With gardens a-sprouting, a warm, wet winter behind us,1 and a hotter-than-average summer for much of the country ahead, we decided to look at whether and how climate change was affecting what plants can grow around the country. The easy data solution — or so it seemed — was to look at a series of maps dedicated to showing Americans what plants can survive in their neck of the woods. These are called plant hardiness zone maps, and they’ve been produced since the 1960s by the U.S. Department of Agriculture.
But then we noticed something weird. The USDA’s website specifically asks people not to use these maps to document climate change. Meanwhile, it looked as if other parts of the federal government were doing exactly that in reports such as the National Climate Assessment.
So what gives? It turns out, the government produces two hardiness zone maps — one made by the USDA and one made by the National Oceanographic and Atmospheric Administration. Both divide the country into segments, each of which represents a 10-degree increment of the average annual minimum temperature. But the underlying data used to build out the zones is different. Those differences are driven by the agencies’ goals, and they affect what the different maps are intended to be used for.
Here, you can see the most current versions of the USDA and NOAA zone maps. They’re drawing from slightly different sets of annual data, with the USDA map using the average temperatures from 1976 to 2005 and NOAA using 1981 to 2010. But the differences go deeper.
The USDA’s map, unsurprisingly, is geared toward agriculture and helping people produce a healthy crop. Its makers want it to be as detailed as possible, because small shifts in local geography — elevation, the slope of the land, the prevailing winds, bodies of water — can make a big difference in average temperature. Historically, a lot of that detail was missing, said Kim Kaplan, a USDA spokeswoman who works on the Plant Hardiness Zone Map. The 1990 version, for instance, relied on readings from National Weather Service temperature stations. Where there wasn’t a station, the mapmakers just drew straight lines — literally, with a ruler — from one data point to the next closest.
That changed in 2012 when the USDA began to make its hardiness zone map using an algorithm produced by researchers at Oregon State University that could account for those local variables and get a better estimate of the annual minimum between weather stations. What’s more, the new map was ground-truthed, checked against the instincts of local weather experts. The result is a more accurate map — but a map that differs a lot from its earlier iterations. The NOAA hardiness zone map, on the other hand, is optimized to show how climate has changed over time by maintaining the same methodology with every update, even if better methodology comes along. “[The USDA map] is always the ‘A,’ and ours is a good ‘B,’” said Russell Vose, chief of the Climate Science Branch at NOAA’s National Centers for Environmental Information. And, for the most part, the two maps match. But you can see some differences in the map above, especially in mountainous regions where the USDA map now captures the way elevation affects average temperature in a more granular way.
To understand how average temperatures have changed over time, on the other hand, you have to look at multiple versions of the NOAA maps. They can’t tell you much about the gardening in coastal San Francisco versus mountainous Lake Tahoe, but they can tell you that northern Texas is getting hotter, creeping toward an average more like that of central Texas.
When the USDA says you shouldn’t use its map for documenting climate change, what the agency means is that it’s impossible to tell what differences between the 1990 and 2012 versions are caused by climate change, and which are artifacts of their methodology upgrade. (Likewise, the folks at NOAA say the USDA map is the best one to use if all you want to do is grow tomatoes.)
But Kaplan and Vose are aware that it’s easy to get the two hardiness zone maps mixed up and that most people probably don’t understand why the distinction matters. News stories about the USDA’s 2012 update even talked about the application of that map to climate change, despite Kaplan’s insistence that it shouldn’t be used for that purpose. USDA “wanted the most detailed approach because that goes on the back of seed packages,” Vose said. NOAA “is just the best guess. But it’s the same approach for each slice in time.”