It is scientifically-improper to graft a thermometer record to a paleoclimate (e.g. tree-rings) proxy for temperature because trees are not thermometers, they are not calibrated to thermometers, there are many more important variables than temperature that can affect tree rings such as precipitation, location, CO2 (plant food) levels, cloudiness, soil quality, solar activity, weather patterns, etc. for which there is no reliable means to sort out. Even if this was proper, how can you possibly decide where to put the thermometer record on the Y axis compared to tree rings? Could just as well put it starting at -1 degree anomaly - nobody knows. Paleoclimate proxies can only be used to compare relative temperatures. For instance, the Medieval Warming Period (MWP) compared to the 20th century, which by the IPCC's own reconstruction does not show a statistically significant difference between the MWP and the 1940s as shown in the photoshopped graph. The tree-ring proxies started to take a dive around 1960 from which they did not recover, so to deal with this so called "divergence problem" the tree-ring data was either truncated at 1960 in 3 of the proxies, or left in as shown in the graph as taking a dive after ~1950 from which they never recover. But they did their best to further hide the decline by putting the thick black Trick line over the decline and making it look like the proxies showed an upturn in the late 70's just like the faulty thermometer record:
So this is why Mike's Nature Trick was just a "clever way to solve a problem" to quote Michael Mann, explaining away this deception as typical 'scientist talk'. No mention that the problem to solve was that the paleoclimate data showed the opposite of what was needed for the "unequivocal evidence" of unprecedented and man-made global warming. If the trees weren't good thermometers after 1960, then why were they good thermometers at any other point in the reconstruction? The IPCC AR4 continues on in this fine tradition, not allowing anyone to look at their graphs without the trick applied.
more on the "divergence problem":
A mathematical analysis of the divergence problem in dendroclimatologyCraig Loehle in Climatic Change, Volume 94, Numbers 3-4 / June, 2009
Abstract Tree rings provide a primary data source for reconstructing past climates, particularly over the past 1,000 years. However, divergence has been observed in twentieth century reconstructions. Divergence occurs when trees show a positive response to warming in the calibration period but a lesser or even negative response in recent decades. The mathematical implications of divergence for reconstructing climate are explored in this study. Divergence results either because of some unique environmental factor in recent decades, because trees reach an asymptotic maximum growth rate at some temperature, or because higher temperatures reduce tree growth. If trees show a nonlinear growth response, the result is to potentially truncate any historical temperatures higher than those in the calibration period, as well as to reduce the mean and range of reconstructed values compared to actual. This produces the divergence effect. This creates a cold bias in the reconstructed record and makes it impossible to make any statements about how warm recent decades are compared to historical periods. Some suggestions are made to overcome these problems.
and from one of the submissions to the UK Parliamentary Committee on climategate:
The Attempt to Present a "Nice Tidy Story" of Unprecedented 20th Century Warmth
2. The CRU emails, however, reveal that the authors of this material did not present a neutral view of the science. In particular, they downplayed the considerable uncertainty inherent in trying to approximate temperatures from proxy data over a 1000-year period, they suppressed contrary information, and they suppressed dissenting views in ways that made even their own colleagues uncomfortable. Thus, in one representative email written during the preparation of the TAR, Keith Briffa stated that "I know there is pressure to present a nice tidy story as regards 'apparent unprecedented warming in a thousand years or more in the proxy data' but in reality the situation is not quite so simple." He went on to say that "I believe that the recent warmth was probably matched about 1000 years ago." Similarly, another key researcher, Ed Cook, in a lengthy email bristling at the effort to eliminate the MWP, wrote that "I do find the dismissal of the Medieval Warm Period as a meaningful global event to be grossly premature and probably wrong."
3. These concerns, however, were brushed aside in the final TAR. The TAR's version of the temperature record of the last 1000 years was based on the now infamous "hockey stick" study of Mann et al., a study that purported to show 1000 years of slightly declining global temperatures followed by a sharp increase in the 20th century. The hockey stick paper concluded that the 1990s were the warmest decade and 1998 was the warmest year in a millennium. The hockey stick graph was the single most important piece of information in the TAR. It was Figure 1 of the Summary For Policymakers of the TAR appearing on page 3, and it was widely relied on by advocates.
4. Despite its prominence in the TAR, the hockey stick has now largely been discredited, with both the National Research Council ("NRC") and the independent Wegman Report rejecting confidence in the conclusion that the 1990s were the warmest decade and 1998 was the warmest year in a millennium. Although the hockey stick paper was cited in AR4, its significance was downplayed, and EPA did not cite the paper in the Endangerment Finding or TSD.
