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 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.
Read more about this topic: Faint Young Sun Paradox
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