What would the sky color be for the Proterozoic Earth during the "Boring Billion" years(1.8 Ga-0.8 Ga) with a Canfield (Euxinic) ocean and a high atmospheric concentration of colorless hydrogen sulfide and very low atmospheric oxygen? Would it really be a light green as proposed by Professor Peter Ward in his book Under a Green Sky, which he states but did not provide a rationale?

  • $\begingroup$ I am not sure but my perception is that the color of sky comes from light scattering by dust particles. $\endgroup$
    – hsinghal
    Commented Jun 13, 2016 at 17:23
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    $\begingroup$ No offense to everybody who wants to close, but I think this is a perfectly physical question. While there is a perception component, it is 100% determined by the physics of the light scattering in the atmosphere. Planetary atmospheres do have a color and it can be calculated from first principles, even though it is not trivial (one has to perform proper ray-tracing trough multiple layers) and we may not be able to actually do it correctly within the scope of SE. I do agree that the earth sciences folks may have the models, though, which we lack. $\endgroup$
    – CuriousOne
    Commented Jun 13, 2016 at 18:06
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    $\begingroup$ I tracked down a copy of Ward's book, and I think he is suggesting that $H_2S$ preferentially absorbs red light - this is absorption not Rayleigh scattering. The absorption is very weak, but strong enough that on the scale of many kilometres it would make transmitted light slightly green. I have Googled for details of visible light absorption by $H_2S$ but without any luck. $\endgroup$
    – John Rennie
    Commented Jun 14, 2016 at 9:28
  • $\begingroup$ @John Rennie Useful. Would be worth double-checking whether Ward implied it was directly related to H₂S, although I doubt it, and haven't done the simple thing and emailed him. HITRAN sadly only has an IR, not visible, spectrum for H₂S. $\endgroup$ Commented May 8, 2023 at 18:22

1 Answer 1


I'm guessing that hydrogen sulfide was not intended as the direct reason for being 'Under a Green Sky', either in the Proterozoic between Huronian and Cryogenian glaciations, or in Phanerozoic extinctions.

The suggested process of the Permian-Triassic extinction in particular is summarised in this 2005 article based on Kump et al, and this 2010 follow-up. Peter Ward's book was published in 2008, while Kump's finding of ozone layer collapse was being reevaluated by Lamarque et al (2007, open access) and Harfoot et al (2008).

The Boring Billion atmosphere was not as oxidising as today, while the suggested P-T emissions of methane from geological processes, ocean die-off and accelerated decomposition may have overwhelmed atmospheric hydroyxl reactions, even if it did not greatly increase ultraviolet reaching the surface. (Ozone supposedly contributes slightly to blue sky colour when the Sun is at a low elevation, but this is not a major factor. The main cause for a blue sky is well-known to be single Rayleigh scattering and this would be unchanged.)

So a less oxidising atmosphere might not be pink, as James Lovelock thought the Archaean might have been, but still laden with significant amounts of hydrocarbons and reddish tholins. UV would increase production of tholins, but may not be necessary. Other factors increasing lightness and decreasing chroma might include volcanic aerosols and sulphates from oxidisation of tropospheric H₂S; and in the Proterozoic one might expect colour from frequent dust storms, unimpeded by any complex life on land, also absorbing blue light.

There is good evidence for widespread volcanism, high CO₂, warming, anoxia and euxinia shortly before or during Palaeozoic extinctions. The high level of sulfides may have killed off many species even if it didn't kill off the ozone layer.


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