It is true that, in Watson’s phrase, in the autumn of 1999 Jones and his colleagues were trying to ‘tweak’ a diagram. But it wasn’t just any old diagram.
It was the chart displayed on the first page of the ‘Summary for Policymakers’ of the 2001 IPCC report - the famous ‘hockey stick’ graph that has been endlessly reproduced in everything from newspapers to primary-school textbooks ever since, showing centuries of level or declining temperatures until a dizzying, almost vertical rise in the late 20th Century.
There could be no simpler or more dramatic representation of global warming, and if the origin of worldwide concern over climate change could be traced to a single image, it would be the hockey stick.
Drawing a diagram such as this is far from straightforward.
Gabriel Fahrenheit did not invent the mercury thermometer until 1724, so scientists who want to reconstruct earlier climate history have to use ‘proxy data’ - measurements derived from records such as ice cores, tree-rings and growing season dates.
However, different proxies give very different results.
For example, some suggest that the ‘medieval warm period’, the 350-year era that started around 1000, when red wine grapes flourished in southern England and the Vikings tilled now-frozen farms in Greenland, was considerably warmer than even 1998.
Of course, this is inconvenient to climate change believers because there were no cars or factories pumping out greenhouse gases in 1000AD - yet the Earth still warmed.
Some tree-ring data eliminates the medieval warmth altogether, while others reflect it. In September 1999, Jones’s IPCC colleague Michael Mann of Penn State University in America - who is now also the subject of an official investigation --was working with Jones on the hockey stick. As they debated which data to use, they discussed a long tree-ring analysis carried out by Keith Briffa.
Briffa knew exactly why they wanted it, writing in an email on September 22: ‘I know there is pressure to present a nice tidy story as regards “apparent unprecedented warming in a thousand years or moreâ€.’ But his conscience was troubled. ‘In reality the situation is not quite so simple - I believe that the recent warmth was probably matched about 1,000 years ago.’
Another British scientist - Chris Folland of the Met Office’s Hadley Centre - wrote the same day that using Briffa’s data might be awkward, because it suggested the past was too warm. This, he lamented, ‘dilutes the message rather significantly’.
Over the next few days, Briffa, Jones, Folland and Mann emailed each other furiously. Mann was fearful that if Briffa’s trees made the IPCC diagram, ‘the sceptics [would] have a field day casting doubt on our ability to understand the factors that influence these estimates and, thus, can undermine faith [in them] - I don’t think that doubt is scientifically justified, and I’d hate to be the one to have to give it fodder!’
Finally, Briffa changed the way he computed his data and submitted a revised version. This brought his work into line for earlier centuries, and ‘cooled’ them significantly. But alas, it created another, potentially even more serious, problem.
According to his tree rings, the period since 1960 had not seen a steep rise in temperature, as actual temperature readings showed - but a large and steady decline, so calling into question the accuracy of the earlier data derived from tree rings.
This is the context in which, seven
weeks later, Jones presented his ‘trick’ - as simple as it was deceptive.
All he had to do was cut off Briffa’s inconvenient data at the point where the decline started, in 1961, and replace it with actual temperature readings, which showed an increase.
On the hockey stick graph, his line is abruptly terminated - but the end of the line is obscured by the other lines.
‘Any scientist ought to know that you just can’t mix and match proxy and actual data,’ said Philip Stott, emeritus professor of biogeography at London’s School of Oriental and African Studies.
‘They’re apples and oranges. Yet that’s exactly what he did.’
Since Warmergate-broke, some of the CRU’s supporters have claimed that Jones and his colleagues made a ‘full disclosure’ of what they did to Briffa’s data in order to produce the hockey stick.
But as McIntyre points out, ‘contrary to claims by various climate scientists, the IPCC Third Assessment Report did not disclose the deletion of the post-1960 values’.
On the final diagram, the cut off was simply concealed by the other lines.
By 2007, when the IPCC produced its fourth report, McIntyre had become aware of the manipulation of the Briffa data and Briffa himself, as shown at the start of this article, continued to have serious qualms.
McIntyre by now was an IPCC ‘reviewer’ and he urged the IPCC not to delete the post-1961 data in its 2007 graph. ‘They refused,’ he said, ‘stating this would be “inappropriateâ€.’
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