Thanks for the feedback, Heather. You're right that using different scales creates the potential for misinterpretation. The problem is, as you alluded to, that using the same scale could create misunderstanding of a different kind. The trouble is that the ranges of the source data in each map vary by nearly an order of magnitude. So using the same scale for both maps would invariably cause one map in each pair to lose most of its geographic detail.

If the author used the plus/minus 10 scale from the 30-year map for the full time period map, nearly the entire U.S. would be colored a single color: the light gray of the 0-2 degrees bin. Meanwhile, if the author used the plus/minus 2 degrees scale of the full-record map on the 30-year map, all the areas with a trend above 2 F would be solid red (most of the West third of the country), and all the areas with a trend lower than -2 F/century would be solid blue (most of the eastern 2/thirds of the country). Our data visualizers talk about this challenge of balancing these two undesirable outcomes as the "blow-out versus wash-out tradeoff."  

I believe that the author decided that being able to compare the exact values of the 30-year trend at a given place to the exact values of the full-record trend was less important than showing the local and regional variability within each time period, and thus chose to use scales that were optimized for each map. The decision to arrange them with a slider was mine, as the editor, and I was thinking mostly of usability and small screens (the slider allows the images to be viewed without scrolling up and down).

In hindsight, I probably should have suggested we iterate on the maps further to see if we could come up with a scale in-between these two extremes that would have provided a reasonable compromise between consistency and geographic detail. What I will do now is update the caption to make sure to call attention to the different scales to potentially head off any misinterpretation. 

Thanks for your feedback!