Plant hardiness zone map change

The USDA divides the U.S.into zones of plant hardiness.  Obviously the southern part of Florida and Texas have far longer growing seasons than my part of Pennsylvania, which has longer growing seasons than Vermont.

My friend Tom, who makes his living farming, just sent me a note that the USDA has changed this area from Zone 6a to 6b.  Our lowest average temperature is now -5 to 0 degrees.  Our average lowest temperature had been between -5 and -10.  This year we did not have a temperature zero or below the entire winter.  

Of course, even if I could convince a Trump follower that climate change was real, she would say something like “We’ve always had climate change” and dismiss the whole idea of human agency.