 × CO2 run.