There may be good reasons not to frame global warming as a certain, imminent catastrophe that requires immediate mobilization -- what Matthew Nisbet calls the "Pandora"s Box" frame. I've argued myself that fear is not the green's friend in the long term. If nothing else, that kind of framing just doesn't seem to be working.
But I'll tell you what's not a good reason: the oft-cited worry that it "opens up" greens to criticism from professional skeptics. This is a classic earnest progressive concern, as though if we just keep all our p's and q's in order, we'll render ourselves immune to criticism.
Guess what? There's a whole class of people with careers and reputations built around criticizing greens and casting doubt on global warming. Many of them are paid to do it.
No amount of scientific rectitude or rhetorical moderation will forestall those criticisms.
The professional skeptics are simply not in the business of truth finding. That's not their concern. They're in a political knife fight. They don't need "openings." They don't need "ammunition." They're happy to make shit up.
The din of criticism does not rise or fall based on the rhetorical precision of greens or scientists. It just doesn't. Stop worrying about it.