On July 1 NOAA’s National Climatic Data Center released the new U.S. Climate Normals based on 1981-2010 data.

“Normals” are a 30-year average of daily and monthly temperatures, precipitation, snowfall, snow depth, and heating and cooling degree days. We hear them every day when the weather forecast is compared to normal and they’re used to forecast energy loads, crop planting times and construction schedules, to name a few.

Every ten years NOAA recalculates the normals using an international protocol that drops the oldest decade and incorporates the most recent. On July 1, the 1970s were dropped and the 2000s included.

The result shows that the average U.S. temperature increased half a degree. 2001-2010 was the hottest decade on record — 1.5oF hotter than the 1970s — but the normals are a three-decade average so it comes to 0.5oF. Surprisingly, most of the temperature gain was through warmer nights, not hotter days. January minimum temperatures rose 5 degrees in the most northern U.S. states.

Does that half a degree matter?

Indeed it does. Warmer nights mean that killing frosts start later in the fall and end earlier in the spring. Wherever you live, the growing season is a little bit longer.

You can see this on the NOAA map above where each colored stripe represents the change in a plant hardiness zone. Pittsburgh is squarely inside Zone 6 so we aren’t in a colored stripe — yet — but northwestern Pennsylvania and the Allegheny Plateau have warmed enough to move from Zone 5 into 6. Michigan changed a lot!

You might think this is great news but there are bad side effects. If you rely on plants at the southern edge of their range, those plants are stressed because it’s too hot for them. Even if you hate winter, you’ve got to admit it’s a great pest control system. Deep freezing nights protect our plants by killing many insect pests. Sadly, pine bark beetles now thrive in Colorado and Canada because their winters are no longer cold enough to kill them.

If you’ve been gardening or farming for the past 30 years you’ve noticed the shift in temperature and you’ll be glad to know that USDA is revising their 1990 Plant Hardiness Map based on the new data.

When the new map comes out, keep it handy — but expect to need a new one in only 10 years. According to NOAA’s Climate Center, plant zones will shift dramatically by 2041. By then Pittsburgh will feel like Tennessee. We’ll be in Zone 7.

Click on the map above to read Plants (and Pests) Respond to Warmer Nights and to see larger versions of the range maps.

(The image above is NOAA Climate Services‘ map of plant zone changes based on 1981-2010 U.S. Climate Normals. Click on the map to see the original.)