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. 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. q      Most Likely: The most likely solution to the faint young  Sun paradox is that Earth’s greenhouse effect was larger in the past.