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https://doi.org/10.5194/egusphere-2025-1133
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2025-1133
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
31 Mar 2025
| 31 Mar 2025
The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event
Abstract. The Great Oxidation Event (GOE) was a 200 Myr transition circa 2.4 billion years ago that converted the Earth's anoxic atmosphere to one where molecular oxygen (O2) was abundant. This rise in O2 is thought to have substantially throttled hydrogen (H) escape and the associated water (H2O) loss. Since the GOE, the amount of hydrogen escaping has been influenced by the methane (CH4) mixing ratio and the diffusion of H2O into the upper atmosphere. In this study we use WACCM6, a three-dimensional Earth System Model to simulate Earth's atmosphere and predict the diffusion-limited escape rate of hydrogen due to varying O2 concentrations based on atmospheric estimations from the GOE onward, ranging between 0.1 % PAL to 150 % PAL, where PAL is the present atmospheric level of 21 % by volume. O2 indirectly acts as a control valve on the amount of hydrogen atoms reaching the homopause in the simulations: less O2 leads to decreased O3 densities, reducing local temperatures by up to 5 K, which increases H2O freeze-drying. For the considered scenarios, the maximum difference in the total H mixing ratio at the homopause and calculated diffusion-limited escape rates is a factor of 3.2 and 4.7, respectively, with the prescribed CH4 mixing ratio setting a minimum diffusion escape rate of ≈2 × 1010 mol H yr-1. These numerical predictions support geological evidence that the majority of Earth's hydrogen escape occurred prior to the GOE. Our work demonstrates that estimations of how the tropical tropopause layer and the associated hydrogen escape rate evolved through Earth's history requires 3D chemistry-climate models which include a global treatment of water vapour microphysics.
How to cite. Cooke, G., Marsh, D., Walsh, C., Sainsbury-Martinez, F., and Braam, M.: The Oxygen Valve on Hydrogen Escape Since the Great Oxidation Event, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-1133, 2025.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
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RC1: 'Comment on egusphere-2025-1133', Anonymous Referee #1, 13 May 2025
This paper addresses the effect of O2 variation on hydrogen escape from early Earth using 3D modeling with a General Circulation Model (GCM). The study explores a very interesting question with a rigorous method. It focuses on a single parameter, the surface abundance of O2, and investigates its impact on the abundance of water vapor, the main contributor to hydrogen escape in this configuration. Despite the rigorous approach, the paper was somewhat confusing in its organization of the results and discussion. Some aspects also remain unclear and need to be described more precisely in the manuscript. I would suggest a major restructuring of the manuscript.General comments:The confusion comes from the fact that, in the results section, the findings are presented alongside the beginning of a discussion that lacks proper context. This context is only addressed later in the discussion section, where the results are not thoroughly analyzed. Instead, the discussion section primarily provides background, raises questions, and offers some perspective analysis. As a result, the information is scattered throughout the paper, making it difficult to follow.In the results section, it is repeatedly discussed that O2 indirectly impacts temperature through O3, which in turn affects the diffusion of water vapor to the upper atmosphere, ultimately driving hydrogen escape. However, this raises several controversies, some of which are addressed later (too late), while others are not addressed at all:(1) H2O is not the only gas that bring hydrogen to escape, what are the others?The justification of why the paper focuses only on water arrive late with figure 7. In the meantime we don't understand why other species such as CH4 are not discussed at all. Especially that CH4 levels could have been higher in the past, which is mention to late in section 4.2. What would happen for higher abundance of CH4? Where is the critical point between H2O and CH4 to dominated hydrogen escape?(2) Since the temperature is the real driver of the H2O abundace, O2 is not the only factor that could impact the temperature, what are the others?The context of this discussion started in the results section is given to late in sections 4.1, or this discussion is started to early.(3) O2 levels drive methane oxidation. What are the consequences for methane concentrations, given that methane acts both as a greenhouse gas and a source of hydrogen escape? Could we therefore expect an impact on hydrogen escape?It is valuable to investigate the isolated effect of O2 by keeping all other conditions fixed. However, this limitation should not be overlooked and needs to be highlighted more clearly. It is only briefly mentioned late in Section 4.2, but it should be made clear from the beginning what the assumptions are, their limitations, and the implications for interpreting past Earth's climate. CH4 and CO2 have varied significantly throughout Earth's history and have also influenced temperature. The discussion of the results should better reflect this approximation and consistently emphasize that the findings relate specifically to variations in O2 levels, not to different climatic periods in Earth's past.One major contradictory aspect of this paper, surprisingly never discussed, is that its main conclusion is that hydrogen escape is less significant at low O2 levels. However, geological evidence indicates that the majority of Earth’s