In climate dynamics the effect of internally generated fluctuations is usually described by augmenting the balance equations through the addition of random forces. In this paper the properties of these forces are investigated. A fluctuation-dissipation theorem relating their covariance matrix to the phenomenological coefficients such as eddy diffusivity is proposed. The theorem is subsequently used to identify the statistical properties of the climatic variables themselves, and thus to characterize climatic variability from the standpoint of the statistical theory of irreversible processes. Applications to a simple thermal convection problem and to a zonally averaged energy-balance model are presented; the possibility of experimental verification is discussed.