The early Sun had a luminosity of some 30 per cent less than now and, over time, luminosity has increased in a steady state. The low lumunosity of the early Sun was such that the Earth's average surface temperature would have been below 0°C from 4500 to 2000 million years ago. But, there is evidence of running water and oceans as far back as 3800 million years ago. This paradox is solved if the Earth had an enhanced greenhouse with an atmosphere of a lot of carbon dioxide and methane.
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