Faint young Sun paradox
The faint young Sun paradox or problem describes the apparent contradiction between observations of liquid water early in the Earth's history and the astrophysical expectation that the Sun's output would be only 70% as intense during that epoch as it is during the modern epoch. The issue was raised by astronomers Carl Sagan and George Mullen in 1972. Explanations of this paradox have taken into account greenhouse effects, astrophysical influences, or a combination of the two.
Early solar output 
Early in the Earth's history, the Sun's output would have been only 70% as intense as it is during the modern epoch. In the then current environmental conditions, this solar output would have been insufficient to maintain a liquid ocean. Astronomers Carl Sagan and George Mullen pointed out in 1972 that this is contrary to the geological and paleontological evidence.
According to the Standard Solar Model, stars similar to the Sun should gradually brighten over their main sequence lifetime. However, with the predicted solar luminosity 4 billion (4 × 109) years ago and with greenhouse gas concentrations the same as are current for the modern Earth, any liquid water exposed to the surface would freeze. However, the geological record shows a continually relatively warm surface in the full early temperature record of the Earth, with the exception of a cold phase, the Huronian glaciation, about 2.4 to 2.1 billion years ago. Water-related sediments have been found that date to as early as 3.8 billion years ago. Hints of early life forms have been dated from as early as 3.5 billion years, and the basic carbon isotopy is very much in line with what is found today. A regular alternation between ice ages and warm periods is only to be found occurring in the period since one billion years ago.
Greenhouse hypothesis 
When it first formed, Earth's atmosphere may have contained more greenhouse gases. Carbon dioxide concentrations may have been higher, with estimated partial pressure as large as 1,000 kPa (10 bar), because there was no bacterial photosynthesis to reduce the gas to carbon and oxygen. Methane, a very active greenhouse gas which reacts with oxygen to produce carbon dioxide and water vapor, may have been more prevalent as well, with a mixing ratio of 10−4 (100 parts per million by volume).
Based on a study of geological sulfur isotopes, in 2009 a group of scientists including Yuichiro Ueno from the University of Tokyo proposed that carbonyl sulfide (OCS) was present in the Archean atmosphere. Carbonyl sulfide is an efficient greenhouse gas and the scientists estimate that the additional greenhouse effect would have been sufficient to prevent the Earth from freezing over.
Following the initial accretion of the continents after about 1 billion years, geo-botanist Heinrich Walter and others believe that a non-biological version of the carbon cycle provided a negative temperature feedback. The carbon dioxide in the atmosphere dissolved in liquid water and combined with metal ions derived from silicate weathering to produce carbonates. During ice age periods, this part of the cycle would shut down. Volcanic carbon emissions would then restart a warming cycle due to the greenhouse effect.
According to the Snowball Earth hypothesis, there may have been a number of periods when the Earth's oceans froze over completely. The most recent such period may have been about 630 million years ago. Afterwards, the Cambrian explosion of new multicellular life forms started.
A minority view, propounded by the Israeli-American physicist Nir Shaviv, uses climatological influences of solar wind, combined with a hypothesis of Danish physicist Henrik Svensmark for a cooling effect of cosmic rays, to explain the paradox. According to Shaviv, the early Sun had emitted a stronger solar wind that produced a protective effect against cosmic rays. In that early age, a moderate greenhouse effect comparable to today's would have been sufficient to explain an ice-free Earth. Evidence for a more active early Sun has been found in meteorites.
The temperature minimum around 2.4 billion years goes along with a cosmic ray flux modulation by a variable star formation rate in the Milky Way Galaxy. The reduced solar impact later results into a stronger impact of cosmic ray flux (CRF), which is hypothesized to lead to a relationship with climatological variations.
An alternative model of solar evolution may explain the faint young Sun paradox. In this model, the early Sun underwent an extended period of higher solar wind output. This caused a mass loss from the Sun on the order of 5−10% over its lifetime, resulting in a more consistent level of solar luminosity (as the early Sun had more mass, resulting in more energy output than was predicted). In order to explain the warm conditions in the Archean era, this mass loss must have occurred over an interval of about one billion years. However, records of ion implantation from meteorites and lunar samples show that the elevated rate of solar wind flux only lasted for a period of 0.1 billion years. Observations of the young Sun-like star π1 Ursae Majoris matches this rate of decline in the stellar wind output, suggesting that a higher mass loss rate can not by itself resolve the paradox.