5. However, the same people who gave that paper such prominence in the TAR - despite the misgivings expressed internally within the group - continued to dominate paleoclimate research and were again the leading authors of the AR4 paleoclimate material. Indeed, perhaps stung by criticisms of the hockey stick and by the appearance of so-called "skeptics" who questioned the central conclusions of the TAR, the drafting of at least the paleoclimate chapter of AR4 became more of a political than a scientific process.
6. Thus, the two coordinating lead authors of Chapter 6 of AR4, Jonathan Overpeck of the University of Arizona and Eystein Jansen of the University of Bergen in Norway, openly coached contributors to produce materials that would serve a public policy agenda. As just a few examples, the CRU emails show that Overpeck instructed his colleagues to make sure that text was "FOCUSED on only that science which is policy relevant" and that would support pre-conceived summary bullet points. The pair also advised authors to include graphics that would be "compelling" and that the "sign of ultimate success" of a graphic would be that it was so compelling that it would be selected for use in the policymaker's summary. They told authors to "pls DO please try hard to follow up on my advice" to only refer to the MWP and the Holocene Thermal Maximum in a "dismissive" way. They expressed satisfaction with a graphic that described the MWP as heterogeneous - meaning that warming was not uniform on a planetary scale - not because it was accurate but because it read "much like a big hammer," driving home the point they wished to make. Moreover, although the hockey stick could no longer be relied on as a principal source of authority, authors were instructed that "[w]e're hoping you guys can generate something compelling enough" for the summary material for policymakers, "something that will replace the hockey-stick with something even more compelling." Yet new research that reexamined the data on which the IPCC relied has challenged the IPCC's dismissal of the MWP as non-heterogeneous, concluding that the IPCC's conclusion in this regard was, at least, "premature" and based on limited data.
7. The examples of this type of behavior abound.
The "Trick" to "Hide the Warming"
8. Much attention has been placed on Jones' now-famous email in which he stated that "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." The trick he and Mann performed was to hide a decline in temperatures appearing in tree ring data in the latter part of the 20th century. Unless this trick were used, their multi-century proxy temperature reconstructions would show an embarrassing decline in temperatures at the end of the reconstruction, a decline that was not paralleled in the record of directly measured temperatures, which showed an increase. To hide the decline in the proxy data, Mann and then Jones grafted on actual temperature data to the end of their proxy reconstructions rather than using the same proxy data as had been used throughout the reconstruction.
9. This trick makes the graphic presentations of the proxy reconstructions misleading, since the effect is to make it seem as if the proxy data shows rising 20th century warming when it doesn't. But the real deception in the trick was in hiding what became known as the "divergence" problem. The accuracy of tree ring data as proxies for temperatures can only be confirmed by comparing the proxy temperatures yielded by the tree rings with temperatures directly measured during the period when direct temperature measurements could be made. If the proxy data are contradicted by actual data, as they are for a significant period of the time when direct temperature measurements exist, the accuracy of the proxy data over the entire period of the proxy reconstruction is called into question. Thus, the divergence problem undermined faith in the ability of the proxy reconstructions to provide conclusive or even meaningful information about paleoclimate temperature conditions, even as the IPCC was relying on these reconstructions to conclude that temperatures in the 20th century had reached unprecedented levels in the last 1000 years. As one email candidly said, "[t]he issue of why we dont show the proxy data for the last few decades (they dont show continued warming) but assume that they are valid for early warm periods needs to be explained." These concerns, however, were given short shrift. Although divergence was discussed in AR4, the conclusion was reached that the results of the proxy temperature reconstructions remained valid and showed that 20th century warmth was likely unprecedented in 1000 years. If divergence was not a significant issue, however, one wonders why it was necessary to perform "tricks" to hide the problem.
10. More importantly, after AR4 was issued, at least three studies have been published reanalyzing the data used in the proxy reconstructions cited in AR4, including two by authors whose reconstructions were used in AR4. These studies concluded that, in fact, the divergence problem makes the reconstructions unreliable. According to one study, the divergence problem "serve(s) to impede a robust comparison of recent warming during the anthropogenic period with past natural climate episodes such as the Medieval Warm Period or MWP." Another study found that the divergence problem makes it "impossible to make any statements about how warm recent decades are compared to historical periods." Another concluded that the divergence problem "is of importance, as it limits the suitability of tree-ring data to reconstruct long-term climate fluctuations, particularly during periods that might have been as warm or even warmer than the late twentieth century."
11. It would seem, therefore, that the IPCC should have been more cautious in dismissing the divergence problem. It would also seem that the IPCC may have understood that there was something to hide after all.