<rant>First, it occurs to me that a good answer for many scientists, including those in the "hard" sciences, is "I don't know." Probably should use that more often. How old is the universe? I don't know. We have these three models, however, which give us some idea... And so on. A little humility can go a long ways here. Guess what? Average Joe Citizen knows you don't know. Trying to oversell the science is not the way to go.
Second, and more important, this horrible public debate is being framed in the most disingenuous terms possible. I am consistently being told that people either believe in imminent danger requiring bold political action -- or they are science deniers. Pardon me, but that's just bullshit. As the GP pointed out, there are an infinite number of intermediate positions; and no amount of hand-waving about the possible dangers is going to change that.
There are a bunch of logical steps a person must take in order to support political action. Is there a change coming? Are we the cause? Is the change going to cause more harm than good? Can we control the outcome? What is the "proper" outcome? How much action is warranted by the change indicated? And so on. I could list these exhaustively, but it would be pedantic.
This list is not just a bunch of rhetorical obstacles to make sure nothing happens. These are logical and discrete steps that must be looked at and evaluated. Many of them are yes/no questions, but some involve qualitative opinions or numerical differences. If any of these steps come up short, i.e., if there's going to be massive change but the change has more of a positive impact than negative? The whole shebang falls apart. That's not moving the goalposts, that's not trying to throw up a bunch of obstacles. That's just how the reasoning works out.
"The climate is changing" is a trivial statement. Of course it is. "What's left today is a proper scientific theory" looks like a pretty big stretch to this layman. Even granting that somehow we have a falsifiable, reproducible (and how would that work, anyway? Got another Earth nearby?) theory, still looks like a bunch of political types at the IPCC voting on what the change will be. Voting. On the numerical result of a supposedly proper scientific theory. Show me the corresponding physics body that is voting on the mass of the proton and maybe we could get somewhere.
And even then, even if there wasn't so much BS -- even assuming something as rock solid as a huge asteroid heading towards Earth -- a democracy still makes decisions by the persuasion of the voters. I've heard many a climate catastrophist talk as if the average voter is too stupid to make life or death decisions for themselves and the society they live in. If that's what they believe, then we need to start talking about the wisdom of setting up a select group of scientists as the new priestly class, able to understand the mysterious ways of the universe and make decisions about what is just and good for the rest of us.
I've said it before: this is a terrible debate, full of scientists who shouldn't be advocates, politicians who are looking for the latest bandwagon to jump on, the vast morass of uniformed folks who want to look knowledgeable and the poor voter stuck in the middle of all of it.</rant>
As an aside, I would really love to see some discussion about the fact that models based on Cellular Automata always tend towards runaway conditions. Anybody who's worked with such models know this, but it seems to have been left out of the discussion.