EARLY EARTH CLIMATE - CLOUD FEEDBACK FROM REDUCED LAND FRACTION AND OZONE CONCENTRATIONS

Two features of early Earth---reduced ozone (O-3) concentration and la nd fraction are investigated with a general circulation model (GCM). T hese features are components of a paradox (Faint-Young Sun paradox) wh ich has intrigued researchers for more than two decades. In this study , land fraction and O-3 concentrations are uniformly reduced by 100 pe rcent. The reduction in O-3 takes place in the troposphere and stratos phere with all other variables held constant including present-day lan d fraction. Two sensitivity tests under global ocean conditions are re ported: one case with implied oceanic poleward transports of heat, the other case with no implied oceanic poleward transports of heat. The r esults show that the removal of land under present-day conditions incr eases cloud fractions and cool surface temperatures, unless heat is tr ansported poleward by oceans. In a third sensitivity test with zero O- 3 concentrations, global mean air temperatures are increased by 2 K be cause of an increase in upper tropospheric and lower stratospheric clo uds. The clouds enhance the greenhouse effect within the troposphere, increasing downward longwave radiation to the surface, melting sea ice and snow. Similar studies using radiative-convective models which do not include interactive clouds do not show such surface warming.