CHANGES in the surface temperature of the Earth throughout its history are important for understanding both the geological development of the Earth's surface and the origin and development of life on Earth. The calculation of variations in the surface temperature with time is complicated by the number and nature of the physical variables involved, as well as by the long period of time over which extrapolation is required. Theoretical studies must be carried out in terms of highly simplified global atmospheric models with fairly lax boundary conditions. Consequently, even the gross features of atmospheric evolution may be explicable in terms of more than one theoretical model. Here we take the model most widely referred to in recent years and point out that a radically different approach can provide an equally supportable result. Too much significance should not, therefore, be read into the details of currently constructable models.