  We examine possible sources of a substantial increase in tree ring 14C
measurements for the years AD 774-775. Contrary to claims regarding a coronal
mass ejection (CME), the required CME energy is not several orders of magnitude
greater than known solar events. We consider solar proton events (SPEs) with
three different fluences and two different spectra. The data may be explained
by an event with fluence about one order of magnitude beyond the October 1989
SPE. Two hard spectrum cases considered here result in moderate ozone
depletion, so no mass extinction is implied, though we do predict increases in
erythema and damage to plants from enhanced solar UV. We are able to rule out
an event with a very soft spectrum that causes severe ozone depletion and
subsequent biological impacts. Nitrate enhancements are consistent with their
apparent absence in ice core data. The modern technological implications of
such an event may be extreme, and considering recent confirmation of
superflares on solar-type stars, this issue merits attention.
