THE EFFECTS OF REDUCED LAND FRACTION AND SOLAR FORCING ON THE GENERAL-CIRCULATION - RESULTS FROM THE NCAR CCM

Land fraction and the solar energy at the top of the atmosphere (solar constant) may have been significantly lower early in Earth's history. It is likely that both of these factors played some important role in the climate of the early earth. The climate changes associated with a global ocean(i.e. no continents) and reduced solar constant are exami ned with a general circulation model and compared with the present-day climate simulation. The general circulation model used in the study i s the NCAR CCM with a swamp ocean surface First, all land points are r emoved in the model and then the solar constant is reduced by 10% for this global ocean case. Results indicate that a 4 K increase in air te mperature occurs with the global ocean simulation compared to the cont rol. When solar constant is reduced by 10% under global ocean conditio ns a 23 K decrease in air temperature is noted. The global ocean warms much of the troposphere and stratosphere, while a reduction in the so lar constant cools the troposphere and stratosphere. The largest cooli ng occurs near the surface with the lower solar constant. Global mean values of evaporation, water vapor amounts, absorbed solar radiation a nd the downward longwave radiation are increased under global ocean co nditions, while all are reduced when the solar constant is lowered. Th e global ocean simulation produces sea ice only in the highest latitud es. A frozen planet does not occur when the solar constant is reduced- rather, the ice line settles near 30-degrees of latitude. It is near t his latitude that transient eddies transport large amounts of sensible heat across the ice line acting as a negative feedback under lower so lar constant conditions keeping sea ice from migrating to even lower l atitudes. Clouds, under lower solar forcing, also act as a negative fe edback because they are reduced in higher latitudes with colder atmosp heric temperatures allowing additional solar radiation to reach the su rface. The overall effect of clouds in the global ocean is to act as a positive feedback because they are slightly reduced thereby allowing additional solar radiation to reach the surface and increase the warmi ng caused by the removal of land. The relevance of the results to the ''Faint-Young Sun Paradox'' indicates that reduced land fraction and s olar forcing affect dynamics, heat transport, and clouds. Therefore th e associated feedbacks should be taken into account in order to unders tand their roles in resolving the ''Faint-Young Sun Paradox''.