In a couple of posts I have mentioned that the sun was fainter in the past. As you might imagine, it had an influence on the climate of the early Earth.
Sagan and Mullen presented this dilemma in 1972. 4.6 billion years ago the Sun was 30% fainter than today. This would mean, considering atmospheric composition and albedo to be the same as in present day, the temperature on the Earth would have been -10°C (263 K). Temperatures would have reached -2°C, the freezing point of seawater, around 2.3 billion years ago (Fig. 1).
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| Fig. 1: Without greenhouse gases the temperature would have been even lower (see the "Effective temperature"). Their influence is so important, that even today, with a stronger Sun, our planet would still be freezing. The "CO2-H2O greenhouse" represents our present conditions. Chyba, 2010. Simplified from Sagan and Mullen, 1972 |
As we have seen in past posts, by 2.3 billion years, there was life on the planet (it had already started with the small task of helping oxygenate the planet). Sagan and Mullen (1972) saw the inconsistency between the theory and the evidence available - it is unlikely that life could appear without the presence of liquid water and, even back in 1972, there was evidence for liquid water 3.2 billion years ago.
And the solution is...
Nothing conclusive so far, though not through lack of trying. There have been several different atmospheric compositions proposed to counter the effects of the young Sun. Sagan and Mullen (1972) themselves proposed an atmosphere with ammonia (NH3) as the greenhouse gas responsible of warming the planet (Fig. 1, "10-5 NH3 greenhouse" line). However, this has since been disproved. As Chyba (2010) mentions, the NH3 would have been photolyzed by the UV radiation from the Sun converting it into N2, thus eliminating its greenhouse effect. Haqq-Misra et al. (2009) proposed an atmosphere with CH4, C2H6, CO2 and H2O. Rosing et al. (2010) argued that the high greenhouse gas concentrations required by this model (as by others) was not consistent with the geological data, and proposed that the deciding factor was not the atmosphere, but the albedo. The albedo would have been much lower due to the presence of huge oceans, causing a higher absorption of heat which together with the presence of greenhouse gases would have been enough to keep the planet warm.
In the end everything seems to go back to greenhouse gases such as CH4 and CO2 as the reason the Earth was warmer, though the exact composition of the atmosphere remains unknown.
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