This is only a link to the abstract, I don't have the time to read the full article right now.

http://www.pnas.org/content/105/36/13252

Cliffs:

1.)
Mann and his colleagues are credited with debunking the "hockey stick graph" because they looked at the data on seeds and tree rings and found it was inaccurate.

2.)
Horner (who is a lawyer by trade and often represents conservative causes) and others with political axes to grind use Mann's article to discredit global warming.

3.)
Mann publishes second article telling them that his findings do not mean that global warming is a myth, they just mean its not as bad as the previous graph states. The rise in temperature is still unprecedented.

Enjoy