http://arborday.org/media/mapchanges.cfmIt doesn't matter if it's a natural cycle, the sun, or greenhouse gas emissions.
Global warming seems to be a reality -- just look at the changes to the "hardiness zones" map! Almost every place in the US has "gone south" by several hundred miles!
If you were in Zone 4 in 1990, you're probably in Zone 5 by now!
Matthew