The theory behind human-caused, or Anthropogenic Global Warming (AGW) is really pretty straightforward. It is based on the following undisputed facts:
1) CO2 is a greenhouse gas
2) CO2 is accumulating in the atmosphere
3) most of the recent rise of CO2 in the atmosphere is from human activities, primarily the burning of fossil fuels
4) global temperatures are rising.
Most "skeptics" will argue that this series of facts do not directly "prove" that humans are causing the current warming and they are absolutely correct. The above four facts are not "proof" of anything. But they lay out a very obvious path to concern - could human activities be causing global warming and, if so, what should we do about it?
So the theory of AGW is a rational theory based on sound scientific principles and observations of the global climate, not some "hoax" or "swindle". As with all theories, the scientific study of AGW tries to better understand the above facts and also tries to disprove the theory through exploring alternate explanations (such as solar radiation) and focusing on observations that cannot be explained by the theory (such as satellite data that fail to show expected warming in the lower atmosphere). For the most part, newer research has increased the understanding of these four basic facts and has shown a greater correlation between them and the AGW theory. And the study of alternate explanations have been disproved and unexplained observations have been explained. This admittedly does not "prove" the theory, but it strengthens it to the point that is is "very likely" to be true, which is all the scientific consensus has claimed.
This lack of certainty is easily exploited by those who favor inaction, especially given a population in the U.S. that is woefully ignorant and easily confused about science in general. As recently as March 2003, political consultants for conservatives were encouraging their clients to exploit the uncertainties in global warming science and focus on how "unsettled" the science is. One such consultant, Frank Luntz, wrote a memo specifically outlining how to deal with the topic in the 2003-2004 election season.
Starting on page 137 (page 7 of the scanned copy of the Luntz Memo), Luntz states:
1. The scientific debate remains open. Voters believe that there is no consensus about global warming within the scientific community. Should the public come to believe that the scientific issues are settled, their views about global warming will change accordingly. Therefore, you need to continue to make the lack of scientific certainty a primary issue in the debate and defer to scientists and other experts in the field. (emphasis in the original)
and on page 138 (8 of the scanned document)
The scientific debate is closing [against us] but not yet closed. There is still a window of opportunity to challenge the science. Americans believe that all the strange weather that was associated with El Nino had something to do with global warming, and there is little you can do to convince them otherwise. However, only a handful of people believes [sic] the science of global warming is a closed question. Most Americans want more information so they can make an informed decision. It is our job to provide that information.(emphasis in the original)
And more recently, it has been reported that while vigorously promoting the uncertainties of global warming throughout the 1990's, the Global Climate Coalition's own scientists were saying that “The scientific basis for the Greenhouse Effect and the potential impact of human emissions of greenhouse gases such as CO2 on climate is well established and cannot be denied".
By now, only the most ardent skeptic clings to the idea that AGW is some sort of hoax or swindle, but that doesn't stop conservatives from continuing to promote the idea that it is all some sort of liberal conspiracy. My firm belief is that more rational minds will prevail.