'You have to decide who you trust before you decide what to believe.' Right. The difference lies in the way that trust is achieved. The "I trust X because Y trusts X" transitivity is very easy to jump on, but often disastrous; just ask Bernie Madoff's investors. If you read in the newspaper that "X is one of the foremost scientists in his field", it is also dangerous to rely on just this pronouncement, and blindly tr…
Despite we are _not_ back on square one. Between a theory that almost works (like evolution) and one that is pure conjecture, the fact that there's a whole in the former all it shows is that we need to improve the theory.
It is the same thing in climate modelling. It takes a nobody to say something which sound somehow plausible for the scientist to need to prove themselves again, often taking several years and a lot of effort. Most scientist, including at the IPCC, do describe hypothetical scenarios with all their doubts, including error bars etc... They also state all their assumptions, and their full method.
Then one guy says: "Hey, it's been crazy cold in England this winter... so much for global warming", or "between 2001-2002 the global temperature went down" and suddenly everyone feel they are back to "we don't know". But why? The data from the IPCC is the result of many different models, with many different assumptions. The opposition has provided no better model: all they need to do to keep the status quo is point out some small unknown or flaw. That's WRONG!
And btw, climate science is possibly one of the hardest disciplines around: you are making a prediction in the future of a chaotic system. But the anti-science groups can find objections just as easily to pretty much any bit of science they dislike.