Faint Young Sun
Scientists struggle to understand how early Earth stayed warm enough for liquid water
By Erin Wayman
Here’s a climate puzzle — one that goes back to Earth’s infancy some 4.5 billion to 2.5 billion years ago. The sun was much dimmer back then. Far less solar radiation reached the planet. Earth should have been a frozen wasteland. But all geologic signs point to a young planet awash in liquid water, with the first life-forms emerging. Scientists call this conundrum the “faint young sun paradox.”
Carl Sagan and George Mullen identified the paradox in 1972. By then, researchers had determined that a newborn star’s brightness gradually increases over time as hydrogen atoms in the star’s core fuse into helium. Working backward, researchers estimated that the sun generated 20 to 30 percent less energy during the first half of Earth’s history than it does now.