PRECAMBRIAN CLIMATE - THE EFFECTS OF LAND AREA AND EARTHS ROTATION RATE

Rotation rate and land area have changed significantly over the Earth's history. The associated climate changes are examined with a general circulation model (GCM) and compared with the current climate state. Unlike previous GCM studies, clouds in the model interact with the radiation field. We show that rotation rate changes can affect mean air temperatures by strongly affecting the cloud field. For example. an Earth day length of 14 hours causes the global mean cloud fraction to decrease 20%. There are also large changes in the wind field; indeed, at some mid-latitudes the time-averaged surface wind changes from westerlies to easterlies. If the land masses are removed from the model and with present-day rotation, the global mean air temperature is 4 K higher than for the present-day simulation: if in addition the day length is 14 hours, the global mean air temperature is 5.5 K. Thus the early atmosphere may not have contained the very large concentrations Of CO2 thought to be necessary to prevent a frozen planet higher.