A Solution to the Faint Young Sun Paradox

When it comes to the origin of life on Earth some four billion years ago, there’s a problem. At that time, the young Sun was approximately 75 percent dimmer than it is now. That would have made the Earth significantly colder, in fact, too cold for liquid water.

However, we know that liquid water is essential for life and we know from the fossil record that life existed on Earth at the time. Liquid water must have been present. So what was keeping the water warm?

This problem, known as the faint young Sun paradox, has troubled astronomers since the 1970s, when it was pointed out by Carl Sagan and friends. He proposed that the Earth’s atmosphere at that time must have been rich in carbon dioxide and that the consequent greenhouse effect was responsible for the warming. Other evidence, however, suggests that the atmosphere could not have had enough CO2 to do the trick. The arXiv Blog has looked at other possible solutions in the past too.

Today, Christoffer Karoff at the University of Birmingham and a mate make a new suggestion based on their study of kappa Ceti, a star some 30 light years away in the constellation of Cetus which is very much like our Sun as it would have been four billion years ago.

It turns out that Kappa Ceti is little more interesting than astronomers once thought. This young star, says Karoff, produces flares and coronal mass ejections at a rate that is three orders of magnitude greater than our Sun today. The implication, of course, is that our Sun must have been just as active when it was the same age as kappa Ceti (about 700 million years old).

But so what? How can coronal mass ejections have made the Earth hotter? The answer lies in a phenomenon known as the Forbush decrease, after the astronomer Scott Forbush who studied galactic comsic rays in the 1930s and 1940s.

Forbush discovered that the number of galactic cosmic rays hitting Earth drops by up to 30 per cent within a day or so of the Sun producing a coronal mass ejection. The reason is that these ejections are giant clouds of ionised gas enveloped in powerful magnetic fields. These fields simply steer the cosmic rays away from Earth.

So if the early Sun was producing far more coronal mass ejections, far fewer cosmic rays would have arrived on Earth.

And that’s where another idea comes into play. In recent years, various climatologists have speculated that cosmic rays seed the formation of clouds in the lower atmosphere. The idea is that they ionise molecules and dust particles which then become focal points for droplets to condense on.

So fewer cosmic rays lead to fewer clouds. There is even some evidence that cloud cover drops during a Forbush decrease, although it’s fair to say there is some dispute over this.

So Karoff’s thinking goes like this. More coronal mass ejections in Earth’s past lead to fewer cosmic rays hitting Earth which lead to less cloud cover. Less cloud cover meant that less sunlight would have been reflected back into space which would have allowed the surface to heat up.

And that’s what kept water liquid on the Earth’s surface four billion years ago.

Got that?

Ref: arxiv.org/abs/1003.6043: How Did The Sun Affect The Climate When Life Evolved On The Earth? - A Case Study On The Young Solar Twin Kappa Ceti