Discussion about global warming

I have read the text The Truth About Greenhouse Gases by William Happer, a physicist at Princeton. I liked it, so I posted it to my Google+. I was surprised by so many emotional responses. The post also made Michael Tobis to write a blog post with his opinion.

I was not satisfied about the overall tone of the discussion. I am really just interested in factual arguments. As such, I browsed through all the arguments against William's paper from the above discussion, and chose one well formulated question, that I think represents the most important objection: "In your paper you state at several places, that doubling the CO2 concentrations will only increase the temperature by 1 C. However, it is claimed (see for example Michael's post above) that the increase will be around 2.5 C. Which number is correct and where is it coming from?" I wrote to William and asked whether he would be willing to answer it. With his permission I am posting his answer here.

Answer: sensitivity.pdf
Images referenced in the answer: image001.png, image002.png.
Link referenced in the answer: http://www.thegwpf.org/best-of-blogs/4247-steve-mcintyre-closing-thoughts-on-best.html

Edit: I was told that Blogger makes it really hard to comment under the article. You can discuss it at my G+ post about this: https://plus.google.com/u/0/104039945248245758823/posts/PJeqx7GKtLg