You all - I have my own favored model which involves milder climates in the arctic and antarctic. <br /><br />1. The vast carbonate deposits in earth's crust. It is generally accepted that these were deposited by the geological carbon cycle, which involves liquid oceans and dissolved CO2 (= carbon dioxide) from the atmosphere, plus 'metalic' ions also in the oceans (Calcium, Sodium,...).<br /><br />Extrapolate back in time and we end up with an atmosphere like Venus today- without the sulphuric acid.<br /><br />2. Geology indicates a hot origin - too hot for liquid water at present atmospheric pressure - but perhaps not at the ancient atmospheric pressure.<br /><br />3. Catastrophism vs. uniformitarianism. I doubt the changes from that past to the present conditions were all gradual and uniform, either on earth or on Venus (which went in reverse direction compared with earth).<br /><br />4. Accretion at origin - Biblical model - Job 38:9. Early earth had thick clouds (perhaps including much dust, perhaps including carbon) producing gloom on earth's surface, perhaps even less light than Venus today.<br /><br />5. Condensation catastrophes, causing fossilization, among other things. As earth's crust cooled some water would condense, dissolving some CO2, lowering atmospheric pressure and lowering the boiling point and causing evaporation. That is a cycle in a process.<br /><br />Now, the above is partly theoretical, and must be modified by the geological evidence, of course.<br /><br />6. The last condensation catastrophe would be the Biblical flood of Noah'sday to which virtually all cultures testify in their ancient history. <br /><br />7. Before that catastrophe, earth's atmospheric (perhaps in the thermosphere, perhaps way beyond at La Grange points, etc.) water vapor was much thicker, and this produced a greater greenhouse effect which caused the polar climates to be tropical.<br /><br />8. Continental Drift- catastrophic version. The plates were sent in violent mot