Modeling Early Earth

Today we are once again returning to the Archean. The geological era from 3.8 to 2.5 billion years ago, the Archean saw the appearance of life, making it rather interesting from a scientific perspective.
As one might imagine, there are not a lot of rocks left from so long ago, but the geological record does indicate that the Archean had periods of extended glaciation, between 2.45 and 2.22 billion years ago, and possibly at 2.9 billion years ago–all near the end of the era. This is perhaps not surprising given that the sun was twenty to twenty-five percent less bright than today, which logically would lead to a colder Earth. However, the records we do have indicate that for the most part, the early Earth was not covered in ice, but had liquid water. This is a bit strange; given the conditions of modern Earth but with the fainter sun, Earth would result in a state of global glaciation–a “snowball Earth”. This has been dubbed the faint young sun paradox.
The geologic time scale. Taken from Portrait of a Planet, page 443.
The geologic time scale. Taken from Portrait of a Planet, page 443.
This brings us to today’s article, “Exploring the faint young Sun problem and the possible climates of the Archean Earth with a 3‐D GCM” by B. Charnay, F. Forget, R. Wordsworth, J. Leconte, E. Millour, F. Codron, and A. Spiga. Published in 2013 in the Journal of Geophysical Reports: Atmospheres, this article tackles the faint young sun paradox by using a general circulation model, or GCM. Most models for the Archean are the simpler 1D models.
The model in the article featured an atmosphere with full water cycle, cloud cover, the ocean, and albedo (a measure of how much sunlight is absorbed vs. reflected), to name a few notable items. First, the model was run with modern-day Earth conditions, to test its accuracy. While there were discrepancies on regional scales, the model proved itself on its intended global scale.
The model, run with present day conditions but the sun twenty percent fainter, as was present in the Archean, resulted in a snowball Earth in only 23 years. Testing within the model revealed that land placement and area had little effect in the overall result. The model suggested that an atmosphere with 0.1 bar (10,000 pascals–just under standard atmospheric pressure) of carbon dioxide, in combination with methane, would provide a sufficient greenhouse effect and yield a temperate climate, even without other methods of warming, to counteract the fainter sun. This is within restraints given by geological data.
The article also looked at the possibility of a colder Archean, with the global average temperature below freezing. These results all should have been snowball Earths according to 1D models, but this was not the case in the 3D model. The results showed temperatures of -25 Celsius (-13 Fahrenheit), and ice from the poles to 25 degrees latitude north/south, before a snowball Earth resulted.
While a different atmospheric composition is the most common means of solving the faint young sun paradox, it does not mean it is the only possible method, or the only method of warming that was at play. For example, clouds and their droplet size can play a role. Cloud droplet size is related to biological activities; currently at 12 micrometers, it has been suggested it was at least 17 micrometers in the Archean (remember that micro is 10-6, so a micrometer is .000001 meters). In a simulation with 17 micrometer cloud droplets, the precipitation patterns changed and the planet warmed by 7.2 degrees Celsius (around 13 degrees Farenheit). However, it is debated that the cloud droplets could have been this large due to various uncertainties, so at best, this helps, but does not solve, the faint young sun paradox.
Another possible item at play was the amount of nitrogen in the air. The Archean may have had much more nitrogen in the air, and thus a higher atmospheric pressure; the nitrogen has since been taken from the atmosphere by other processes. A higher pressure would have warmed the planet, but also changed the albedo in a way that cooled it. The model saw a 7 degree Celsius (12.6 degree Fahrenheit) climb. However, data implies that the atmospheric pressure due to nitrogen was not as high as it could have been, so perhaps the nitrogen left the atmosphere earlier than realized; in any case, this may not have been a notable effect.
One last item to consider is the fact that the Earth used to spin faster. 4 billion years ago, a day lasted around 14 hours, instead of 24. This has a limited impact on an already temperate climate, but warmed a cold or warm climate by 1.5 to 2 degrees Celsius (2.7 to 3.6 degrees Fahrenheit). For the most part, it only made a snowball Earth harder to achieve, according to the model.
In conclusion, the atmospheric composition was the most effective method of explaining the faint young sun paradox in this model, though it may not have been the only factor. With a pressure of 10 millibars (around one tenth of modern atmospheric pressure) due to carbon dioxide and 2 millibars (one-fiftieth of modern pressure) from methane, the Earth would have been 10 to 14 degrees Celsius (50 to 57.2 degrees Fahrenheit), only slightly cooler than today. However, there is still much more to do, including finding constraints of atmospheric composition through geological data if possible, and using a 3D GCM with a more complex ocean model.

 

Bibliography
Charnay, Benjamin, et al. “Exploring the faint young Sun problem and the possible climates of the Archean Earth with a 3‐D GCM.” Journal of Geophysical Research: Atmospheres 118.18 (2013).
Today’s main article.
Marshak, Stephen. Earth: Portrait of a Planet. 3rd ed. W.W. Norton & Company. Print
My geology textbook, used for the image of the geologic eras.
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