Ancient climates: investigation with climate models

Mathematical models of the climate system have recently become an important aspect of the study of ancient climates. Three geologic climate problems have received particular attention: (i) the faint early Sun paradox; (ii) the warm apparently ice-free Cretaceous period and (iii) the glacial-interglacial cycles of the last 700000 yr. There are three basic limitations in investigating geologic climate problems: (i) limitations in reconstructing the climate state; (ii) limitations in specifying the forcing factors which influenced the climate and (iii) limitations in determining the climate sensitivity to specific forcing factors. The three climate problems which are described here are widely different in terms of the degree to which the climatic record can be reconstructed, the importance of specific factors and in the types of models which have been applied to investigate climate sensitivity. Each time period is sufficiently different in terms of the climatic record and in terms of the evidence for a particular forcing factor to be a well-posed climate modelling problem. For the most part a simulation or replica of these climates is not yet possible, but based on sensitivity experiments there has been a convergence toward understanding the climatic response forcing factors and describing the causes of climatic change during Earth history.