THE FAINT YOUNG SUN CLIMATIC PARADOX - INFLUENCE OF THE CONTINENTAL CONFIGURATION AND OF THE SEASONAL CYCLE ON THE CLIMATIC STABILITY

A quasi-three-dimensional climate model is used to study the early sta te of the Earth when the solar luminosity was 70% of the present value , Usually, climatic simulations going back to this period lead to a co mpletely frozen planet contrasting with the geologic evidences of sedi mentary rock formation and thus of the presence of liquid water at the surface of the continents during the Archean (4.6-2.5 billion years b efore present). Here, several model simulations are performed for sola r luminosities varying between 0.7 and 1 times the present value. Usin g the present-day continental configuration and taking the seasonal cy cle into account, a steady state is found in which glaciation is compl ete but snow covers only some oceanic coasts, leaving the continents e ssentially snow-free. As a result, the albedo of the continental area is strongly reduced compared to that of the frozen ocean, Some contine ntal temperatures can almost reach the freezing point of water in summ er (-1 degrees C in the center of Eurasia). This result can be explain ed by the behavior of the detailed hydrologic cycle included in the mo del. During the decrease of the solar luminosity, the jump to a comple tely frozen Earth occurs when the solar luminosity reaches 0.86 times its present value. The behavior of the climatic system is substantiall y different with a global ocean configuration. In the absence of land surfaces, the meridional heat transport, explicitly calculated, is les s effective and the glaciation of a model latitude zone does not lead to the glaciation of its equatorward neighbor. The climate instability is relatively local and the jump to the completely frozen state is mu ch more progressive than in the case of the modem continental configur ation. The role of the seasonal cycle in the paleoclimatic simulation is also studied. Due to the non-linearity of the model, removing the s easonal cycle drives the system to an increase of the annual mean plan etary albedo and to a decrease of the relative value (0.82) of the cri tical solar luminosity at which the jump to the completely frozen solu tion occurs.