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Comparing crop acreage harvested per county, in US, 1948-1952 vs. 2008-2012. The article was posted in USA Today with the title Climate Change Changing Agriculture. It is an interesting visual presentation (in USA Today) as you can superimpose the two images for better comparisons. Here, you can see 1948-1952 at the top, and 2008-2012 at the bottom.
Clearly, there is an expansion towards North, but I also notice expansion to the South and East - indeed as far South as the southern tip of Texas (many new counties where corn is harvested, although acreage is small as the color is light). Or was Texas not tracked back in 1948, making comparison meaningless? Maybe the Texas corn species is different from the one now growing in North Dakota.
So, how do you measure, which metric should you use to prove that there is clearly a trend? And it's clearly North, despite slight expansions southwards and eastwards as well. Clearly, a question for statisticians. Maybe different people will have a different opinion if you based your judgement on these two maps alone.