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The fluctuation-dissipation theorem of statistical mechanics, proved in this note for a system with two quadratic integrals of motion such as that of two-dimensional or geostrophic turbulence, relates, the mean response to impulsive external forcing of a dynamical system to its natural unforced variability. The utility of this theorem in providing an estimate of the response of climatic means to changing external influences is discussed.
C. E. Leith (Wed,) studied this question.