Early Earth’s Atmosphere

Today’s post will discuss a topic I’ve mentioned previously–the faint young sun paradox. This is the issue arising from there being evidence for liquid water during the Archean eon (3.8 to 2.5 billion years ago) even though the sun was about 25% dimmer (due to its younger age). So how can these two things possibly work together?
The usual solution is that the atmosphere of the early Earth had more carbon dioxide, which in tandem with other greenhouse gases, warmed the planet enough for liquid water. That brings us to today’s article, Faint young Sun problem more severe due to ice-albedo feedback and higher rotation rate of the early Earth, by Hendrik KienertGeorg Feulnerand Vladimir Petoukhov. Don’t worry if you do not know all those terms; I will explain them in a minute. Published during 2012 in the American Geophysical Union’s Geophysical Research Letters, this paper uses a three-dimensional model to look at the early Earth, while most other models have used only one-dimensional models.
The geologic time scale. Taken from Portrait of a Planet, page 443.
The geologic time scale. Taken from Portrait of a Planet, page 443.
The three dimensional model took into account the effects of the ocean, atmosphere, and the faster rotation rate of Earth during the Archean (1.4 to 1.5 times faster than today). This model started as an ice-free version and was allowed to run until it reached equilibrium after about 5000 years of simulated time. The authors modeled various amounts of carbon dioxide in the an atmosphere with 0.8 bar nitrogen partial pressure. Partial pressure is the pressure that would be exerted by a particular gas within a mixture; a bar is a unit of pressure, with sea-level atmospheric pressure just over one bar. 
The result was that 0.4 bar partial pressure from carbon dioxide was needed to keep liquid water on the modeled Earth. They noted that the albedo, a measure of how much sunlight is reflected away, was larger than present day values. This points to the noted ice-albedo feedback mentioned in the title of the paper. Ice and snow both have high albedo. Since they are more reflective, more sunlight is reflected away; it is cooler, and this leads to more ice and snow, that reflects even more and makes it even cooler, and so on. This is a positive feedback loop (feedbacks are ‘positive’ if they are self-reinforcing in this way, while ‘negative’ feedback loops will trend towards a steady state without further change).
The other part of the title is the rotation rate of Earth. Notably faster in the Archean, the rotation rate in this study had a slight cooling effect. Together with the ice-albedo feedback, they seem to only exacerbate the paradox of the faint young sun. We do have estimates of the amount of carbon dioxide in the Archean, but the modeled carbon dioxide partial pressure needed to keep liquid water on the surface is above what these records indicate. Clearly, the paradox is not solved here. It is likely that other greenhouse gases played a significant role, including methane, and the nitrogen partial pressure may have been higher, which would also impact the results. There may never be a certain solution to the faint young sun paradox, but people will continue to look into the possibilities.

 

Bibliography
Kienert, H., G. Feulner, and V. Petoukhov (2012), Faint young Sun problem more severe due to ice-albedofeedback and higher rotation rate of the early  Earth, Geophys. Res. Lett., 39, L23710, doi:10.1029/2012GL054381.
Today’s main article.
Marshak, Stephen. Earth: Portrait of a Planet. 3rd ed. W.W. Norton & Company. Print.
My geology textbook, used for the diagram of the geologic time scale as well as background on albedo.
National Oceanic and Atmospheric Administration. What are positive feedbacks? NOAA Paleoclimatology. 20 Aug. 2008. Web.
A short page discussing feedback loops, including the ice-albedo feedback as an example. Aimed at non-scientist audiences, it is easy to understand.
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