New Scientist ran a lengthy article on the Hockey Stick. They seem to have talked to everyone involved except Ross and I.
In 2004, even before our GRL article published, a freelancer for New Scientist had got interested in the story and spent a lot of time interviewing me on the telephone. It got to a very advanced stage and then got spiked by the New Scientist editor, following some ExxonMobil type disinformation of the type that Mann sent to Natuurwetenschap to try to prevent publication there
The editors decided not to publish it after all. Your connections with the oil industry raised doubts in their minds about your disinterested independent researcher status and the scientific corroboration from other groups for Mann’s findings persuaded the editors that the story simply did not stand up.
First of all, New Scientist argued that the hockey stick itself doesn’t “matter” to the AGW debate.
The hockey stick has been repeatedly misrepresented as the crucial piece of evidence when it comes to industrialization and global warming. It is not. Even if the hockey stick were shown to be a doodle that Mann did on a napkin during a night out, the evidence that the world is getting warmer and that this warming is largely due to human activities would still be overwhelming.
Having done so, they pose the multiproxy problem as follows:
Leaving that aside, did Mann get it right? . There is no doubt that reconstructing past temperatures from proxy data is fraught with danger. Take tree ring records. They sometimes reflect rain or drought rather than temperature. They also get smaller as a tree gets older so annual or even decadal detail is lost.
To reveal the “signal” behind the noise of short-term and random change, a proxy record for one region must be based on as many tree ring records as possible. It must also correlate with direct measurements of temperature during the period of overlap” which adds another layer of complication as in some cases human factors such as pollution might have affected recent tree growth.
It’s not the worst statement of the problem. However, using “as many tree ring records as possible” is not a sensible strategy if your methodology is a data mining methodology, such as both stages of MBH98, the tree ring PC methods and their multivariate step. They then ask the question about whether the proxies are any good, quoting Jacoby on a series from central China “making tree ring people angry”
So the first question is whether the proxy records Mann chose are reliable indicators of temperature. Some have been questioned. He has a series from central China that we believe is more a moisture signal than a temperature signal, says Jacoby. He included it because he had a gap. That was a mistake and it made tree ring people angry.
Mann accepts that some of the measurements he used do not directly represent temperature change. His argument is that, for instance, coral records showing rainfall records in the Pacific are proxies for El Nino cycles and so for changes in ocean temperature. Jacoby is not convinced”
I’ve never seen any criticism by tree ring people of this series from central China (which doesn’t affect MWP or 15th century issues.) I think I know which one is involved, but it would be nice to confirm. And why would tree ring people dispute this Chinese series and not dispute the 11 instrumental precipitation used by Mann, which include the delicious mislocations of French precipitation records to North America and the still unlocatable precipitation record assigned to Bombay (or many other precipitation-related records in MBH98). The New Scientist then quotes Mann on regional aspects (following a theme of Crowley’s):
Indeed, the proxy records suggest that high temperatures in one region tend to be balanced out by low temperatures in another. The tropical Pacific for instance appears to have cooled in the Medieval Warm Period and warmed during the Little Ice Age, “The regional temperature changes in our reconstruction are quite large; it’s simply that they tend to cancel out” says Mann.
Mann’s reconstruction in the MWP and the 15th century only reconstructs one PC. Hence it has no regional properties in its early portions. (From the mid-18th century, his reconstructions generate regional patterns, but not in the early going.) I wonder what proxy records that Mann has in mind for this claim about the tropical Pacific in the MWP and LIA. The other issue with proxies cancelling out is that many of the proxies are simply noise (or precipitation related) and that might be the reason that they cancel out. Mann goes on to puff his error bars:
Mann also points out that he was one of the first to include error bars, which show how much variance is lost due to smoothing,
Of course, these error bars were calculated in MBH98 based on calibration period residuals, which were hugely overfitted. Had verification period residuals been used, the confidence intervals would have exceeded natural variability since r2 was ~0. Then New Scientist gets to the MM dispute as follows:
A more serious accusation has come from two non-climate scientists from Canada, who claim to have found a flaw in Mann’s statistical methodology. McIntyre and McKitrick claim that the way Mann applied this method had the effect of dampening down natural variability, straightening out the shaft of the hockey stick and accentuating 20th century warming.
There is one sense in which Mann accepts that this is unarguably true. The point of his original work was to compare past and present temperatures so he analysed temperatures in terms of their divergence from the 20th century mean. This approach highlights differences from that period and will thus accentuate any hockey stick shape if, but only if, he insists, it is present in the data.
