qSolution 1:
Additional heat sources must have been presented
q Unlikely: The geothermal heat from the early Earth is sometimes suggested one such additional heat source
to warm Earth. However, the geothermal heat flux is not big enough to supply
the required energy.
qSolution 2: The planetary albedo must have been
lower in the past
q Unlikely: It would require a zero albedo to keep the present-day surface temperature with the 30% weaker
solar luminosity in the early Earth.
qSolution 3: Greenhouse effect must have been
larger
q Most Likely: The most likely solution to the
faint young Sun paradox is that Earth’s greenhouse effect was
larger in the past.
q But (1) why and (2) why that
stronger greenhouse effect reduced to the present-day strength?