I'm just a lay person reading this stuff, but here goes:
1. The site Watt's Up With That is wonderful--truly on a quest for information, totally lacking in snark. Editor Anthony Watts and others on the site probably tend green in many ways, and respect science. There was recently a post stressing that the people who actually developed computer models for climate over large regions truly made a great new discovery. No wonder they were excited, and no wonder the excitement spilled over to someone like Gore--and beyond him to many intellectuals. Instead of climate being a larger version of the weather guessing game--in which no one can say for sure if it's going to rain this weekend--computers hold out the promise of being able to include all relevant factors, then let the model work to predict actual conditions. It's nothing against the real scientists involved to say they probably don't have a very complete model yet, and it remains unlikely that the tail of man-made CO2 wags the dog of global climate.
2. Taking this further, it is understandable if real scientists applying for grants says something like: I may have identified a serious problem which will get worse if no action is taken. My research can be expected to point to some possible actions, and will also help to understand the possible disastrous results if nothing is done. There is a diplomatic way for scientists to say: give me my money, and no one gets hurt. It is a different thing for a political hack like Gore (abetted by James Hansen, who is perhaps less of a scientist, and more of a hack, than he used to be) to knit together a number of worst-case scenarios and say: do what we say immediately or the results will be disastrous.
3. Data problems: Freeman Dyson among others points out that there is remarkably little data behind any sweeping generalization about global climate. There are a relatively small number of temperature monitoring stations, even today, and the Watts people take pains to show that many of them are old and poorly designed.(In one case, there used to be trees around the station; the trees were cut down, and lo and behold, the temperatures that were recorded went up dramatically.) The twentieth-century data, which may or may not indicate that that was a warm century, is spotty and inconclusive; but before the twentieth century there is even less real data of any kind. Hence tree rings, etc., all very ambiguous.
4. Was the twentieth century a warm century? Perhaps somewhat warmer than the medieval period, although that was also a warm one, but almost certainly not warmer than a period before the Christian era--the blink of an eye in geologic time. Of course warming periods before the 19th century would have had nothing to do with man-made CO2, or man-made anything.
5. The best evidence that the twentieth century was unusually warm is probably the geographically isolated glaciers, in several parts of the world, retreating relatively rapidly. Of course, massive melting has happened before--that's how ice ages end--and has never previously had anything to do with man-made anything.
6. Even if the twentieth-century was warm (and even if the warming has continued past 2001, which is doubtful), is man-made CO2 the likely cause? No because even within the phenomenon of greenhouse gases (the reality of which no one doubts) CO2 does not seem particularly significant. To make it significant, the modelers have to include some reasoning that CO2 causes a multiplier through the really significant gases like water vapour. Interesting speculation based on very little data. And no, it's not enough to say: let's spend billions just to be on the safe side. We need to know not only how serious the risk is if in fact it develops, but how likely it is. Lomborg has pointed out for years that for a while the IPCC included a cost-benefit analysis in their reports; then they stopped doing so.
7. What's the most significant factor influencing global climate? The sun. Second? Probably the earth being struck by meteorites. The sun we can really do nothing about; for meteorites there may be some precautions we can take.
8. It's hard not to see the politics alongside the science. Many people who are proud to be known as progressive have criticized the burning of fossil fuels, particularly in the West, for years--since before the serious talk about global warming. They think the burning of fossil fuels, often wastefully and in a way that produces emissions, is proof of our materialism, our failure to be stewards of the environment, our lack of concern about the future. Global warming ostensibly caused by CO2 gives them a powerful vehicle to say we should cut back on our burning of fossil fuels.
9. There's also a kind of crypto-Marx and crypto-Rousseau that keeps coming up in environmentalism. Crypto-Marx: the very thing that seems most immoral in capitalism is the thing that will destroy it, and possibly destroy us. Capitalists think primarily of short-term profits; long-term issues such as preservation of the parts of the environment that are essential for life will be neglected, and the result will be an environmental disaster. This is very much like believing there is a big Dr. David Suzuki in the sky, ensuring that justice prevails. Unlikely.
10. Crypto-Rousseau: non-human nature good; human nature evil.