How much global warming is due to human activity?

In a recent comment, I was asked how much global warming was due to human activity, as opposed to natural climate fluctuations, errors in measurement, and other phenomena beyond our control. (This Wikipedia article explains global warming well, with many links. It allows that there is, indeed, uncertainty over its cause.) My short answer, of course, is that I don't know, and I don't think anyone else knows for sure, either. However, it seems to me that if there is a case to be made for human acceleration of global warming, we don't need to know exactly how much we are doing, if what we are doing has bad effects. We don't know for sure if a particular smoker will die of lung cancer, but that doesn't mean that we shouldn't try to stop smoking. We do know that some smokers do die of lung cancer.

Is there any such thing as global warming at all? There is, indeed, some controversy over this, but, like some other controversies, my impression is that there shouldn't be--some people are misinformed, or are deliberately producing misinformation, and encouraging others to disbelieve global warming. For whatever reason, those who disbelieve global warming tend to be conservative Christians or Republicans. (The two are not necessarily the same!) As an example, I saw Jerry Falwell on the CBS Early Show, on October 14th, 2005. He used the phrase ". . . the myth of global warming." (Harry Smith, the moderator, was doing a segment "Is God Mad at Us?" and Falwell was one of the guests. He said that God was not mad at us. Bill Nye, the Science Guy, another guest, said that he expected more hurricanes because of global warming.)

Back to the question. Is there really global warming? I believe so. (See the New Scientist report, updated occasionally, on Climate Change. See the Wikipedia article for data going back to 1860.)

And, again, to the question that started all this. Assuming there is global warming, how much of it is due to human activity? All I can say is that some of it is, and that, if we can do so, we should take action to cut back on such human activity, by, for example, cutting down on the emission of greenhouse gases in burning, by making transportation more efficient, and the like. I don't think we want to be responsible for the melting of the polar icecaps. This would change the environment radically, to say nothing of putting many of earth's large cities, which are located at or near sea level, in jeopardy. Apparently it would lead to more hurricanes.

Would there be effects on the economy? Yes, of course. A change, such as, say, giving up gasoline-powered automobiles entirely, would cause some change, and it is understandable that it would be resisted by people who would expect to be negatively affected by such changes. Radical changes in how we do things are never easy, and care should be taken to minimize the effects on people as much as possible. However, changes are not necessarily all bad. It is almost certain that new industries, and new jobs, would arise related to any changes in how we transport ourselves, heat our homes, etc.