Steve McIntyre explains the “trick” referred to in the CRU emails. The trick is subtle, which allows the scientists to weasel out, saying things that are technically true but in essence false and misleading.
Most of the proxy series are smoothed in some way. Most smoothing algorithms adjust a data point by averaging in data both forwards and backwards in the series. A simple algorithm puts high weights on nearby data points in this averaging and relatively lower weights on data points further away.
The problem occurs when the series reaches its end. There are not points forward in the data series to average. By the last point in the series, fully half the data necessary for smoothing does not exist. There are various techniques for handling this, all of which have trade-offs and compromises (at the end of the day, you can’t create a signal when there is no data, no matter how clear one’s math tends to be).
The trick involved taking instrumental temperature records and using these records to provide data after the end point for smoothing purposes. This tends to force the smoothed curves upwards at the end, when there is no such data in the proxy trend to substantiate this. The perpetrators of this trick can argue with a semi-straight face that they did not “graft” the instrumental temperature record onto the data, but the instrumental temperature records does in fact affect the data series by contributing as much as half of the data for the smoothed curve in the end years.
I have always considered the “we-don’t-graft” claim disingenuous for another reason. This is driven in large part because I have spent a lot of time not just manipulating data, but thinking about the most effective ways to represent it in graphical form.
To this end, I have always thought that while folks like Mann and Briffa have not technically grafted the instrumental data, they have effectively done so in their graphical representations — which is the form in which 99.9% of the population have consumed their data.
Below is the 1000-year temperature reconstruction (from proxies like tree rings and ice cores) in the Fourth IPCC Assessment. It shows the results of twelve different studies, one of which is the Mann study famously named “the hockey stick.”
All the colored lines are the proxy (tree ring, ice cores, sediments, etc) study results. The black line is the instrumental temperature record from the Hadley CRU. There is no splice here – they have not joined proxy to instrument. But they have effectively done so by overlaying the lines on top of each other. The visual impact that says hockey stick is actually driven by this overlay.
To prove it, lets remove the black instrumental temperature line as well as the gray line which I think is some kind of curve fitted to all of the above. This is what we get:
Pretty different visual impact, huh? The hockey stick is gone. So in fact, the visual image of a hockey stick is driven by the overlay of the instrumental record on the proxies. The hockey stick inflection point occurs right at the point the two lines join, raising the distinct possibility the inflection is due to incompatibility of the two data sources rather than a natural phenomenon.