I think the case for climate skeptic is a bit overblown. In his own words:

http://archive.randi.org/site/index.php/swift-blog/806-i-am-not-qdenyingq-anything.html

The relevant quote:

My remarks, again, are directed at the complexity of determining whether this GW is anthropogenic or not. I do not deny that possibility. In fact, I accept it as quite probable.

Not sure what to make of the claim that he debunked people who never signed up for his challenge. There are a number of psychics and others that he has debunked that never signed up for the challenge (for example Uri Geller or Sylvia Browne) which this could be referring to, and I feel are valid debunking.