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.

Methods
The data was originally produced using the Earth System Model WACCM6. The corresponding paper that describes WACCM can be found at the following DOI: https://doi.org/10.1029/2019JD030943. WACCM6 is a model configuration of the Community Earth System Model version 2 (CESM2). We used CESM2.1.3, which can be downloaded from the following URL: https://www.cesm.ucar.edu/models/cesm2/release_download.html. 

The atmospheric data from each simulation was output in terms of monthly means (h0 files; h for history), and 5 day averages (h2 files). For both h0 and h2 files, the last 4 years of each simulation was averaged using netCDF operators. ncra for time average (h0 and h2 files), and ncwa -alon for zonal mean (h2 files). 

The data has been processed using the Python programming language. Two .py files are included named O3_FYSP_paper.py and Analysis_Functions.py. The python files may not be well-commented in all places.

To produce the solar file which represent the Sun 2 billion years ago (2 Gyr), we used an existing solar model by Claire et al. 2012 (https://doi.org/10.1088/0004-637X/757/1/95). The model can be downloaded here: http://depts.washington.edu/naivpl/content/models/solarflux

The datasets Kasting_Catling_Fig10_data.csv and ROCKE3D_Fig3_O3_col.csv are from the following papers: https://doi.org/10.1146/annurev.astro.41.071601.170049 and https://doi.org/10.3847/1538-4365/aa7a06, respectively. The data was recovered using WebPlotDigitizer (https://automeris.io/WebPlotDigitizer/).