That's what I was thinking. It's a variation on a known theme, but it can't be done on Earth, insofar as we know.

It would require one of:
* a planet with an oscillating atmosphere that changed composition periodically
* a planet with different atmospheres in different places
* a way of creating small areas of a different atmosphere than the usual
* migrating from a planet with one type of atmosphere to a planet with a different one
... or something of the sort.