The charge from McIntyre and McKitrick is however is that Mann’s computer program does not merely accentuate this shape but creates it. To make the point they did their own analysis based on looking for differences from the mean over the past 1000 years instead of from the 20th century mean. This produced a graph showing an apparent rise in temperatures in the 15th century as great as the warming occurring now. The shaft of the hockey stick had a big kink in it. When this analysis was published last year in GRL, it was hailed by some as a refutation of Mann’s work.
The data mining from Mann’s PC methodology has been well-publicized, but people tend not to get the nuances right. You can get a hockey stick shaped PC1 from series in which there is no hockey stick shape in the underlying data. His method will pick out and overweight series with 20th century trends and flip the series so that the trends are all in the same direction. In the shaft of the stick as you get away from the common 20th century feature, the noise cancels out. Since the PC1 is a weighted average of the various series, the noise features cancel out. The variance of the average is small in the shaft, but large in the blade.
In the empirical situation of the North American tree ring data set about which there’s been so much dispute, there actually are some hockey stick shaped series in the data set (bristlecones). Given that the “Artificial Hockey Stick” effect exists with red noise, it doesn’t take much imagination to contemplate that it’s really enhanced by actual hockey stick shaped series. But of you take out the bristlecones, there’s no hockey stick (and bristlecones were known beforehand to be problematic)
New Scientist fairly states that we do not present an alternative reconstruction. It’s too bad that Climatic Change seems unable to address this error in Wahl and Ammann (even though it’s been pointed out to them):
M&M say their work is intended only to show that there are problems with Mann’s analysis. They do not claim their graph accurately represents past temperatures.” We have repeatedly made it clear that we offer no alternative reconstruction”, McIntyre states on his Climate Audit blog.
Then they get to Wahl and Ammann:
The work of Eugene Wahl of Alfred University and Caspar Ammann of NCAR in Boulder CO raised serious questions about the methodology of Mann’s critics. They found that the reason for the kink in the McIntyre and McKitrick graph was nothing to do with their alternative statistical method. Instead it was because they had left out certain proxies, in particular, tree ring studies based on bristlecone pines in the SW of the US.
“Basically the MM case boiled down to whether selected North American tree rings should have been included, and not that there was a mathematical flaw in Mann’s analysis” Ammann says. The use of the bristlecone pine series has been questioned because of a growth spurt around the end of the 19th century that might reflect higher CO2 levels rather than higher temperature and which Mann corrected for.
Once again, Ammann has misrepresented matters. Wahl and Ammann have a nasty habit of taking results that we reported, claiming them as their own and then reproaching us for not reporting them – a point that we made in our GRL Reply to A&W. After all the ink that we’ve spilled on bristlecones, it rankles that Ammann says that we’d “left out” the bristlecones. We didn’t “leave out” the bristlecones. We showed that MBH results were sensitive to the presence/absence of bristlecones or to the presence/absence of the bristlecones in the PC1. We were completely explicit about bristlecones. We did not claim that the MBH results came simpliciter from their PC method, but that the bad method interacted with the worst proxies. As to the famous CO2 adjustment, Mann did not make any adjustment to MBH98 for CO2 – that is pure disinformation. Plus his CO2 adjustment – based on 19th century saturation and low-frequency wiggle matching – is nonsensical.
Then New Scientist gets to the “other studies”:
What counts in science is not a single study.. here Mann is one a winning streak – upwards of a dozen studies, some using different statistical techniques, different combinations of proxy records (excluding the bristlecone pines) have produced reconstructions more or less similar to the original hockey stick”
Whatever the flaws in the original work, it seems the broad conclusion is correct. McIntyre is not impressed. “There is a distinct possibility that researchers have either purposefully or subconsciously selected series with the hockey stick shape”, he told one reporter.
Readers of this blog are familiar with my position on the other studies. They are not “independent” in authorship. The proxies are not “independent”. Bristlecones are used repeatedly despite their problems.
Since MBH, a couple of other hockey stick shaped series have turned up (e.g. Yamal, Jacoby in Mongolia). It only takes a few series to imprint a hockey stick shape on a small subset. Osborn and Briffa 2006 tried to show that their reconstructions could survive 3 series deletions, but they started off with 2 bristlecone/foxtail series, Yamal, Mongolia and Dunde- all the stereotypes.
The Hockey Stick studies all have nearly identical statistical problems – failed Durbin-Watson in calibration; collapse in verification r2 values and a spurious RE statistic.