An issue I discussed in my recent eBook is how Mark Steyn, widely admired figure within the Skeptic community, claims Michael Mann spliced instrumental temperature records onto his (in)famous hockeystick to cover up the fact proxy temperatures (estimated from things like tree rings) were going down. I discussed this because that claim is entirely false.
However, that Mann did not do this does not mean other people have not done it. I was recently surprised to see it is, in fact, an accepted practice within the paleoclimate community these days. This surprised me because years back when a user said:
Whatever the reason for the divergence, it would seem to suggest that the practice of grafting the thermometer record onto a proxy temperature record – as I believe was done in the case of the ‘hockey stick’ – is dubious to say the least.
Apparently holding the same incorrect belief as Steyn (misinformation tends to spread when nobody corrects errors like Steyn's), Mann responded:
No researchers in this field have ever, to our knowledge, "grafted the thermometer record onto" any reconstruction. It is somewhat disappointing to find this specious claim (which we usually find originating from industry-funded climate disinformation websites) appearing in this forum.
Mann said this in late 2004 so I can't fault him for being unaware of what would happen after 2010, but given the response Mann shows to this accusation, I find it strange this practice would be an accepted one a mere ten years later. Plus, I thought it was interesting nobody has pointed out any recent examples of it happening despite at least one being easy to find.
Before getting to a recent example, I want to revisit a bit of history. Mann's (in)famous hockey stick papers contained this figure (MBH Figure 7):
Some people, including Steve McIntyre, claim this shows Mann spliced instrumental temperatures onto reconstructed temperatures. The caption for it does say:
Relationships of Northern Hemisphere mean (NH) temperature with three candidate forcings between 1610 and 1995. Panels, (top to bottom) as follows. “ÅNH’, reconstructed NH temperature series from 1610–1980, updated with instrumental data from 1981–95. “
Which certainly supports that interpretation. However, the "splice" is clearly identified in the figure's caption, and if you look closely at the top chart of the figure, you can see the instrumental data is plotted differently than the reconstructed data. It takes a careful eye to see, but it is there. One might be concerned about using spliced data like this for calculations like those used in correlation coefficients in the bottom chart, but at least the splicing is made rather clear. That's not the case in some examples.
A paper which came out a bit later than Mann's contains a troubling example of splicing instrumental data onto reconstructed data. This paper is Crowley (2000). It's freely available here, though unforunately, I was unable to find a free copy of Crowley and Lowery (2000) which it takes the series in question from. Here is Figure 1 of the paper:
In the caption for it, we see:
Fig. 1. Comparison of decadally smoothed Northern Hemisphere mean annual temperature records for the past millennium (1000–1993), based on reconstructions of Mann et al. (Mn) (11) and CL (12). The latter record has been spliced into the 11-point smoothed instrumental record (16) in the interval in which they overlap. CL2 refers to a new splice that gives a slightly better fit than the original (12).
The author, Thomas Crowley, openly states he spliced instrumental temepratures onto the reconstructed temperatures. If you dig into the two papers, you can find the splice begins at ~1870. That means ~120 years of instrumental temperature data were spliced on, and unlike when Mann did it, they were plotted without any visual distinction. Interestingly, Crowley fails to note he has spliced data onto Mann's reconstruction as well, causing the red line in this chart to track the blue line better and further into the future than it would have otherwise.
Now, that was a 2000 paper. Mann was the lead author of a 2003 paper in the Eos journal with 12 other authors. It included this figure:
One might think the inclusion of the line labeled "Crowley and Lowery" would indicate Mann was familiar with the work done on and around this reconstruction and thus, familiar with how instrumental data was spliced onto reconstructed data.
I'm not bringing that up as a "Gotcha!" though perhaps such would be justified. I bring it up because it there is an interesting factoid most people have never noticed. That yellow line labeled "Crowley and Lowery" is not actually taken from any work done by Crowley and Lowery. Instead, it is... Mann's (in)famouse hockey stick. Somehow, this figure includes the same temperature reconstruction from Michael Mann (and two co-authors) twice, once labeled as Crowley and Lowery.
