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.
Evidence of the paradox comes from clues in the rock record that indicate the presence of flowing water as far back as the Archean eon, 3.8 billion to 2.5 billion years ago. Geologists have found ancient pillow lavas — knobby volcanic rocks that form only when lava erupts under water — and ripple marks etched by waves on sedimentary rocks. No such rocks are known from the earlier Hadean eon (SN: 5/19/12, p. 22) 4.5 billion to 3.8 billion years ago, but the chemistry of Hadean-aged zircon minerals recycled into younger rocks suggests that liquid water must have been present by at least 4.2 billion years ago.
By this time, much of the heat from Earth’s formation would have dissipated, so it couldn’t account for the warm temperatures. The only explanation is that some unknown factor helped warm the planet. The dilemma seems impossible to resolve because data on fundamental climate factors are missing for this primordial period, says planetary scientist Robin Wordsworth of the University of Chicago. “The Earth has such an active system that the evidence gets erased quickly.”
There has been no dearth of theories, however. Over the last 40 years, climate scientists have offered a range of explanations — everything from high concentrations of insulating greenhouse gases in the atmosphere to changes in Earth’s proximity to the sun. Some ideas are more plausible than others, but even the most probable hypotheses present roadblocks for scientists.
Still, as researchers continue to mine the ground for more geologic clues and refine their simulations of early Earth’s climate, they inch closer to answers.
“I’m rather confident that we can have a much clearer picture of what can solve the faint young sun problem in the next few years,” says Georg Feulner, a paleoclimate scientist at Germany’s Potsdam Institute for Climate Impact Research.