The short version: you need to be peer-reviewed to be viewed as a reputable scientist. So in order to prevent reputable arguments against global warming canon, heretics were simply denied peer review. Ta-da! No more "reputable" scientists coming out against global warming... even though some 35,000 have signed petitions against it.
See how easy it is to make it look settled when you simply control the data?
From the AGW site: More specifically, 97% of climate scientists actively publishing climate papers endorse the consensus position.
With no recognition from the church of global warming, dissenters were all heretics spouting apocrypha. And of course, as the data is shown to be manufactured, it's very easy to dispute.
Now I'll do it:
All scientists are opponents of anthropogenic global warming. And in order for any counterclaim to be reputable and worthy of consideration by science, you'll have to be published in a peer-reviewed professional blog. Namely this one. See how easy it is to manufacture consensus?