Climatic and biogeochemical effects of a galactic gamma ray burst

It is likely that one or more gamma ray bursts within our galaxy have strongly irradiated the Earth in the last Gy. This produces significant atmospheric ionization and dissociation, resulting in ozone depletion and DNA-damaging ultraviolet solar flux reaching the surface for up to a decade. Here we show the first detailed computation of two other significant effects. Visible opacity of NO₂ is sufficient to reduce solar energy at the surface up to a few percent, with the greatest effect at the poles, which may be sufficient to initiate glaciation. Rainout of dilute nitric acid could have been important for a burst nearer than our conservative ``nearest burst''. These results support the hypothesis that the characteristics of the Late Ordovician mass extinction are consistent with GRB initiation.