From
http://curious.astro.cornell.edu/question.php?number=523
quote:
One effect that may occur during a magnetic reversal is that the Earth may not be protected from charged particles streaming from the sun. {snip} If the Earth's magnetic field is weakened during a reversal, more of these particles will get through to the upper atmosphere. This could be a problem, but most likely the atmosphere is thick enough to protect the Earth's surface.
In the process of finding the above, I did see a comment (elsewhere) that no effect of magnetic refersals, has been found in the fossil record.
The above cited also gives a link to an outline of the development of plate tectonic theory, including discussion of magnetic refersals and the resultant "stripes" on the oceans floors. That site is at
http://pubs.usgs.gov/publications/text/developing.html
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