USDA issues new plant hardiness zone map, corroborating evidence of climate change

Five years ago, the Arbor Day Foundation changed its plant-hardiness zones because of warmer winters. Now the U.S. Department of Agriculture has followed suit, but "made clear that it doesn't ascribe the trend to climate change," reports Bart Ziegler of The Wall Street Journal. "The 30 years of weather data used to create the map weren't sufficient to smooth out weather cycles and determine if there is any underlying climate change," according to USDA spokesman Kim Kaplan.

Also, Ziegler writes, "The agency said the methodology used to build the new map was more sophisticated than that for the 1990 version, so the maps aren't directly comparable. The new map relies on data from 8,000 weather stations and also takes into account topography, prevailing wind, elevation, proximity to large water bodies and other factors not used to create the 1990 map." The map is based on the average low temperature.

However, Cornell University professor David Wolfe, who studies climate change, said "doesn't prove climate change" by itself, but corroborates other evidence, "including shifts in animal migration patterns, changes in snow cover and other temperature readings," Ziegler writes. For the USDA press release, with a link to an interactive map that shows climate zones at the county level, click here.