You'll see someone like this in every global warming discussion that happens with an educated audience. They'll put on all the plumage of reasonable dissent: "I don't think that the argument for AGW takes into account <pet theory>." In an audience of non-experts, they will go unanswered, and appear to have a point, until you spend an hour or two digging through the relevant IPCC reports and discover that not only has <pet theory> been investigated, it has been extensively investigated by dozens of labs across hundreds of studies using methodologies you never dreamed possible, along with 3 or 4 variants of the argument, all of which were quite thoroughly put to rest.
If you confront them with this evidence, citing all the reports and papers, they'll just switch from <pet theory> to <pet theory 2>, possibly after firing off a final riposte defending <pet theory> that makes it abundantly clear they couldn't even be bothered to read the links you dug up, forcing you to ask yourself if you really want to do all that homework again.
Fake-reasonable arguments are easy. Countering them is hard. If you don't discard them as a matter of policy (peer review or STFU), they will dominate the decision making process with 100% certainty.