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Originally Posted by NeilBlanchard
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Those two plant hardiness maps are not directly comparable. They were compiled by two different organizations (USDA and National Arbor Day Foundation) who likely used different databases and statistical analyses to get their results. The 1990 map is compiled from 13 years (1974 to 1986) worth of data, the Arbor Day map from 18 years (1980 to 1997) worth of data. Both of these are just a blink in time and wouldn't capture any normal long term variations in the climate. For that you would need at least a couple of centuries of data to make a map from. A better indicator of what is normal climate for a region is to look at the cold and heat tolerances of plants native to a region, especially of the long lived one's.
That "zone changes in the next 30 years" map isn't worth the paper it is printed on considering all of the variables it can't anticipate such as changes in solar activity, volcanic eruptions, etc. Back in the 1980's and 1990's they were predicting that by this time there would be an ice-free north pole and a noticeable increase in sea level, neither of which have happened yet.