Well, this was interesting. Alarming, too, at least at first. It seems that our magnetic field isn't protecting our atmosphere from the solar wind as much as we thought. In fact, our atmosphere is being carried off at a rate faster than that of either Mars or Venus. (!) It seems we have plenty for a long, long time, but still...

http://dsc.discovery.com/news/2009/06/02/solar-wind-atmosphere.html