USDA’s map, released on January 25, revealed that planting zones have been creeping northward as temperatures warm. Each zone is based on a 10-degree difference in average annual minimal temperatures for that region. For example, in zone 6, which runs through parts of Pennsylvania, northern New Jersey, southern New York and New England, the average minimal temperatures range from 0 to 10 degrees Fahrenheit.
The recent map was based on an analysis of 30 years of data stretching from 1976 to 2005, and showed that most parts of the country are on average five degrees warmer – equal to a “half-zone” — than they were in 1990, when the previous map was released.
But an analysis by Nir Krakauer, an assistant professor of civil engineering at City College, reveals that planting zones have shifted even further, according to Science Daily. More than one-third of the country has warmed by a half zone, and over one fifth has shifted a full zone, Krakauer wrote in the journal “Advances in Meteorology.”
Krakauer faulted USDA’s analysis of data, which relied on average annual minimum temperatures over the 30-year period from 1976 to 2005. The problem, he said, is that winters have warmed considerably during that period, so calculating the average rise over the course of the last three decades doesn’t reflect recent warmer temperatures, particularly in winter.
In addition, the analysis showed that warming is occurring faster in the Eastern interior and slower in the Southwest.
Source: Science Daily, 9/13