  Our model involving cosmic ray acceleration in supernova remnants has been
used to predict cosmic ray intensities over long periods of time on a
statistical basis. If, as is highly probable, extensive air showers caused by
PeV cosmic rays are needed to initiate terrestrial lightning then past dramatic
changes in PeV intensities may have had important biological effects.
  The model has been used to estimate the manner in which the PeV cosmic ray
intensity at Earth has varied over the past tens of thousand years.
