There seems to be lot of interest, and also a lot of confusion, in trying to interpret what the stolen CRU e-mails really mean. In some of the following posts we will try to place some of these emails in their proper background and include some explanations for those interested. I will try to do it in a neutral way, and let the reader make his/her own judgement.
The email alluding to the 'Mike's Nature trick' has been perhaps the one most frequently quoted.
Background: temperatures during the past few centuries and millennia are reconstructed from the so-called climate proxy data, of which the rings of old or fossil trees are the best example. In some places, at high altitudes or high latitudes, the growth of trees is severely constrained by cold temperatures, so that in warmer than normal summers, trees tend to grow thicker rings or build wood of higher density, for instance. By applying statistical methods, the treering width can be interpreted in terms of past temperatures being above or below normal on those locations. The statistical methods themselves are subject to considerable debate, but this is not what this particular 'trick' is about. It happens that some trees - how many and where is also debated - show a 'divergence' from their local temperature since 1980 or so: they do not reflect the warming experience in some of these locations in the last 20-30 years. The reasons for this divergence are so far not well known, but several hypothesis concerning other environmental factors, such as air pollution, have been put forward. To explain this divergence is important, because as long as it remains unexplained, it can be suspected that this divergence may have happened also in the past, thus increasing the uncertainty in the reconstructions of past temperatures.
The 'trick' was to not show this mismatch between treering records and temperatures in the late 20th century. Instead of showing the treering records drifting away from the instrumental temperatures, the instrumental temperatures were substituted for them, 'hiding the decline' of the treerings (and not of the temperatures). In doing so, a potential problem of temperature reconstructions was ensconced and an artificial sense of robustness of these reconstructions was conveyed. An important consequence is that claims about the record level of warmth of particular recent years, say 1998, against the backdrop of the past millennium are not really meaningful, because data of different nature are being compared: instrumental versus treerings.
By the way, this divergence problem was and is well known by dendroclimatologist. Some accept this 'trick' as as a makeshift solution until the real explanation for the divergence is found. But the current situation would have been avoided if, from the very beginning, these difficulties had been openly presented and discussed.