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

TL;DR

This study uses advanced climate modeling to revise estimates of Earth's ozone levels during key oxygenation events, revealing lower ozone columns than previously thought and implications for surface UV exposure and habitability.

Contribution

It provides the first 3D chemistry-climate simulation-based lower estimates of ozone during Earth's oxygenation, impacting understanding of ancient surface conditions.

Findings

Lower ozone columns by up to 4.68 times compared to previous estimates.

Higher UV fluxes could have affected surface life during Earth's oxygenation.

Methane's greenhouse role during the Mesoproterozoic is unlikely due to ozone reduction.

Abstract

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 $\sim 2.4$ 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 (CH$_4$). We show that a CH$_4$ 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.