I'm not sure I'd say there's a compelling alternative hypothesis--there's no such hypothesis that compels me to believe it rather than the AGW one. I do agree that sometimes the claim of consensus is used that way (i.e. to shut down debate), but that's not how it's always used.
Something pretty much everyone in my profession recognizes is that it's always possible to formulate a skeptical position strong enough to deny any claim or proposition. And it is always possible to make such a skeptical position sound reasonable. Of course, if we believed even a small percentage of those skeptical positions, we'd quickly have to surrender way too much for comfort. Evaluating those positions, and arguments in general, then, goes beyond merely looking at which ones have apparently true premises and valid reasoning. It involves a certain amount of judgment, and in the political sphere particularly, it also involves prudential concerns.
I have access to one of the better research libraries in the world, and I've done a fair bit of reading on this topic late at night when insomnia strikes. There's just very little reason any more to believe the skeptical position. We know that CO2 is a greenhouse gas. We know how it works as such. And we know that we're putting enough into the air to have an effect. Additionally, even now the lifestyle changes we would have to make to mitigate the worst effects of climate change wouldn't be all that great--we'd all still enjoy long healthy lives full of stimulation, intrigue, love, good food, and so on. It's a comparative small price to pay. On the other side, the macguffin we're risking is huge--if the skeptics are wrong, and we do nothing, the proverbial brown stuff is going to hit the proverbial spinning blades. In terms of practical decision making, there really is no decision here.