Good Post on Global Warming at Volokh
I don't really have much to add to this; Professor Zywicki pretty much nails it. The precautionary principle is a virtually useless guide, as the environmentalists typically formulate it: it doesn't tell us what we should precautionate against, and there's no guarantee that acting bears any risk than not acting. The kind of action the Greens generally demand often has potentially severe economic consequences (therefore we shouldn't take these actions. The chance of totally derailing our economy by implementing silly standards is too great to ignore; the precautionary princple demands that we not risk it. So there). Further, if we don't really understand what we're doing—and the climate system is so complicated that no one has any legitimate claim to understand what we're doing—we could easily exacerbate the harm we're trying to prevent.
The real problem with most Greens' analysis is a complete failure to weigh the costs and benefits of different courses of action (this doesn't apply to all environmentalists. There's a big difference between people who want to protect the environment and conserve resources effectively and people who buy into the Green ideology). In many cases, they see protecting the environment not as an economic tradeoff, but as a moral imperative. If you're worried about the trees-in-and-of-themselves, then it doesn't matter what the economic benefits of logging are. If you're trying to build the kind of world that most people would want to live in, it matters very much.
The worst example of this kind of thinking is recycling: in many cases recycling is far, far more wasteful than just throwing the stuff away, but many people seem to possess a complete inability to evaluate it this way. When I've pointed this out to people, they've typically either refused to believe me or claimed that we should encourage recycling even if it's bad for the environment and the economy, because it promotes good attitudes. I suppose it does, if by "good attitudes" you mean "complete disregard of facts in service of ideology." But I doubt that's what most people mean.
Okay, I take it back. I guess I did have something to add.