Examination of Archaean sediments appears inconsistent with the hypothesis of high greenhouse concentrations. Instead, the moderate temperature range may be explained by a lower surface albedo brought about by less continental area and the "lack of biologically induced cloud condensation nuclei". This would have led to increased absorption of solar energy, thereby compensating for the lower solar output.
See also 
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- Gough, D. O. (1981). "Solar Interior Structure and Luminosity Variations". Solar Physics 74 (1): 21–34. Bibcode:1981SoPh...74...21G. doi:10.1007/BF00151270.
- Windley, B. (1984). The Evolving Continents. New York: Wiley Press. ISBN 0-471-90376-0.
- Schopf, J. (1983). Earth’s Earliest Biosphere: Its Origin and Evolution. Princeton, N.J.: Princeton University Press. ISBN 0-691-08323-1.
- Veizer, Jan (March 2005). "Celestial climate driver: a perspective from four billion years of the carbon cycle". Geoscience Canada 32 (1).
- Walker, James C. G. (June 1985). "Carbon dioxide on the early earth". Origins of Life and Evolution of the Biosphere 16 (2): 117−127. Bibcode:1985OLEB...16..117W. doi:10.1007/BF01809466. Retrieved 2010-01-30.
- Pavlov, Alexander A.; Kasting, James F.; Brown, Lisa L.; Rages, Kathy A.; Freedman, Richard (May 2000). "Greenhouse warming by CH4 in the atmosphere of early Earth". Journal of Geophysical Research 105 (E5): 11981−11990. Bibcode:2000JGR...10511981P. doi:10.1029/1999JE001134.
- Ueno, Y.; Johnson, M. S.; Danielache, S. O.; Eskebjerg, C.; Pandey, A.; Yoshida, N. (August 2009). "Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox Ueno, Y.; Johnson, M. S.; Danielache, S. O.; Eskebjerg, C.; Pandey, A.; Yoshida, N.". Proceedings of the National Academy of Sciences 106 (35): 14784−14789. Bibcode:2009PNAS..10614784U. doi:10.1073/pnas.0903518106.
- Veizer, J. (1976). In B. F. Windley. The Early History of the Earth. London: John Wiley and Sons. p. 569. ISBN 0-471-01488-5.
- Zeebe, Richard (April 28, 2008). "Before fossil fuels, Earth’s minerals kept CO2 in check". University of Hawaiʻi at Mānoa. Retrieved 2010-01-30.
- Walker, J. C. G.; Hays, P. B.; Kasting, J. F. (October 20, 1981). "A negative feedback mechanism for the long-term stabilization of the earth's surface temperature" (PDF). Journal of Geophysical Research 86 (C10): 9776−9782. Bibcode:1981JGR....86.9776W. doi:10.1029/JC086iC10p09776. Retrieved 2010-01-30.[dead link]
- Hoffman, Paul F.; Kaufman, Alan J.; Halverson, Galen P.; Schrag, Daniel P. (August 28, 1998). "A Neoproterozoic Snowball Earth". Science 281 (5381): 1342−1346. Bibcode:1998Sci...281.1342H. doi:10.1126/science.281.5381.1342. PMID 9721097.
- Shaviv, N. J. (2003). Toward a solution to the early faint Sun paradox: A lower cosmic ray flux from a stronger solar wind 108 (A12). p. 1437. arXiv:astro-ph/0306477. Bibcode:2003JGRA..108.1437S. doi:10.1029/2003JA009997. Text " Journal of Geophysical Research " ignored (help)
- Caffe, M. W.; Hohenberg, C. M.; Swindle, T. D.; Goswami, J. N. (February 1, 1987). "Evidence in meteorites for an active early sun". Astrophysical Journal Letters 313: L31–L35. Bibcode:1987ApJ...313L..31C. doi:10.1086/184826.
- Gaidos, Eric J.; Güdel, Manuel; Blake, Geoffrey A. (2000). "The faint young Sun paradox: An observational test of an alternative solar model". Geophysical Research Letters 27 (4): 501–504. Bibcode:2000GeoRL..27..501G. doi:10.1029/1999GL010740.
- Rosing, Minik T.; Bird, Dennis K.; Sleep, Norman H.; Bjerrum, Christian J. (April 1, 2010). "No climate paradox under the faint early Sun". Nature 464 (7289): 744–747. Bibcode:2010Natur.464..744R. doi:10.1038/nature08955. PMID 20360739.
Further reading 
- Bengtsson, Lennart; Hammer, Claus U. (2004). Geosphere-Biosphere Interactions and Climate. Cambridge University Press. ISBN 0-521-78238-4.
- Solar Evolutionary Models and Precambrian Temperatures