Faint Young Sun Paradox: Part 1

There is something very strange and very interesting about the history of life on planet Earth. According to the Standard Solar Model, stars get hotter as they get older. When the Earth first formed, and after its surface cooled, it should have been a very cold world -- because the sun was about 30% cooler than it is today. Any water on the surface should have been frozen solid. Earth should have been covered in ice.
But as the sun grew warmer, the ice would melt, the oceans would fill, and the planet would start to resemble the world we know today.
But, there is a big snag in this simple scenario. Despite the cooler sun of this early era, there is evidence that liquid water did flow.
When the sun was much cooler billions of years ago, the Earth was warm enough to have liquid surface water. As the sun grew warmer, Earth also should have gotten warmer and, eventually, the water should have boiled off to space. But that’s not what happened.
Instead, as the sun grew hotter, planet Earth stayed about the same temperature - which was not too cold for liquid surface water, and not too hot for liquid surface water. This is an unexpected history.
This logical conundrum is known as the Faint Young Sun Paradox.
This video introduces the Faint Young Sun Paradox and describes how the sun and how the planet could have changed together over billions of years.