The tree ring data only changes to a limited degree with temperature so it lessens the extremes and flattens out the data. Not to mention many other problems with tree rings as a temperature indicator such as lack of data and sensitivity to temperature independent parameters. Mann's claim of the Little Ice Age and Medieval Warming Period being regional phenomena is debated.

Mann used tree ring data up to about 1980 then used measured temperature data for the rest of his graph. Measured temperature data has lots of problems but flattening out the extremes is not one of them. Regardless, the hockey stick is awful science as it uses one data source for a broad period then suddenly switches to a second source and the entire conclusion drawn is solely based on the differences in the two data sources. Not the kind of thing you'd want to do on your statistics homework.

Of course all that would need to be done is replace the measured temperature data with more current tree ring data. Amazingly this hasn't been done and the main reason cited is that it would be too expensive to get updated data!

Steve McIntyre can be found on the web and he has done a-lot of work rigorously investigating the science around the topic of global warming. There is certainly reason to doubt whether climate change is occurring at all before even considering whether human activity could be the cause.

Gerv, you ask good questions and obviously have an open mind. I think if you continue to research into this issue you'll find there is much to be questioned and feel a lot less concern over our impending doom. Well, as far as global warming. My $$$asteroid$$$ is still out there though.