The cooling of the Earth after a large sudden change in the solar insolation is buffered by the large heat reservoir in the ocean. For the present day temperature distribution it takes at least one decade to cool down the ocean to the freezing point of seawater. With a simple coupled climate model the influence of the radiative forcing expected to follow an extra-terrestrial impact or from extensive volcanic activity is determined. We show that the radiative forcing associated with an impact of a similar size to the one which occured at the K/T-boundary, has a significant influence on the climate system, and that the outcome of such an impact is critically dependent upon the initial thermal state of the ocean. Ultimately, such an impact could have led to a complete ice-covered Earth due to the combined effect from the decreased insolation and the ice-albedo feedback.