Warmth in the dark age
Lower reflectivity kept Earth from freezing under a fainter young sun
By Sid Perkins
Though the sun was so much dimmer billions of years ago that the young Earth should have been literally freezing, the planet remained largely covered with liquid water. That was thanks to a substantially darker surface and a dearth of light-scattering clouds, a new study suggests.
“All other things being equal, Earth should have been frozen over for the first half of its existence,” says James F. Kasting, a geoscientist at Pennsylvania State University in University Park who was not involved in the research. “But it wasn’t.”
Previously scientists have explained the presence of liquid water at that low-light time, during the Archean eon of geologic history, by suggesting that Earth’s atmosphere held large amounts of planet-warming greenhouse gases such as carbon dioxide and methane. But new analyses show that greenhouse gases weren’t dramatically higher then compared with today, a team of earth scientists reports in the April 1 Nature. The researchers now propose that early Earth stayed above freezing because the planet was darker then and therefore absorbed more of the sun’s energy — the same phenomenon that renders dark vinyl car seats scorching hot while light-colored seats stay relatively cool.
Early in the sun’s lifetime, the portion of solar core where the light- and heat-generating fusion reactions take place was much smaller than it is today. So, for an extended period, the sun could have been up to 30 percent dimmer than it is now, says Minik Rosing, a geologist at the University of Copenhagen’s Nordic Center for Earth Evolution. Although Earth’s surface temperature should have been well below freezing, geological signs of liquid water in that era abound — a puzzler that scientists have dubbed the “faint young sun paradox.”