It is a common debate tactic, especially when arguing in favour of fringe theories, to point to perceived flaws or unexplained phenomena in the dominant theory, and claim that the dominant theory can't possibly be true. This is particularly popular for conspiracy theories, such as the "moon landing hoax" or the more recent September 11, 2001 conspiracy theories. The key is to focus entirely minor points that remain unexplained, and to overstate their importance, while ignoring, and making no case to counter, the vast amounts of data supporting the dominant theory. The aim is not to develop a coherent alternative - the incongruity of the various minor points of dispute normally preclude that - but simply to imply there is a need for debate. This lack of coherent alternative is important because it provides little to be refuted. Indeed the large amounts positive evidence for the dominant theory would stand as immediate refutation of any significant alternative theory, but by ignoring all such evidence and failing to posit any solid alternative the minority seek to shift the perceived burden of proof to the middle ground and create a perception of active and necessary debate.
Equally telling is the fact that most of the popular talking points for climate change skeptics use the same rhetorical tricks and fallacies as conspiracy theorists. Let's have a look at a few examples:
There is global warming on Mars and elsewhere in the solar system!
This is, in many ways, akin to the September 11th conspiracy theorists' claims that the collapse of the World Trade Center towers looked like a controlled demolition, or arguments that because other buildings, even after prolonged fires, did not collapse the WTC could not have collapsed due to fire. Essentially it is an effort to argue by analogy when ultimately the claimed related cases are ultimately quite distinct situations. Yes the towers' collapse bears some superficial resemblance to some controlled demolitions but ultimately, when analysed by experts, the differences are clear and a controlled demolition isn't an adequate explanation for the collapse. And yes, there are buildings that have had sustained fires but not collapsed as the WTC towers did, however those were very different buildings with different structures and quite different fires: the analogy simply isn't that robust.
Similarly, Mars, and the rest of the solar system, are different planets with their own quite different climates with different factors affecting their climates: the analogy isn't that robust. More importantly, when analysed by experts, the warming of other bodies in the solar system can be well accounted for by the natures of their orbits and variation in solar intensity. The warming on Earth, which is more severe than elsewhere, however, cannot adequately be accounted for via solar variation and other natural factors alone. Ultimately warming elsewhere says little or nothing about the nature of the warming on Earth.
We know the Earth has been warmer than this in the recent past - the Vikings colonised Greenland during the Medieval Warm Period!
This is simply a false statement with supporting evidence that relies on a lack of context to be at all convincing. It is true that the Norse colonised Greenland around 1000 CE. This is meant to imply that, at that time, Greenland must have been much warmer than it is today with its 3km thick ice pack. The reality is that there were only 2 Norse settlements in Greenland, located in fjords on Greenland's west coast. The areas of both those settlements are quite green and hospitable today, and there are several farms there now, so Greenland need not have been any warmer than today to have provided the Norse with sustainable settlements. Further the archaeological evidence from the sites implies the Norse hardly had an easy life: Greenland cows were the smallest ever known, largely due to malnutrition, and it seems the cows and sheep may have had to be force fed seaweed over the winter to keep them alive. The Norse also tended to rely on trips to the Eastern Canadian coast, most likely the Labrador area, for many things, most particularly wood. In practice the apparently marginal Norse Greenland colonies say little or nothing about the temperatures a millennium ago.
So what do we know of temperatures from around that time? In practice such historical temperature records are reconstructed from proxy data such as tree rings, data from glaciers, and the isotopic data from ice cores, corals, and stalactites. The aim is to collect as much data from different sites around the world, and different types of data sources, then attempt to reconstruct global temperatures by cross referencing all the varying data series. Almost all such reconstructions show that, while there was a period around 800 CE to 1300 CE that was warmer than the period from 1300 CE to 1800 CE, current temperatures are significantly warmer than in any period in the last 1200 years or more. Wikipedia provides a chart that compiles 10 different temperature reconstructions of the last 1000 years, and while there is some variability the trends are clear, and Medieval Warm Period shows as significantly cooler than today.
This sort of approach - using a lack of context to give a false impression and imply support for an otherwise unsupported statement - is quite common in a lot of other climate change skeptics' arguments. It is, of course, also very common in conspiracy theorists' arguments. A classic example is the claim that United flight 93 didn't crash in Pennsylvania. This rather extraordinary claim is supported by such things as a quote from the local coroner that "I stopped being coroner after about 20 minutes, because there were no bodies there". Of course given the full context of the interview it becomes clear that what he meant was that there were no identifiable bodies. There were human remains, quite a lot of human remains, and that same coroner went on to identify 27% of the passengers based on DNA testing of those remains, just nothing that could in any way be called a body. Taken out of context, however, it sounds quite damning and seems to be a statement in support of the claim that no remains of the passengers were found.
They were worried about a coming ice age back in the seventies!
I think we can all remember the confusion and uncertainty that occurred on the morning of September 11, 2001. News reports were constantly changing and contradicting themselves as new information came in. In their effort to report something, there wasn't an awful lot of fact checking going, so, for example, reports of how many hijacked planes were still in the air and unaccounted for fluctuated up and down for several hours before finally settling on zero. With enough points of view available it was not hard to find a source claiming whatever you wanted. The media being as they are, reports tended to lean toward the sensational.
