Stellar evolution models predict that the solar luminosity was lower in the past , typically 20-25 \% lower during the Archean ( 3.8-2.5 Ga ) .
Despite the fainter Sun , there is strong evidence for the presence of liquid water on Earth ’ s surface at that time .
This “ faint young Sun problem ” is a fundamental question in paleoclimatology , with important implications for the habitability of the early Earth , early Mars and exoplanets .
Many solutions have been proposed based on the effects of greenhouse gases , atmospheric pressure , clouds , land distribution and Earth ’ s rotation rate .
Here we review the faint young Sun problem for Earth , highlighting the latest geological and geochemical constraints on the early Earth ’ s atmosphere , and recent results from 3D global climate models and carbon cycle models .
Based on these works , we argue that the faint young Sun problem for Earth has essentially been solved .
Unfrozen Archean oceans were likely maintained by higher concentrations of CO _ { 2 } , consistent with the latest geological proxies , potentially helped by additional warming processes .
This reinforces the expected key role of the carbon cycle for maintaining the habitability of terrestrial planets .
Additional constraints on the Archean atmosphere and 3D fully coupled atmosphere-ocean models are required to validate this conclusion .