  We present in this work a unified, quantitative synthesis of analytical and
numerical calculations of the effects that could be caused on Earth by a
Gamma-Ray Burst (GRB), considering atmospheric and biological implications. The
main effects of the illumination by a GRB are classified in four distinct ones
and analyzed separately, namely: direct gamma Flash, UV Flash, Ozone Layer
Depletion and Cosmic Rays. The effectiveness of each of these effects is
compared and distances for significant biological damage are given for each
one. We find that the first three effects have potential to cause global
environmental changes and biospheric damages, even if the source is located at
Galactic distances or even farther (up to 150 kpc, about five times the
Galactic diameter of 30 kpc). Instead, cosmic rays would only be a serious
threat for close sources (on the order of a few pc). As a concrete application
from a well-recorded event, the effects on the biosphere of an event identical
to the giant flare of SGR1806-20 on Dec 27, 2004 have been calculated. In spite
of not beeing a classical GRB, most of the parameters of this recent flare are
quite well-known and have been used as a calibration for our study. We find
that a giant flare impinging on Earth is not a threat for life in all practical
situations, mainly because it is not as energetic, in spite of being much more
frequent than GRBs, unless the source happens to be extremely close.
