There is an interesting argument I have seen with regard to Mann's hockey stick reconstruction of past temperatures. I am not sure if it is correct. The hockey stick has been the subject of a lot of heated controversy pro and con and I am not in a mood at the moment to try to wade through it, so I thought I would sketch the argument and see if anyone here can give me a good reason to believe that it is either true or false.
It goes as follows:
Most of the reconstruction is based on proxies for temperature such as tree rings, since nobody was producing reliable instrumental data in 1500 A.D. The recent part, on the other hand, is based on instrumental data. The claim I have seen is that Mann's graph shows proxies up to one date, proxies and instrumental up to another date, instrumental only starting sometime around 1960-80. That fits what sense I can make of the graph in his paper, the exact date at which the proxies disappear being hard to make out. The further claim, and the critical part, is that if you run the proxies up all the way to the present they do not show as sharp a rise as the instrumental data.
The instrumental data is a more reliable source of information than the proxies for the period when we have it, so if the only objective is reconstructing what has happened to global temperatures over the past six hundred years or so it makes sense to use it where available. But part of the point of Mann's paper is that the recent temperature rise is unprecedented. To show that he needs an apples to apples comparison, a comparison between the proxy record for the past and the proxy record for the recent period of rising temperature or a comparison between the instrumental record for the past and the instrumental record for the recent period. The latter is impossible, since there is no instrumental record for most of the period he is looking at, but the former is not.
If the proxy record for the past century fails to show the rapid temperature rise in the instrumental record, that is evidence that the proxy record is an unreliable source of information, in particular unreliable as a way of detecting rapid rises. And it is probably more unreliable the farther back we go, since the farther back we go the less relevant data is available. If so, the fact that the proxy record shows no rapid rises in the past may not mean that they did not happen.
Can anyone tell me if this argument is correct? Does the proxy record for the second half of the 20th century show a substantially milder rise in global temperature than the instrumental record?