I was thinking of what is perhaps the most cited email of SwiftHack; this one by Phil Jones:
From: Phil Jones
To: ray bradley ,firstname.lastname@example.org, email@example.com
Subject: Diagram for WMO Statement
Date: Tue, 16 Nov 1999 13:31:15 +0000
Dear Ray, Mike and Malcolm,
Once Tim’s got a diagram here we’ll send that either later today or first thing tomorrow. I’ve just completed Mike’s Nature trick of adding in the real temps to each series for the last 20 years (ie from 1981 onwards) amd from 1961 for Keith’s to hide the decline. Mike’s series got the annual land and marine values while the other two got April-Sept for NH land N of 20N. The latter two are real for 1999, while the estimate for 1999 for NH combined is +0.44C wrt 61-90. The Global estimate for 1999 with data through Oct is +0.35C cf. 0.57 for 1998.
Thanks for the comments, Ray.
Some people seem to be under the impression that the "decline" refers to the decline in temperatures shown in some databases when you cherry-pick 1998 as the starting year in temperature slope calculations. The email was written in 1999, so that can't be it, first of all.
Second, how exactly do you fudge data by adding in "real" data? That doesn't make any sense on the surface.
Once you parse the statement further, you realize that "Mike's Nature trick" is a reference to Figure 5b of Mann, Bradley & Hughes (1998), a paper published in Nature, and whose first author is Mike Mann. The figure follows.
The figure is not very clear, unfortunately, because it's in black and white, but you can see it's simply a graph of multiple time series: (1) A historical temperature reconstruction up to 1980, (2) The instrumental record (referred to as "actual data" in the figure, and "real temps" by Phil Jones), and (3) a 50-year low-pass filter of the reconstructed series. Really, there's nothing underhanded or nefarious about plotting multiple series in one graph. It's not much of a "trick" at all.
In addition to this, Phil Jones says he added the real temps to Keith Briffa's series, starting in 1961, in order to "hide the decline." That's the interesting part, isn't it? After some digging (hat tip Deltoid) it's clear the "decline" after 1961 refers to something related to tree-ring data that was documented in Briffa et al. (1998). Figure 6 of this paper follows.
It's pretty clear why you'd want to add the real temps starting at about 1961. Again, I don't think there's anything wrong with this.
Some people might insist, however, that throwing away the last part of a series because it doesn't show what you think it should show is just wrong. It's not valid scientific methodology, and so on and so forth, ad nauseum.
Is it wrong? Are you sure? Let me show you something that I bet most readers of this blog have not seen.
This is a graph of the administrative prevalence of autism by birth year. The X axis is the year of birth. For example, the administrative prevalence of autism for children born in 1992, according to special education counts, was 34.5 in 10,000 as reported in 2003. The prevalence is much lower for children born in the year 2000: 12 in 10,000.
Is it actually true that the prevalence of autism is that low for children born in 2000? Absolutely not. That's what the 2003 report said. The 2009 report will tell you something very different. You see, it takes time for children to get diagnosed with autism. If they are too young, they might not have any label at all, or they might be classified under a different label.
Autism prevalence by birth year series will always decline in recent years. They also change as the series are revised year after year. (That's why I suggest using prevalence at age by report year instead, but I digress.)
If you must use a prevalence by birth year series, there's a "trick" you can use to "hide the decline," and this "trick" has a name: You left censor the series. In an administrative autism series, you probably should not consider children below the age of 8.
I have used this "trick" myself in a blog post titled Is Precipitation Associated with Autism? Now I'm Quite Sure It's Not. I said: "I'm left-censoring autism caseload starting at 2000." So there you go. Joseph, from the Residual Analysis blog, has admitted to using a "trick" to "hide the decline." Feel free to quote me on that.
Once again, I think AGW deniers need to grow up a little.