Early Earth's climate: Cloud feedback from reduced land fraction and ozone concentrations

Two features of early Earth—reduced ozone (O₃) concentration and land fraction are investigated with a general circulation model (GCM). These features are components of a paradox (Faint‐Young Sun paradox) which has intrigued researchers for more than two decades. In this study, land fraction and O₃ concentrations are uniformly reduced by 100 percent. The reduction in O₃ takes place in the troposphere and stratosphere with all other variables held constant including present‐day land fraction. Two sensitivity tests under global ocean conditions are reported: one case with implied oceanic poleward transports of heat, the other case with no implied oceanic poleward transports of heat. The results show that the removal of land under present‐day conditions increases cloud fractions and cool surface temperatures, unless heat is transported poleward by oceans. In a third sensitivity test with zero O₃ concentrations, global mean air temperatures are increased by 2 K because of an increase in upper tropospheric and lower stratospheric clouds. 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.