Dial back the clock nearly 4 billion years, to a time called the Archean, and the sun would appear about 30% dimmer than it is now. That's a problem: It couldn't have warmed Earth enough to keep the seas from becoming permanent ice sheets. Yet overwhelming geological evidence indicates that liquid water has existed on our planet since the seas formed more than 4 billion years ago, even during the deepest ice ages. What could have provided the added warmth?
In 1972, famed astronomer Carl Sagan proposed that the answer lay in the atmosphere. Sagan and his co-author George Mullen hypothesized that carbon dioxide (CO2) concentrations were much higher during the Archean—possibly 100 times higher than they are today—and therefore the atmosphere could retain enough heat to keep the planet from freezing. But so far, no one has found convincing data that Earth was once a super greenhouse. And now researchers have uncovered strong evidence to the contrary.
A team led by earth scientist Minik Rosing of the University of Copenhagen analyzed iron-bearing rocks in southwestern Greenland that were 3.8 billion years old. They focused on two minerals, magnetite and siderite, that can provide a bellwether of the CO2 concentrations in the atmosphere. Too much CO2, and magnetite can't form, whereas the opposite is true for siderite. Based on the ratio of the minerals, the team reports in tomorrow's issue of Nature that CO2 levels during the Archean could have been no higher than about 1000 parts per million—about three times the current level of 387 ppm and not high enough to compensate for the weak sun.
The results were "very surprising," Rosing says. As to the question of what kept the planet warm instead of CO2, he says his research points to two possibilities. First, Earth's land masses were much smaller billions of years ago, meaning that the oceans, which generally are darker than continents, could absorb more of the sun's heat. Second, because life was brand new, organisms were manufacturing little of the gases that help clouds form. So, more sunlight reached the surface.
There are bound to be other factors, Rosing says. "I think that our paper is just one link in a long chain of further refinements of our understanding of the early Earth and of the dynamics of our planet."
Earth scientist James Kasting of Pennsylvania State University, University Park, thinks it's premature to discard the greenhouse-gas option. Temperatures during the Archean were at least as high as they are today, despite the weaker sun, he says. It's difficult to account for the warmth using just the mechanisms suggested in this paper, Kasting says. "So, I think there is still a need for additional greenhouse gases."