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Since the Great Oxidation Event approximately 2.4 billion years ago, oxygen (O₂) concentrations have fluctuated until reaching modern day levels, with O₂ now comprising 21% of the atmosphere. Given the geological history of terrestrial bodies in the solar system, telescopes will likely find an exoplanet in a particular geological regime, with the Proterozoic being the longest geological period from Earth’s history. Chemistry-climate simulations using the Whole Atmosphere Community Climate Model (WACCM) will be presented, where oxygen concentrations were altered from pre-industrial levels to possible Proterozoic levels of 10%, 1%, 0.5%, and 0.1% of the present atmospheric level. These simulations represent not only Earth through time, but possible Earth-analogue exoplanets that orbit a Sun-like (G-type) star. Reduced O₂ changes the chemical regimes for these atmospheres, limiting the ability for atmospheric CH₄ to accumulate. With a reduced O₃ shield, enhanced photolysis produces increased chemical destruction in a Proterozoic-like atmosphere, such that possible transmission spectra observations of these inhabited exoplanets will likely result in a false-negative (a null result) for detecting the presence of life. Thus, at least for transmission spectra, dedicated larger telescopes would be needed to confirm biosignatures on these Earth-analogue exoplanets during an equivalent Proterozoic-like period.

Oxygen’s 2.4 billion year control on Earth’s atmosphere with consequences for exoplanet biosignatures