Varied oxygen simulations with WACCM6 (Proterozoic to pre-industrial atmosphere)

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 ∼ 2.4 Gyr ago,
resulting in a time-varying ozone (O3) layer. Using a three-dimensional
(3D) 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 3D
chemistry-climate simulations when assessing paleoclimates and the
habitability of faraway worlds.