A two-dimensional atmospheric chemistry modeling investigation of Earth's Phanerozoic O sub(3) and near-surface ultraviolet radiation history

• Published in: Journal of Geophysical Research. D. Atmospheres 2007-04, Vol.112 (D7)
• Main Authors: Harfoot, Michael BJ, Beerling, David J, Lomax, Barry H, Pyle, John A
• Format: Article
• Language: English
• ISSN: 0148-0227
• DOI: 10.1029/2006JD007372

We use the Cambridge two-dimensional (2-D) chemistry-radiation transport model to investigate the implications for column O sub(3) and near-surface ultraviolet radiation (UV), of variations in atmospheric O sub(2) content over the Phanerozoic (last 540 Myr). Model results confirm some earlier 1-D model investigations showing that global annual mean O sub(3) column increases monotonically with atmospheric O sub(2). Sensitivity studies indicate that changes in temperature and N sub(2)O exert a minor influence on O sub(3) relative to O sub(2). We reconstructed Earth's O sub(3) history by interpolating the modeled relationship between O sub(3) and O sub(2) onto two Phanerozoic O sub(2) histories. Our results indicate that the largest variation in Phanerozoic column O sub(3) occurred between 400 and 200 Myr ago, corresponding to a rise in atmospheric O sub(2) to similar to 1.5 times the present atmospheric level (PAL) and subsequent fall to similar to 0.5 PAL. The O sub(3) response to this O sub(2) decline shows latitudinal differences, thinning most at high latitudes (30-40 Dobson units (1 du = 0.001 atm cm) at 66 degree N) and least at low latitudes (5-10 du at 9 degree N) where a 'self- healing' effect is evident. This O sub(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 sub(3) and UV changes were exacerbated when we incorporated a direct feedback of the terrestrial biosphere on atmospheric chemistry, through enhanced N sub(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.