A Revised Lower Estimate of Ozone Columns During Earth's Oxygenated History

The history of molecular oxygen (O) in Earth's atmosphere is still debated; however, geological evidence supports at least two major episodes where O increased by an order of magnitude or more: the Great Oxidation Event (GOE) and the Neoproterozoic Oxidation Event. O concentrations have likely fluctuated (between 10 and 1.5 times the present atmospheric level) since the GOE ∼2.4 Gyr ago, resulting in a time-varying ozone (O) layer. Using a three-dimensional chemistry-climate model, we simulate changes in O in Earth's atmosphere since the GOE and consider the implications for surface habitability, and glaciation during the Mesoproterozoic. We find lower O columns (reduced by up to 4.68 times for a given O level) compared to previous work; hence, higher fluxes of biologically harmful UV radiation would have reached the surface. Reduced O leads to enhanced tropospheric production of the hydroxyl radical (OH) which then substantially reduces the lifetime of methane (CH). We show that a CH supported greenhouse effect during the Mesoproterozoic is highly unlikely. The reduced O columns we simulate have important implications for astrobiological and terrestrial habitability, demonstrating the relevance of three-dimensional chemistry-climate simulations when assessing paleoclimates and the habitability of faraway worlds.