The faint young sun climatic paradox: A simulation with an interactive seasonal climate-sea ice model

A seasonal one-and-a-half-dimensional (1 1/2-D) energy-balance climate model including a detailed sea ice calculation and an interactive albedo formulation has been developed and is used to investigate the faint young sun climatic paradox. This model is shown to reproduce the present climate and sea ice observations. In spite of its greater complexity, its behavior is globally similar to simple energy-balance models with highly parameterized ice-albedo feedback used in previous studies of this question. It is found that when the solar luminosity drops by more than about 5% below its present value, the ice albedo feedback causes a global irreversible glaciation. Several sensitivity experiments show that the value of the critical solar constant and associated global surface temperature are only little sensitive to the set of model parameters describing the ice and snow albedo and meridional heat transport. In contrast, the absence or polar location of the continental mass introduce a nearly 10% decrease of the critical luminosity. The minimum level of atmospheric CO2 needed to prevent a global glaciation through enhanced greenhouse warming is calculated as a function of the solar luminosity. A 30% drop in solar output requires a 2 x 10(4)-fold increase in atmospheric CO2, an unacceptably large value. However, in the absence of continents, a carbon dioxide partial pressure of 2000 times the present level is found to be sufficient to stabilize the climate. The effects of a reduced continental area, paleogeographic changes and higher CO2 greenhouse effect combine to ensure a larger stability of the non-frozen configuration. Their cumulated and interactive effects may be able to solve the young sun paradox.