A revised lower estimate of ozone columns during Earth's oxygenated history

The history of molecular oxygen (O-2) in Earth's atmosphere is still debated; however, geological evidence supports at least two major episodes where O-2 increased by an order of magnitude or more: the Great Oxidation Event (GOE) and the Neoproterozoic Oxidation Event. O-2 concentrations have likely fluctuated (between 10(-3) and 1.5 times the present atmospheric level) since the GOE similar to 2.4 Gyr ago, resulting in a time-varying ozone (O-3) layer. Using a three-dimensional chemistry-climate model, we simulate changes in O-3 in Earth's atmosphere since the GOE and consider the implications for surface habitability, and glaciation during the Mesoproterozoic. We find lower O-3 columns (reduced by up to 4.68 times for a given O-2 level) compared to previous work; hence, higher fluxes of biologically harmful UV radiation would have reached the surface. Reduced O-3 leads to enhanced tropospheric production of the hydroxyl radical (OH) which then substantially reduces the lifetime of methane (CH4). We show that a CH4 supported greenhouse effect during the Mesoproterozoic is highly unlikely. The reduced O-3 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.