  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 is 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.
