Every month when he announces the UAH lower troposphere global temperature anomaly for the previous month, he includes on his graph a 3rd-order polynomial fit
and writes, "The 3rd order polynomial fit to the data (courtesy of Excel) is for entertainment purposes only, and should not be construed as having any predictive value whatsoever."
Recently some people (including me) have complained, in part because the caveat doesn't always get replicated with the graph is reproduced, and in fact Spencer himself doesn't always replicate the caveat. That's misleading.
Some people defend it. Someone named "Bill A." even thinks it's on the short list of those approaching a realistic possibility."
Hmm. Let's see.
If you use Excel to do a 3rd-order polynomial fit to the UAH LT data you get
temperature anomaly = aD3 + bD2 + cD + d
where D is the date (as an integer; in Excel, D=1 is January 1, 1900), and where up through March 2012 the best fit is
a = -1.13719E-12
b = 1.21004E-07
c = -0.004228872
d = 48.51822318
This function peaks in November 2008. Hmm. For January 2020 it "predicts" the temperature anomaly will be -0.13°C, and it only gets worse from there.
By January 2035 it "predicts" an anomaly of -2.1°C
By Jan 2050, it predicts -7.0°C
and by January 2171 the anomaly will be -287.45°C, which, if you take the baseline to be 14°C, is below absolute zero and violates the 3rd law of thermodynamics.
Of course, it's absurd to project a fit that far, no matter what degree polynomial it is. But that's part of the point -- the 3rd order fit seems selected for the one that shows the data peaking and decline in the near future. (Actually, Excel says a 6th-order polynomial is a better fit, with a slightly higher R2 value.)
On the other hand, a linear fit "predicts" the anomaly will be about 0.7°C in 2050.
So, if you had to make a fit, which seems more realistic: 0.7°C, or -7°C?
But besides any of that, I think the data keepers should, above all, not be the ones using it for entertainment, especially in a way that seems to support their inclinations. Not everyone is going to get the joke.