What was done
The authors investigated 20th century changes in (1) the intensity of cosmic rays incident upon the earth's magnetopause and (2) their transmission through the magnetosphere to the upper troposphere.

What was learned
It was determined, in the words of the authors, that (1) "the intensity of cosmic rays incident on the magnetopause has decreased markedly during this century" and (2) "the pattern of cosmic ray precipitation through the magnetosphere to the upper troposphere has also changed."

With respect to the first and more basic of these changes, they note that "at 300 MeV the difference between the proton flux incident on the magnetosphere at the beginning of the century and that incident now is estimated to be a factor of 5 decrease between solar minima at the beginning of the century and recent solar minima" and that "at 1 GeV the change is a factor of 2.5."  With respect to the second phenomenon, they note that the part of the troposphere open to cosmic rays of all energies increased by a little over 25% and shifted equatorward by about 6.5° of latitude.

What it means
With the great decrease in the intensity of cosmic rays incident on earth's magnetosphere over the 20th century, one would expect to see a progressive decrease in the presence of low-level clouds and, hence, an increase in global air temperature, as has indeed been observed.  Likewise, with the equatorward shift in the region of most effective cosmic ray transmission through the magnetosphere to the troposphere, one might expect to see an equatorward shift in storm tracks.  The authors say this latter expectation "requires further studies."