I think you are wrong when you insist that greens have to engage the dissenters and convince them of their errors. It is one thing to express scepticism as a result of sincere doubt; it is another thing entirely to use scepticism as a way to avoid changing your behaviour. One is capable of being convinced through providing evidence and coherent argument; the other is simply using that doubt as a socially acceptable way to continue on with 'business as usual' without paying the social penalty of blatant selfishness. The result is already locked in stone in the dissenters' minds; the arguments are merely the way to create that result, rather than sifting evidence to come to a conclusion that was not pre-determined.
You say that it is 'not a good sign when greens seem eager to discredit dissent rather than engage it', but what do you do when the 'dissent' is the public domain version of a filibuster - not meant to achieve anything on its own, but instead cause paralysis so that the status quo can continue unchecked? There is nothing there to engage - there is no possibility of changing minds or convincing those people otherwise. All that is left is to show how bankrupt their arguments are so that others will not follow along. That is why discrediting and debunking is the focus, not engagement.