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Well it isn’t clear to me if it is slow. This article on the study says :” The field geometry of reversed polarity, with field lines pointing into the opposite direction when compared to today’s configuration, lasted for only about 440 years, and it was associated with a field strength that was only one quarter of today’s field,” explains Norbert Nowaczyk. “The actual polarity changes lasted only 250 years”

Read more at: http://phys.org/news/2012-10-extremely-reversal-geomagnetic-field-climate.html#jCp