The Hot Zone

A faint young sun

Posted by Alison Hawkes on May 4, 2010
Category : Climates of the Past

Some 3.8 billion years ago was a mystery that scientists have long attempted to solve. Way back then, the Earth was a completely different place, and so was the solar system. The sun shined with less luminescence — as much as 30 percent weaker — which meant the Earth should have been really cold. So cold, in fact, that liquid water would not have existed.

But the geologic record shows that water was, indeed, present and provided the foundation for the proverbial “primordial soup” that gave rise to life. How come? This is what’s called the “faint young Sun problem.”

There are many theories, among them that the Earth’s reflectivity was lower because of smaller continents, allowing more sunlight to be absorbed.  But one of the leading theories examines the atmosphere of the Archaean period, specifically the presence of greenhouse gases like carbon dioxide and methane that might have warmed the atmosphere to temperatures at or above today’s.

The same greenhouse gases that, in abundance, are getting us into trouble today, may have been fundamental to the Earth’s life-creating conditions. As geochemist James Kasting of Penn State University points out in Chapter 8, The primitive Earth of Prebiotic Evolution and Astrobiology, methane and carbon dioxide should have been abundant in the first several hundred million years of Earth’s history because of degassing during the planet’s formation.

The concentrations would have declined over time, methane converting to carbon dioxide, and carbon dioxide converting into carbonate rocks. But the storage of carbon in rocks would have slowed with dropping temperatures. Meanwhile, continually-spewing  volcanoes would have then provided the carbon dioxide boost to the atmosphere to spike temperatures back up again. The feedback loop, called the carbonate-silicate cycle, goes on to this day.

Clearly more went on than we know. CO2 concentrations would have had to be about 10 times higher than today’s values, an unlikely scenario given that past research estimates the concentration no larger than three times that of today. Methanogens, bacteria-like organisms that lived in oceans and marine sediments, may have gassed enough methane to make up at least some of the difference.

Why does this matter to modern day climate change? “One thing that paleoclimate research definitely does do is to put modern day climate change into perspective,” says Kasting.

Let’s compare the numbers. A 30 percent change in solar luminosity, up to today’s levels, corresponds to an extra 80 watts/m2 in atmospheric heat. Every doubling of anthropogenic CO2 amounts to an extra 4 watts/m2. If we get up to 12 watts/m2, well within our capabilities, we’re staring down a temperature change of between 6 to 12 degrees C. That’s still nowhere near the sun’s radiative forcing, but also not that far off.

Early Earth needed CO2 to warm up, but we sure don’t anymore. There can be too much of a good thing.

– Alison Hawkes

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