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

We use the Cambridge two‐dimensional (2‐D) chemistry‐radiation transport model to investigate the implications for column O 3 and near‐surface ultraviolet radiation (UV), of variations in atmospheric O 2 content over the Phanerozoic (last 540 Myr). Model results confirm some earlier 1‐D model investigations showing that global annual mean O 3 column increases monotonically with atmospheric O 2 . Sensitivity studies indicate that changes in temperature and N 2 O exert a minor influence on O 3 relative to O 2 . We reconstructed Earth's O 3 history by interpolating the modeled relationship between O 3 and O 2 onto two Phanerozoic O 2 histories. Our results indicate that the largest variation in Phanerozoic column O 3 occurred between 400 and 200 Myr ago, corresponding to a rise in atmospheric O 2 to ∼1.5 times the present atmospheric level (PAL) and subsequent fall to ∼0.5 PAL. The O 3 response to this O 2 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 O 3 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. O 3 and UV changes were exacerbated when we incorporated a direct feedback of the terrestrial biosphere on atmospheric chemistry, through enhanced N 2 O 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.