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

Abstract

The history of molecular oxygen (O2) in Earth's atmosphere is still debated; however, geological evidence supports at least two major episodes where O2 increased by an order of magnitude or more: the Great Oxidation Event (GOE) and the Neoproterozoic Oxidation Event. O2 concentrations have likely fluctuated (between 10-3 and 1.5 times the present atmospheric level) since the GOE ~2.4Gyr ago, resulting in a time-varying ozone (O3) layer. Using a three-dimensional chemistry-climate model, we simulate changes in O3 in Earth's atmosphere since the GOE and consider the implications for surface habitability, and glaciation during the Mesoproterozoic. We find lower O3 columns (reduced by up to 4.68 times for a given O2 level) compared to previous work; hence, higher fluxes of biologically harmful UV radiation would have reached the surface. Reduced O3 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 O3 columns we simulate have important implications for astrobiological and terrestrial habitability, demonstrating the relevance of threedimensional chemistry-climate simulations when assessing paleoclimates and the habitability of faraway worlds.