Earlier this year, the USDA released an update of its 1990 Plant Hardiness Map. This map breaks the U.S. into horizontal bands or zones so we can all figure out which plants might do well in our area.
What the 2012 update shows is that those bands have shifted north by hundreds of miles since 1990. Indiana, for example, now has the climate that most of Tennessee had in 1990, and Tennessee now has the climate that most of Alabama had then. As Mother Jones tells it:
Compared to the 1990 version, zone boundaries in this edition of the map have shifted in many areas. The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States. This is mostly a result of using temperature data from a longer and more recent time period.
Despite the climate change they reveal, the maps look similar. Some sources have suggested that this was intentional in order to hide the evidence of climate change. But in 2006, the Arbor Day Foundation put together a nice map (above right) using the same scale as the 1990 USDA map (above left). It shows the shifting climate zones quite clearly.