Will Robinson wrote:I get the impression that the MWP is central enough to cause the alarmists on the other side to go way out of their way to make it go away.
I think any attempts - of which I have yet to see a convincing one - are more founded in an attempt to protect one's career. For some researchers, the absence of a MWP was a career defining moment, or has been made one (a large part because of the sceptic uproar about the matter).
Somebody stating that the presence of the WMP would be a nail in the coffin of the AGW theory is usually a dead giveaway that they have not read enough about the matter, or get their information only from denier sites (here, the term "denier" is quite appropriate --- for the website, not the person doing the reading that is). The reason is that you can always go back far enough and see larger temperature variation than those that we are seeing now. It would not change the distinct fingerprint of the current warming.
There are two reasons why the MWP might be relevant for a theory of AGW;
1) A very large MWP would imply that there is a natural forcing that happened in this time that (a) nobody has considered yet (very unlikely given the 10000+ researchers working on this topic), or (b) that is not well recorded in the reconstructions (more likely).
2) This mystery forcing would have to have been very subtle for researchers to miss it till now. This does not suggest very good things for the current warming. This means that the climate system is much more sensitive to small variations than thought so far, and can be brought out of balance quite easily.
What the MWP would NOT change, however, is our knowledge about how CO2 affects warming. This knowledge about how much CO2 is necessary for a temperature increase is derived from basic physics, and from (independent) calibrations of both the reliably recent temperature record, or using the drastic changes in the ice ages.