Phenomena related to the natural carbon cycle as the 14C distribution between atmosphere and ocean and the atmospheric response to the input of fossil fuel CO2 and of 14C produced in nuclear weapon tests have been quantitatively discussed by other authors using box models. However the exchange coefficients derived from the natural 14C distribution do not agree with those valid to describe the short-term phenomena. A model consisting of a well mixed atmospheric box coupled to a long-term biosphere, of an ocean surface box and a diffusive deep ocean is discussed. The dynamic parameters were derived from the preindustrial 14C distribution in atmosphere and ocean. A consistent description of phenomena with completely different characteristic times is possible, because in the box diffusion model the flux from mixed layer to deep sea increases for decreasing time constants of the perturbations. This is in contrary to box models where it is essentially independent of the time constants if they are smaller than a few hundred years. Due to this fact our model is valid for predictions of the atmospheric CO2 response to the various possible future CO2 input time functions.