[Just discovered this post by Ron Broberg dedicated to me. Reproduced with slight editing.]
Peer-Reviewed Survey Finds Majority Of Scientists Skeptical Of Global Warming Crisis (Feb 2013)
Regarding ad homininum (circumstantial) 
Let X be AGW
- Person A makes claim ~X.
- Person B asserts that A makes claim ~X because it is in A’s interest to claim ~X.
- Therefore claim ~X is false.
If Person B’s assertion is that ~X is false simply because the persons surveyed are petroleum engineers, I agree that argument is fallacious.
But there is a deeper problem.