Conspiracy theorists love to take advantage of this period to mine for quotes, often from reputable news sources. For example there are the news reports that the planes were "not commercial airliners" and similar. Often these were preliminary news reports based on unreliable witnesses (one witness who was certain it wasn't a commercial jet because he didn't see any windows on the side of the plane was over 2 miles away at the time). There was, for example, some uncertainty as to what hit the Pentagon; once more witnesses had been interviewed consensus quickly settled on a commercial flight (over 100 witnesses testified to this). There was the report that United flight 93 landed in Cleveland which was simply a false report that was immediately corrected by the AP. There were initial reports of bombs in the buildings based on misinterpretations of various secondary explosions from within the WTC from the fire. The list goes on. All such reports are regularly cited by conspiracy theorists. The aim is to imply that there was a consensus which was subsequently covered up.
The very same quote mining from sensationalist media reports is what drives the meme about scientists predicting an ice age. The intention is to imply that in the seventies there was a similar scientific consensus about the looming threat of an ice age as there is today about global warming. This simply isn't true - there was no consensus. Serious climate research was effectively still in its infancy in the sixties and seventies, and there were plenty of opinions to go around. The media, loving sensationalist doom-sayers, and general fear-mongering (consider the recent round of TV specials on the threat of bird flu), seized upon whatever sounded the worst. In practice there simply wasn't any significant scientific claims of an impending ice age in the seventies. There were, at most, only a few papers ever published making such claims, and nothing even resembling a consensus. Compare that to the number of papers published on global warming, and the hundreds of scientists that collaborate on each IPCC report. The reality is that the "scientists fears of an impending ice age" was largely a matter of hype and misreporting from the media - though it does provide good material for quote mining.
Water vapour makes up 97% of greenhouse gases - why don't the scientists ever take it into account?
The ploy used here is to simply lie boldly. Water vapour accounts for around 80% of greenhouse gases by mass, or 90% by volume. But even that's somewhat deceptive because what really counts is how effectively it acts as a greenhouse gas to trap heat. In terms of percentage input to the warming effect of greenhouse gases, water vapour is somewhere between 36% and 70%, though most studies tend to find it to be around 65%. I have been completely unable to track down any solid reference for the 97% figure that is regularly bandied about (and even sometimes claimed to be water vapour's percentage input to greenhouse warming) in climate change skeptic circles. It seems to be something that was simply made up. Tell a lie boldly enough and often enough, however, and a lot of people will simply start believing it.
Still, 65% is a very significant portion, so why don't climate scientists ever take the effects of water vapour into account? The answer to that question is surprisingly easy: they do take water vapour into account. The difference is that water vapour, unlike carbon dioxide or methane, has a very short residence time in the atmosphere (around 10 days). This means that water vapour will very quickly find an equilibrium point and can only act as a feedback rather than a forcing with regard to climate change. None the less water vapour represents an important feedback and you'll find no shortage of scientific papers detailing its effects on climate change. You'll also find that tropospheric water vapour is a vital component in IPCC climate models, while stratospheric water vapour is treated specifically in IPCC reports. Again the key is to lie boldly and repeat the lie often.
This tactic of bold lies is also remarkably common in conspiracy theorist circles. Some popular ones include: the World Trade Center towers fell at free fall speed (they didn't); wreckage and human remains from United flight 93 was found in Indian lake, 6 miles from the crash site (some very light weight debris was found in the lake, which is actually only 1.5 miles downwind from the crash site); no significant wreckage of American Airlines flight 77 was ever found (a great deal of the plane was recovered); World Trade Center 7 was undamaged when it fell (on the contrary there's plenty of video evidence of significant damage). and so on. Of course these falsehoods just keep getting recycled amongst the conspiracy theorist community until they're widely held to be fact.
Both conspiracy theorists and climate change skeptics also make a point of completely ignoring, and trying to never mention, any of the vast amounts of evidence that stacks up in favour of the dominant theory. In the case of September 11, 2001 conspiracy theorists they conspicuously avoid any mention of the trail of evidence the hijackers left behind (including records at flight schools, ties to and meetings with Al Qaeda members, etc.), nor wreckage of the planes, and remains of the passengers that have been recovered and identified - almost all the passengers from all the flights have now been identified from remains, often by DNA).
Climate change skeptics similarly ignore the close historical correlation (using ice core records covering the last 650,000 years) of atmospheric carbon dioxide levels and global temperature; the fact that there are very good reasons to anticipate causation based on absorption spectra of carbon dioxide; the dramatic increase of atmospheric carbon dioxide in the last 150 years which (via isotopic composition) can be tied to human activity; and the fact that detailed climate models (which can accurately reconstruct historical climate data they weren't trained on) clearly demonstrate that natural causes alone are insufficient to account for the current warming.
I do have some sympathy for the skeptics: I too am frustrated by the shrill claims of doom from some on the "pro" climate change side who are just as uninterested in facts and science as the skeptics. You'll find bizarre, and equally unsupported, claims of conspiracies by the Government, oil companies, "big business" and so on. The "debate" has become increasingly politicised, and as such only the most vocal get heard. This has reduced things to a bullet point yelling match between polar extremes. The problem is that this leads many to believe that the truth must lie roughly in the middle - in reality the best evidence and science we currently have lends far, far more credence to the position that climate change is real, and that human activity is a significant causal factor. As long as we give any time to the unreasoned arguments of conspiracy theorists (on either side) this reality will remain obscured.