hydrogen escape occurred before the GOE, when O2 levels were indeed very low. This point is mentioned in the introduction, but it is never addressed again, and the paper ultimately focuses only on post-GOE conditions. Can the findings of this study be extrapolated to lower O2 levels? If so, what then drove hydrogen escape before the GOE?Specific comments:l.11-12: "These numerical predictions support geological evidence that the majority of Earth’s hydrogen escape occurred prior to the GOE."This appears to contradict the statement in lines 8–9, which claims that lower O2 levels lead to reduced O3, resulting in lower temperatures and, consequently, reduced diffusion of H2O. This would imply that before the GOE, when O2 levels were lower, hydrogen escape was less efficient, since it depends on the diffusion of H2O to the homopause.l19: "As all life requires liquid H2O"We don’t truly know whether life can or cannot emerge without liquid water, this reflects an Earth-centric perspective. For example, on Titan, where there are lakes and rivers of liquid methane and ethane, it is conceivable that life could emerge based on hydrocarbons instead.l.23: "It’s possible that Venus was never habitable (Constantinou et al., 2024)"Constantinou et al. (2024) focus on atmospheric chemistry, but several studies more relevant to the question of whether Venus was ever habitable are cited in their introduction. For example, Turbet et al. (2021) demonstrates that an ocean would not have condensed on early Venus.l.43-45: "For these mechanisms, with the exception of impacts, the hydrogen escapes from the top of the atmosphere, from the exosphere."There seems to be a missing verb, should it be "the hydrogen escapes [come] from"?l.110-111: "Imposed mixing ratios and fluxes follow the PI settings, apart from O2 which is varied."How is this approximation valid up to 2.4 Gyr, which is the range of O2 levels that are supposed to be modeled?l.114-115: "once the middle atmospheric trend in total hydrogen atoms has halted"Why middle? Does it mean that the atmospheric trend in total hydrogen atoms is not converge everywhere in the atmosphere?Figure 5 caption: "pressures of 100 – 30 hPa", in the figure the legend shows 120 to 30 hPa.l.201-202: It could be better explained why these specific pressures, 88 hPa and 50 hPa, were chosen. In general, the paper does not clearly justify how the pressure ranges are precisely selected, and the criteria often appear approximate. What are the precise pressures (altitudes) of the cold trap for the simulations?Figure 6 caption: "The tropical average is for latitudes +-20 degree from the equator."Should it be +-24 degree for tropical latitudes?l.211-214: "Fig. 6 suggests that the warmest TTL temperatures are the controlling factor in each atmospheric scenario, instead of the atmospheric composition. Yet because composition (i.e. the O2 mixing ratio) is the variable that is altered in each scenario, the atmospheric composition is the controlling factor for the TTL temperatures. Hence, the oxygenation state of the atmosphere is indirectly controlling the upward flow of hydrogen atoms and affecting the diffusion-limited hydrogen escape rate"O2 is the only variable tested here, so naturally all the resulting effects stem from variations in O2, even though the actual parameter controlling hydrogen escape is temperature. However, many other factors can influence temperature, and thus hydrogen escape, such as variation of greenhouse gases (CO2, CH4), stellar flux evolution (faint young sun), or glaciation events. These influences should not be overlooked and variation of temperature solely attributed to the "oxygenation state of the atmosphere".I noticed that this is addressed later in the discussion (l.272), but the way the paper is written is confusing. It begins an explanation in the results section, but leaves the reader with many questions that are only addressed later in the discussion.l.223-224: "the four species that carry the majority of hydrogen atoms (H, H2, H2O, and CH4)"This could be specified earlier in the method and used to justify why their is a focus on H2O to explain hydrogen escape and not other species.l.228-229: "CH4 is never the dominant carrier in the scenarios we present. As we will discuss later, this may not have been the case for much of the Proterozoic."I would have expect this explanation earlier following previous comment where we can question why the focus on H2O and not other species such as CH4.l.329: Other temperature dependencies are finally cited.l.246: This paragraphe could be more conclusion/perspectives, right?Citation: https://doi.org/
10.5194/egusphere-2025-1133-RC1 - RC2: 'Comment on egusphere-2025-1133', Anonymous Referee #2, 12 Jul 2025
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Institute of Astronomy, University of Cambridge, CB3 0HA, UK
School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
Dan Marsh
School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
Catherine Walsh
School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
Felix Sainsbury-Martinez
School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK
Marrick Braam
Center for Space and Habitability (CSH), Bern, Switzerland
Short summary
During the Archean eon (4–2.4 Gyr ago), Earth's atmosphere lacked oxygen (O2) but contained nitrogen, carbon dioxide, and methane. As cyanobacteria evolved, they produced O2, while hydrogen (H) escaped, making Earth more oxidized. Around 2.4 billion years ago, oxygen levels rose, limiting hydrogen loss. Using 3D computer simulations, we found that oxygen concentrations affect the upward diffusion of water vapor (H2O). We therefore quantify the rate of hydrogen escape as O2 changes.
During the Archean eon (4–2.4 Gyr ago), Earth's atmosphere lacked oxygen (O2) but contained...