  The Galactic cosmic ray (GCR) intensity has been postulated by others to vary
cyclically with a peak to valley ratio of ~3:1, as the Solar System moves from
the Spiral Arm to the Inter-Arm regions of the Galaxy. These intensities have
been correlated with global temperatures and used to support the hypothesis of
GCR induced climate change. In this paper we show that the model used to deduce
such a large ratio of Arm to Interarm GCR intensity requires unlikely values of
some of the GCR parameters, particularly the diffusion length in the
interstellar medium, if as seems likely to be the case, the diffusion is
homogeneous. Comparison is made with the existing gamma ray astronomy data and
this also indicates that the ratio is not large. The variation in the intensity
is probably of order 10-20% and should be no more than 30% as the Solar System
moves between these two regions, unless the conventional parameters of the GCR
are incorrect. In addition we show that the variation of the GCR intensity, as
the trajectory of the Solar System oscillates about the Galactic Plane, is too
small to account for the extinctions of species as has been postulated unless,
again, conventional assumptions about the GCR parameters are not correct.
