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?