Climate-Gate has certainly dealt a tremendous blow to the validity of the Anthropogenic Global Warming (AGW) theory, but some die-hard ignoramuses continue to assert it is true. Aside from outright reductio ad absurdum, AGW enthusiasts also frequently justify their belief using logical fallacies. I will cover a few of the more common logical fallacies (note that these are commonly used to defend many other ill-contrived ideas):
Fallacy of Affirming the Consequent
This is arguably the most oft used fallacy in the AGW debate. It's exhibited in statements like, "Global Warming is obviously happening because glaciers all over the World are melting." However, there are a lot of reasons why glaciers melt. In fact, glaciers have been melting ever since the ice age concluded - far earlier than the supposed commencement of man-caused global warming.
In fact, the assertion that the Earth is warming as a result of man made greenhouse gases (namely CO2) is itself a fallacy of affirming the consequent because there may be many reasons the Earth is warming (assuming, of course, that the Earth is actually warming). This fallacy can be spotted wherever someone is making the claim that a consequence is occurring as a result of a specific event or condition when there are other explanations that have not been eliminated from possibility.
The Fallacy of Appeal to Authority
The AGW faithful commonly claim that their position is valid because, "like, all the scientists say it's true!" This is the fallacy of appeal to authority (and is also flatly inaccurate). There is only one truth; even if every single living person earnestly believes something to be true doesn't necessarily make it so. It is not enough for "experts" to simply say something is true - the truth stands on its own and only through the use and understanding of incontrovertible facts can an expert, or anyone, come to understand the truth. Experts are individuals with a vast amount of knowledge in a particular area, but they're not infallible. People don't make the truth, they discover the truth.
A good example is NASA's Dr. James Hansen, a favorite AGW expert of Al Gore's. If relying on the expertise of "experts" such as Dr. Hansen was reliable, then we should be in the midst of an ice age because Dr. Hansen was among the "experts" declaring we were on the verge of it in the 1970s. Apparently, his flawless scientific mind made a goof and we were really on the cusp of a global warming epoch - or, perhaps, he was just confused about the freezing temperature of water.
The Fallacy of Reification
"The science clearly says mankind is causing the Earth to warm catastrophically." No, science does not, and cannot, say anything. People say things; as such, people must interpret scientific knowledge, observations, and data. Then they present their findings. Clearly, what is really intended by the quote is that an "expert" says something which refers back the previous fallacy, but what is connoted is that somehow the facts are self-evident although they most certainly are not.
The Fallacy of Ad Hominem
This fallacy is always presented when someone has no viable argument nor logical basis for his or her position and is a favorite among AGW fanatics. "You're just not evolved enough to understand the science!" Demeaning an individual, group, or competing idea is the lowest form of debate because it both fails to support the person's argument or defeat his opponent's position, and it has no relevance to the issue. If someone is resorting to the fallacy of ad hominem he is doing so in a feeble attempt to discredit the person - most likely because he believes appeals to authority are valid proofs and if an opponent is discredited then she would be unfit as an authority.
The Fallacy of the False Dilemma
This fallacy can be more difficult to spot, but it is also very prevalent in the AGW debate. For example, "either you realize that AGW is occurring or you don't think humans have any affect on their environment." In this case, the false dilemma is between agreeing with AGW or believing that humans have no impact on the environment. Realistically, you can comprehend that humans affect the environment, but causing calamitous worldwide warming isn't part of some make believe, all inclusive humans-ruin-the-Earth package.
Another example would be, "If you don't believe in AGW then you don't believe in science." Quite to the contrary actually.
While I could fill books with example after example of logical fallacies in the AGW debate, this should provide a succinct summary of the most common classification of fallacies often relied upon by the AGW faithful. Should you find yourself in a debate on this topic be sure to point out to your opponent his use of these fallacies. When you consider all the arguments presented as proof of AGW, they all fit within one of these categories (or possibly another logical fallacy not listed here). The two tenets of AGW are the correlation between atmospheric carbon dioxide concentration and global average tempertaure and the climate computer models. While not strictly a logical fallacy, correlation never proves causation - and the correlation is pitiful; it is only made presentable by using annual global average temperature variants because using raw data would render it completely statistically void. Computer models are only worthwhile if they can be validated and a global climate model can never be validated, not to mention that pesky little detail that the models have already failed to correctly predict average global temperatures. Finally, never forget the most important question, "What exactly is the correct temperature for the Earth?"