What really jumped out at me when I saw CC's maps was that they show exactly (and not surprisingly) what my own trend-line graphs of Extreme Minimum Temperatures (EMT) indicate, which is (1) that the whole northern tier of states has gotten significantly warmer and (2) that the further north you go the sharper the change. The difference between CC's work and mine is that the Minnesota Rose Gardener graphs cover a longer period, i.e. since 1962 vs.1970, and that my graphs are dynamic, in that the trend lines, being statistically valid, are predictive of the future.
Here is CC's map of the United States:
Note that the upper-midwest generally has warmed more than the rest of the country and that the areas around and just north of the Twin Cities, and in the northwest corner of Minnesota, have warmed more than just about anywhere else in the country (my upstate NY and New Hampshire readers will be happy to note they are paralleling the upper-midwest).
This is very clear when you look at my 53 year EMT trend-line graph for the Twin Cities. Note the relatively sharp slope of the line and how it points upwards to USDA Zone 6. I will show several other cities' EMT graphs below, which show definite warming, but with shallower trend lines than Minnesota, just as the CC map shows.
Here is a regional close-up of the upper-midwest from CC's article:
And now let's look at the EMT chart for Milwaukee to see the difference in the slope of their warming trend-line:
Note that the slope of the trend-line, while definitely upward, is not quite as sharp as the Twin Cities' line and, while they've had two winters solidly in USDA Zone 7 in this decade, the line predicts that it will be quite a few years before they move into Zone 7.
Here is Detroit, which has now crossed into Zone 7, as predicted by the trend line, albeit with a very shallow slope over quite a few years.
Here is CC's regional close-up of the Ohio Valley:
And here is my EMT graph of St. Louis, which has moved solidly into Zone 7, with a fairly sharp upward trend:
And here is Chicago, which perennially was in Zone 5 but now has moved solidly into Zone 6. Note that the slope of its trend line is very similar to Milwaukee's, which should be no surprise, but they too are quite a long way from being consistently in Zone 7.
Finally, here is Indianapolis, which has a trend line a bit shallower than Chicago or Milwaukee, but the line projects that it is almost in Zone 7 (and actually had a Zone 8 winter in 2012).
So What Happened Last Winter?
That's a perfectly logical question, in the face of all the trend line evidence of warming. Most climate scientists believe that the "polar vortex" phenomenon we experienced last year was (or is) a product of climate change and that it is unlikely to repeat itself with regularity. However, that is certainly not to say that it won't happen again or that we might not see variations of it. Note in the graphs above that last winter was a big departure from all trend lines, especially in some of the warmer midwest cities like Detroit, St. Louis and Indianapolis. With a developing El Nino, which is also a child of climate change, it seems unlikely that we could see another major vortex incursion in the winter of 2014-15. For a more detailed explanation of the polar vortex and its presumed causes, please see my October 2014 blog: "Winter Protecting Roses in a Climate Change Environment".
Finally, I would like to emphasize the statistical validity of the trend-lines generated on the graphs in this article. As mentioned earlier, these trends are both predictive and dynamic, in that you should be able to extend them into the future with a fair degree of accuracy, regardless of one year anomalies in either direction. For example, it is logical to conclude that the Twin Cities will see more winters in Zone 6 than in Zone 4 in coming years and that, within the next seven or eight years we will see consistent Zone 6 winters. This can be clearly seen on the St. Louis graph, as it progressed along its trend line to where it crossed into Zone 7. And the same can be seen on the Detroit chart, although its change was more gradual and over more years.
Having said all of this, I am not implying that winter-protecting roses isn't important to prevent the inevitable freeze-thaw cycle. Rather, I am saying that extreme measures, such as the Minnesota Tip, are unnecessary, if sensitive roses are properly planted with bud unions below ground. Please see the article cited above, as well as my September 2013 article "Winter Protecting Your Roses" for more complete explanations: