Understanding the Faint Young Sun Paradox
Since the mid-1960s, scientists have argued about an apparent contradiction in our understanding of the early years of planet Earth. At issue was the overwhelming evidence for a warm Earth in its first billion-ish years, alongside evidence that the Sun would have been something like 25% dimmer then than it is today. How could Earth be warm if our star was dim? This is known as the faint young sun paradox.
One possible explanation has to do with greenhouse gases, and that's what Carl Sagan and George Mullen suggested in 1972. Over the decades since, multiple models have been developed that could explain the contradiction, though it's hard to be certain which (if any) of our current theories is correct.
In the short video below, MinuteEarth digs into the problem. Have a look:
If video isn't your thing, this National Geographic article is a good summary.