Let's look at how you're looking at the data:
... look at the red average line - it shows a clear upward trend over the last 30 - 40 years. Importantly, it shows an increase of about 8 degrees (from about -8 to about 0). If I go back a 100 years, it still shows 6 degrees warming.
Right there. You can only get the 8 degrees of warming if you choose the lowest possible point on the chart and compare it to the highest possible point. You're cherry picking your comparison period to give the result you want. Now, the avg coldest temps in the 2000's have been historically warm but that's all you can say and it's a mistake to jump to a conclusion about the cause, particularly when you look at the overall chart -- note the decline from the 1950's to the early 80's.
On to the earlier chart of avg hottest temps:
Now let's look at the chart your referred to. Again looking at the average line, over the same period not only it shows a mostly sideways trend, when you look at the recent tail end where the drop occurs, over the same period, it shows a drop of only about 1 degree - going from about 98 to about 97.
Wow, earlier you were looking only at recent years ... now you decide we need to look at 100 years ago! I would suggest what stands out on this chart is the decline in hottest temps from the 30's to today. This is the period when fossil fuel use has really taken off. Yet the hottest temperatures were back in the 30's when people were just starting to buy cars. This should be a problem for the overall fossil fueled warming idea.
In fact, you can look at the average line and compare today's numbers to that of 100 years ago and you will see a warming of the coldest day by +6 degrees while the hottest day has hardly moved at all (maybe 1 degree cooler?). So even using your own method it comes out with a net warming of at least 5 degrees over a hundred years.
So the entire century of warming consists of winter cold extremes being higher. Not from the warm parts of the year posting higher temperatures.
I'd say that shows temperatures in the US are NOT becoming more extreme. In fact, if anything, temperature highs and lows are becoming less extreme.
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