Four billion years ago, when the sun was young and dim, Earth may have stayed warm in part because it was spinning faster.
The sun, astronomers believe, wasn’t always as bright as it is today. Four billion years ago it was probably about 30 percent dimmer. Although the sun had the same mass then as it does now, it was a little larger and less dense. Over the eons, as it contracted under its own gravity, it grew hotter and brighter. Astronomers have no problem with this picture--it seems to be the typical way stars evolve--but geologists do. If the young sun was so feeble, Earth should have been a frozen wasteland. And yet the oldest sedimentary rocks--which were deposited in water--are themselves 4 billion years old. Somehow Earth’s surface was warm enough for liquid water despite a wan sun.
Geologists have weaseled out of this faint young sun paradox by assuming that hyperactive volcanoes erupted vast clouds of heat-trapping carbon dioxide into the atmosphere, warming the young Earth. Some models call for carbon dioxide levels 1,000 times higher than today’s. But to Gregory Jenkins, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colorado, that solution has always seemed ad hoc. You have to put so much carbon dioxide into climate models in order to get an effect that it doesn’t seem realistic, he says. There is no direct evidence to show that carbon dioxide levels were ever a thousand times higher.
Jenkins thinks he has a better way out of the paradox. He and two colleagues at the University of Michigan, Hal Marshall and William Kuhn, propose that Earth’s rapid rotation rate and the absence of large landmasses 4 billion years ago were the keys to the planet’s warmth.
Four billion years ago Earth may have spun on its axis at least once every 14 hours. (Since then the moon’s tidal drag has gradually slowed the planet’s rotation and thereby lengthened its days.) Jenkins and his colleagues have constructed a sophisticated computer model to study the effects the revved-up rotation would have on Earth’s climate. The model allowed them to follow the movements of air masses in three dimensions-- something that had never been done before for early Earth. None of the previous studies had looked at how the entire system would be changed under different rotation rates, says Jenkins. There was a big gap.
In their model, Jenkins and his colleagues used a 14-hour day and also assumed that Earth had no land. That is not a bad approximation, says Jenkins: most landmasses formed less than 3.5 billion years ago. Before then, Earth’s surface was probably one vast ocean dotted with volcanic islands.
This landlessness itself would have made the planet warmer; water reflects less sunlight than land and absorbs more, which means it has more energy to radiate to the air as heat. What surprised Jenkins and his colleagues, though, was the dramatic effect of the rapid rotation rate. On their fast-spinning model Earth, most storms and clouds were confined to the equatorial and subtropical regions, and global cloud cover was 20 percent less than today’s.
There were fewer clouds in temperate latitudes, Jenkins explains, because the rapid rotation rate was making it difficult for warm, moisture- laden air to spread from the tropics--where evaporation is greatest--toward the poles. Through the so-called Coriolis effect, Earth’s rotation deflects moving air to the right in the Northern Hemisphere and to the left in the Southern. The faster the rotation, the greater the deflection. With Earth rotating once every 14 hours, Jenkins’s model suggests, moist, cloud- forming air would get deflected back toward the equator even before it left the tropics.
And with clear skies over more of the planet, more sunlight would have reached the surface. Jenkins and his co-workers did include some carbon dioxide in their model, but the amount was only eight times higher than current levels, a tremendous reduction from the standard scenario. Even without a massive greenhouse effect, their model suggests, and even in the face of a faint young sun, the fast young Earth would have been warm enough to keep its ocean from freezing. In fact, it would have been about 10 degrees warmer than it is today.