> The 2012 USDA hardiness zones were calculated using the average lowest winter temperature for the observation period of 1976-2005. The new zones are calculated using the years 1991-2020. These two observation windows overlap. Colors show the difference between the two 30-year averages for each place on the map.
So while climate change may or may not be the explanation for broader trends in the map, you cannot rule out noise for any particular location, especially given that the methodology uses the lowest winter temperatures in that window as the basis for the zone. That's a fundamentally noisy metric. For example, this drops the blizzard of 1978 -- one of the largest ever in US history -- from the dataset. It's hard to know how that affects the zones without careful inspection, but major weather events like that can easily affect simple outlier analysis. You can see this in the dot plots for Raleigh (the default), which had two outliers in the winters around 1995, and then an otherwise flat series of data. What was the prior dataset? We don't know. Just look at the two default examples:
It would only take a small number of cold or warm winters to shift those means in either direction.
I don't want to overemphasize a silly little article about plants, but IMO, the fact that NPR buried this information while hyper-focusing on specific locations is just another failure of basic journalism. Telling people that an arbitrarily defined metric has been changed is meaningless, unless you tell people the specific change that was made. It should be the first or second thing they tell you, not buried in a hidden footnote.