qSolar luminosity was much weaker
(~30%) in the early part of
Earth’s history (a faint young Sun).
qIf Earth’s albedo and greenhouse effect
remained unchanged at that time, Earth’s mean surface temperature
would be well below the freezing point of water during a
large portion of its 4.5 Byr history.
qThat would result in a “snowball” Earth,
which was not evident in geologic record.