I read an article in the Mother Earth News about how hardiness zones have changed. The referenced a map from the National Arbor Day Foundation. http://www.arborday.org/media/map_change.cfm
While it is mathematically possible to have that much change with the 2 degrees or so of actual global warming we have had, I wouldn’t expect it. So I did some checking I noticed that the new hardiness map says Raleigh NC is zone 8. So Raleigh should have a low temperature between 10 and 20 degrees at least every five years. Yet Raleigh had a day with 9 degree and another day with 7 degree temperature in January of 2005. That would keep it in zone 7 not the zone 8 shown on the new map. (http://www.weather.gov/climate/local_data.php?wfo=rah)
Using the same website I found Greensboro had a 7 in 2004 so it remains in zone 7 not the zone 8 showed on the map. Fort Wayne had a -15 in December of 2004 which puts it in zone 5 not the zone 6 show on the map. http://threadex.rcc-acis.org/threadex/program/process_records
Using the same website I found that Sioux City had a -14 in 2003 so it hasn’t changed from zone 5. Wichita Falls had a 7 in 2004 which makes it zone 7 not the zone 8 shown on the new map.
In summary, 100% of the locations I checked (5 out of 5) have had a minimum temperature within the last five years below the zone listed on the hardiness zone change map. My conclusion is that the map from the Arbor Day Foundation over states the zone changes for the last 16 years.
Actually each temperature I quoted was in the top three of all time minimum temperature records for the day it occurred. My conclusion is that the global warming has increase minimum temperature extremes. This is what I would predict from bigger winter storms caused by global warming. They are sucking the colder air farther south.
I bounced this off a real climatologist and he said the maps may be accurate but 10 years of data was not enough to draw conclusions or maps. He thinks we need to use every year we have data and then some. Be curious to hear what you think.