A two‐dimensional atmospheric chemistry modeling investigation of Earth's Phanerozoic O3 and near‐surface ultraviolet radiation history

Abstract

We use the Cambridge two‐dimensional (2‐D) chemistry‐radiation transport model to investigate the implications for column O3 and near‐surface ultraviolet radiation (UV), of variations in atmospheric O2 content over the Phanerozoic (last 540 Myr). Model results confirm some earlier 1‐D model investigations showing that global annual mean O3 column increases monotonically with atmospheric O2. Sensitivity studies indicate that changes in temperature and N2O exert a minor influence on O3 relative to O2. We reconstructed Earth's O3 history by interpolating the modeled relationship between O3 and O2 onto two Phanerozoic O2 histories. Our results indicate that the largest variation in Phanerozoic column O3 occurred between 400 and 200 Myr ago, corresponding to a rise in atmospheric O2 to ∼1.5 times the present atmospheric level (PAL) and subsequent fall to ∼0.5 PAL. The O3 response to this O2 decline shows latitudinal differences, thinning most at high latitudes (30–40 Dobson units (1 DU = 0.001 atm cm) at 66°N) and least at low latitudes (5–10 DU at 9°N) where a “self‐healing” effect is evident. This O3 depletion coincides with significant increases in the near‐surface biologically active UV radiation at high latitudes, +28% as weighted by the Thimijan spectral weighting function. O3 and UV changes were exacerbated when we incorporated a direct feedback of the terrestrial biosphere on atmospheric chemistry, through enhanced N2O production as the climate switched from an icehouse to a greenhouse mode. On the basis of a summary of field and laboratory experimental evidence, we suggest that these UV radiation increases may have exerted subtle rather than catastrophic effects on ecosystem processes.