You have reached an internally inconsistent conclusion. You stated initally that there is no way to determine temperatures 1000 years ago.. Then you go on to say that the climate constantly changes. How do you know that? If you can't determine the emperature in the past then your conclusion should be "Climate may or may not change, I don't know."
The biggest problem with the AGW position is that there has been no baseline established for what is normal or abnormal temperature fluctuations.
The relevant part of the temperature reconstructions is not just the last 15-20 years. Nor does the temperature reconstruction "predict" anything. It attempts to estimate the temperatures using proxies such as tree rings. Nor is Mann's the only reconstruction. There have been many. They all reach similar conclusions. Most have current temperatures higher than the MWP.
The advent of numerous periods of glaciation in addition to the intermittent warming periods is concrete proof that there are severe fluctuations..... Regardless of the conclusions and models stating gloom 'n doom, the facts lend far more support to natural cycles than to man made inputs