Krakauer conducted his own analysis of climatic data and determined that much of the country is already a half zone to one full zone warmer than the new USDA map. Why the difference? The USDA zone estimates were based on a simple average of annual minimum temperatures from the past thirty years. Dr. Krakuar's used an analysis to account for more recent warming. One of the key observations of the new analysis is that winter minimum temperatures are warming much faster than average temperatures, leading to the new hardiness zone projections.