  We investigated the potential biological impacts at Earth's surface of
stratospheric O3 depletion caused by nearby supernovae known to have occurred
about 2.5 and 8 million years ago at about 50 pc distance. New and previously
published atmospheric chemistry modeling results were combined with radiative
transfer modeling to determine changes in surface-level Solar irradiance and
biological responses. We find that UVB irradiance is increased by a factor of
1.1 to 2.8, with large variation in latitude, and seasonally at high latitude
regions. Changes in UVA and PAR (visible light) are much smaller. DNA damage
(in vitro) is increased by factors similar to UVB, while other biological
impacts (erythema, skin cancer, cataracts, marine phytoplankton photosynthesis
inhibition, and plant damage) are increased by smaller amounts. We conclude
that biological impacts due to increased UV irradiance in this SN case are not
mass-extinction level, but might be expected to contribute to changes in
species abundances; this result fits well with species turnover observed around
the Pliocene-Pleistocene boundary.
