I have some remarks concerning the â€œhydrosphere spending hypothesisâ€, specially about the supposed mechanism of decomposition of methane in the exosphere and the escape of the resulting hydrogen atoms into outer space. In the link you provided, which covers the hypothesis in some detail, an alternative to this mechanism is described and dismissed as â€œnot validated strictlyâ€:
Bates et al in 1950 proposed that the methane would be oxidized gradually by hydroxyl. However, the hydroxyl oxidizing process was not validated strictly, and academic cycle often considered that whereabouts of the methane in the atmosphere is still a misty with necessary of further research, though the explanation of the methane oxidized gradually by hydroxyl is popular in the world now
Please note, the conclusion, that more research is needed, is based on an article from 1980. When we compare the quoted paragraph with the following survey from 2001 , which sums up the current knowledge about the methane cycle - including a table (4.2) detailing the methane budget based on three relatively recent studies (1991, 1997,1998) - we find that methane loss in the atmosphere is mainly caused by its reaction with OH in the troposphere, expressed by:
OH + CH4 -> CH3 + H2O
Minor losses are reported for the stratosphere by reactions with OH, Cl, and O(1D). Losses in the exosphere are not mentioned.
Re: the evidence of methane being decomposed in upper atmosphere
Thank you for your interesting answer. First a question, Iâ€˜m not sure what you exactly mean with upper atmosphere, the layers above the mesopause? Please clarify! Besides that, I have some additional remarks.
When I understand your model for hydrogen escape correctly, you are assuming temperatures of 1000Â° K of the hydrogen atom (deduced from this post  ). I agree that at this temperature a considerable part of the atoms would escape into outer space. But a quick look at this link shows that such temperatures are only reached in the thermosphere at about 300 km altitude, temperatures in the stratosphere are much lower, less than 300 Â°K. In other words to defend your hypothesized mechanism you would have to demonstrate a substantial methane flux from the stratosphere at least to the thermosphere.
When we accept the budget data that was presented in , only 20-40 Tg/yr are candidates for the above mentioned flux. Could this amount produce the effect you are predicting, that is a reduction of the original mass of the ocean by a factor of 20? Letâ€˜s do a quick calculation. When we assume that all methane is generated by methane producing bacteria and that life on earth exists for less than 4*109 years, when we further assume that to produce one molecule methane we need one water molecule, then we can conclude that we loose about 4*109 * 40 Tg methane, which would roughly correspondent to 1.6 * 1011 Tg water. The present day ocean on the other hand contains 1.4*1012 Tg water.
When we further consider that your model assumes that the first land masses appeared only in the Cambrium, we have to reduce the estimated methane loss even more, with an ocean to atmosphere flux of 0.4 Tg (see  ), we get 3.5*109 *0.4 Tg + 0.5*109*40 Tg, that is 2.14 * 1010 Tg, which amounts to 1.5 % of the mass of the current ocean.