I have no idea how that happened. The error in the paper has never been corrected. It hasn't even been acnkowledged. Mann published a 2004 paper in which he said:
The Crowley and Lowery  series shown
here replaces an incorrect version of the series shown in similar previous comparisons [e.g., Mann et al., 2003a].
But somehow I don't think it's reasonable to say that's acknowledging the error. If Mann publishes his reconstruction while labeling it "Crowley and Lowery," that is not publishing "an incorrect version" of Crowley and Lowery. This isn't important for the issue of how acceptable paleoclimatologists feel splicing instreumental temperatures onto reconstructed temperatures is, but it's too funny not to bring up.
With that diversion out of the way, let's look at another publication from the same general time period. In 1999, Mann was involved in the creation of a figure for the cover of a World Meterological Organization (WMO) report. This figure:
Shows three paleoclimate reconstructions that are remarkably coherent in the modern period. It turns out the reason they agree that much is the people who made the figure spliced instrumental data onto the reconstructions and pretended they were one and the same. This was particularly egregiouos for the green line as Keith Briffa's reconstruction had also been truncated to 1960 because it supposedly produced bad results after that. That means data was deleted and then replaced with instrumental data... and it was impossible for an onlooker to know.
That was a pretty extreme example of splicing instrumental temperatures onto reconstructed temperatures. It has received a fair amount of attention. Given how negative that attention was, one might expect researchers would avoid splices like these in the future. At a minimum, you might think they'd do what Mann did in 1998 and plot the two types of data in a manner which made them visually distinctive.
However, I recently read a 2012 paper which I hadn't paid much attention to before. I was trying to track down what tree ring series had been used in another paper, and that paper cited this one for some of its series. I can't find any listing or archive of the data as used in the paper (I plan to contact the authors in the next few days). The authors say:
The tree-ring chronology network available for reconstruction of summer temperatures is shown in Fig. 1 as a series of filled red triangles. It is comprised of 323 annual tree-ring chronologies previously identified as potential predictors of the Palmer Drought Severity Index (PDSI) over monsoon Asia (Cook et al. 2010a) and 99 new treering records contributed by members of the PAGES Asia2k project, for a total of 422 chronologies.
This is a bit strange as even this paper notes:
Although neither property is desirable for our specific application, and almost certainly limits the quality of the temperature reconstruction that is currently achievable, this new tree-ring network is actually denser than the 327-chronology network used to successfully reconstruct drought over monsoon Asia (Cook et al. 2010a).
The Cook et al. 2010 data set contains 327 series. Why the 2012 uses only 323 of them is unexplained. I don't know what, if anything, that might mean. Perhaps a reader can shed some light. In the meantime, the reconstruction for this paper can be seen here:
Looking at this figure, there is nothing to suggest instrumental data has been spliced onto anything. The caption notes it:
Fig. 9 Asia2k summer temperature reconstruction from tree rings for temperate East Asia with 1 RMSE uncertainties (a) and the changing number and average replication of the chronologies used (b). The annually resolved values are in red, the 10 year low-pass spline filtered values are in dark blue, and the uncertainties are in pale blue. The time period covered is 800–2009 C.E., but the 1990–2009 data are instrumental data for comparison to the past estimated temperatures
As does the text:
The full period covers 800–2009 C.E., with the data since 1990 being instrumental for easy comparison of recent warming to earlier reconstructed temperatures.
For which the authors deserve credit. It's not like this was completely hidden as in the 1999 WMO report. Still, I can't help but wonder, why would you ever publish a chart showing instrumental and proxy temperatures on the same line with nothing to distinguish them? In 2004, Michael Mann thought it was "disappointing to find [the] specious claim" this happens on his website. Only, the claim is clearly true. Respected Paleoclimate researchers do this.
I don't have a concluding thought on all this. Personally, I think splicing series together like this is a bad idea. I don't know that it is necessarily "wrong" though. If I'm being honest, I'm more curious about those 323 series.