I look at this and first think… wow that is cold. Then I look at it again and wonder “how would I figure out an average temperature” of the US and Canada from a series of maps like this. How would I figure out if it was getting warmer or colder. It isn’t obvious and lots of problems immediately crop up.
One example being… it was 21 degrees when I woke up this morning, and it is now 46 degrees. Which temperature should I use? Should I average the temperature taken each hour (minute, second) over 24 hours and use it for the day? Another, is I have two weather stations at my house… one said 21, the other 28. Two very accurate digital yielded very different readings 50 yards apart. I have two because I updated to a new one and never took down the old one.
This is why I don’t trust any climate science paper that will not share data and exact methods. The methods, instruments, station location, and what data is used can make a big difference!