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Liquid water has flowed on Earth for some 3.8 billion years, since not long after the planet formed. The evidence comes from rocks that date from that period which seem to have formed under the action of water.

But this presents palaeontologists and geologists with a problem. At that time, the Sun was some 30 per cent dimmer than it is today and would not have provided enough heat to keep water liquid on the surface.

This is known as the faint young Sun paradox and it has puzzled scientists since the 1970s when astronomers first pointed it out. But it didn’t really worry anybody. The obvious solutions are that the Earth was warmer because it reflected less heat from the Sun, it had a lower albedo, or that it was victim of a runaway greenhouse effect. One of these must be right but nobody was sure which.

But last year, a group of researchers claimed to have solved the paradox. They said that the make up of rocks from that time exclude the possibility that the atmosphere was rich in a greenhouse gas such as methane or carbon dioxide.

Instead, the Earth must have had a lower albedo and therefore must have absorbed more heat from the Sun than it does today. The lower albedo, they argued, was the result of fewer biological particles in the atmosphere. These nucleate water droplet formation. So without them there would be fewer clouds and less sunlight reflected into space.

These guys published their solution in Nature and the problem was thought to have been solved. (We looked at another mechanism that may have prevented cloud formation in the early atmosphere about a year ago.)

But today Colin Goldlatt and Kevin Zahnle at NASA’s Ames Research Center in Moffett Field re-ignite the controversy.

They’ve looked at this problem again and studied the effect of fewer clouds. They say that however you do the numbers, this could not have made the Earth hot enough to allow the existence of liquid water.

Clouds have two effects. In general, high clouds trap heat while low clouds reflect it. “Therefore the absolute upper bound on warming by decreasing cloud reflectivity would be found by removing low clouds entirely,” they say.

When you do that in a computer model of the Earth’s early climate, you get no more than half the heating necessary to maintain liquid water on the surface.

“We show that, even with the strongest plausible assumptions, reducing cloud and surface albedos falls short by a factor of two of resolving the paradox,” say Goldlatt and Zahnle.

So the paradox is alive and well; and more puzzling than ever. Last year we discovered that a greenhouse effect can’t explain the paradox. Now we know that a lower albedo wouldn’t have done the trick either.

So the race is back on to nail this problem once and for all. Get your thinking caps on.

Ref: arxiv.org/abs/1105.5425: Faint Young Sun Paradox Remains

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