I thought I'd reply to Linda Glass' statement someway below where she stated "Michael Mann was investigated no less than 8 times, and every single person found him to be innocent of ANY wrong doing." I'll then go onto more general matters.
Linda's statement is incorrect. Michael Mann and his hockey-stick methodology was found wanting by Edward J Wegnman some years ago in his report to the Committee on Energy and Commerce (in the USA). This criticism was echoed (although using much more conciliatory language) in another report by North. Pressed later on the matter, North stated "We don’t disagree with their [Wegman's] criticism. In fact, pretty much the same thing is said in our report." These criticisms were bizarrely reported in the media as an exoneration of Mann's methods and therefore his hockey-stick findings. To this day, those on the AGW side of the argument still insist that there is nothing wrong with Mann and his hockey-stick representation of the last 1,000 year temperature record, and often cite so called independent papers by Mann's serial co-authors Wahl and Ammann as evidence of that, but those papers turned out to be flawed in much the same way (by using many of the same discredited proxies and statistical techniques for example).
The point here is that Mann's reputation among many both within and outside of the scientific community is very low. His shenanigans over disclosing his data, methodology and computer codes (over many years) so that his findings could not be reproduced was (and still is) a disgrace. I suspect that even some on his own side are embarrassed by him, but he continues to be feted by the media, including, so it seems, the National Geographic. Indeed, it seems that Mann's disregard for the scientific method (that all results should be reproducible) seems fairly widespread within the paleoclimatalogical community, often hiding behind intellectual property arguments, even where they, or the data they use, has been publicly funded. I find it really odd that so many pro AGW climatologists find this acceptable.
As for the quality of the scientific debate regarding AGW, it is sadly lacking. Vast amounts of money are being spent on the premise that the science is settled, whilst at the same time, very few, if any of the vast number of peer reviewed papers that aren't useful to the AGW viewpoint are discussed by the media, or I suspect by many of those posting here. The truth is, the IPCC 5 yearly reports are not balanced, or at least as balanced as they should be. By their own criteria, they by and large exist to assess the case for anthroprogenic global warming (AGW), and are therefore much less interested in peer reviewed scientific papers that provide alternative explanations. All the IPCC do is collate the "evidence for" and they don't check it's scientific validity. Many of the lead authors of the various chapters are from a relatively small AGW community, and so they naturally tend to cite their own papers and those of their colleagues that have published supportive papers. For a more balanced review, you need to go to the NIPCC website, where they present a much more balanced view taking into account the vast amount of scientific literature on BOTH sides of the argument. It makes extremely interesting reading and is thoroughly referenced throughout.
Of course, I fully understand that many reading my post will be dyed-in-the-wool AGW believers, much more content with visiting websites that reinforce their own beliefs (such as Michael Mann's RealClimate website), than genuinely making an effort to seek out more balanced views. I judge no-one for that. But I still think exposure to some of the counter arguments would be very worth while for many, even if it does't change minds.
For my own part, I believe that the global warming that has taken place in the last 200 years (and of course before that!) has been due in large part to natural cyclical changes in climate following what is now thought to have been the coldest spell in the last 5,000 years - the little ice age. Clearly, some of the more recent temperature increases can be attributed to CO2, because everyone (surely by now!) accepts that CO2 is a greehouse gas. But the contribution to this more recent increase (between about 1978 and 1998) is, for reasons given elsewhere in this thread, necessarily fairly tiny. All the observed changes in sea level (isostatic changes aside), glacier retreat, and changes in minimum arctic ice coverage are a direct result of this continuing temperature adjustment. They are not (as is commonly believed by many who have only a tenuous grasp of the issues) proof of the anthroprogenic part of AGW.
The real area of disagreement between sceptics and believers in catastrophic AGW (CAGW) is Climate Sensitivity, the amount that average global temperature will increase in response to a doubling in atmospheric CO2. Without any feedback mechanism in operation, climate sensitivity is around 1.0 to 1.2 degrees C. But those who believe in CAGW think that positive feedbacks will increase global temperatures by several more degrees and a tipping point will be reached meaning global temperatures will race away catastrophically. Sceptics by and large however, believe that the evidence for this scenario is wholly inadequate, and is likely to be wrong. And there you have the AGW debate in a nutshell. The fact is that no-one really knows exactly which feedback mechanisms operate , and how they interact with each other. Climate models all contain the assumption that any feedback that occurs will be positive, but this is by far the most controversial and least understood aspect of climate science. For my own part, I look back at the temperature record over geological history and note that increased global temperatures have always preceded increasing CO2 levels (by several hundred years on average). This implies that temperature increases in the past have been caused by natural climate variables that have then acted to expell CO2 from the oceans etc. But more importantly (and probably more relevantly) I've seen no evidence of positive feedbacks causing run-away global warming either. There have been times in geological history where CO2 concentrations have been far higher than today, with comparable global temperatures to today's. This implies to me that negative feedbacks are also important and act to dampen (and likely reverse) any positive feedbacks. Don't forget that as CO2 increases, it's ability to act as a greenhouse gas diminishes.
Lastly, I am disappointed to note some on here using the term "denier" to disparage those holding a counter view. It's a pretty childish way to dismiss those you don't agree with, and to some, offensive. I'm not even sure what us skeptics are supposed to be denying? If two sides disagree about a part of science that is still very much in its infancy and far from settled (as per above) then how could either party be said to be denying anything? It's an irrational statement to make and I suggest we all get a grip and discuss the science and leave the silly name calling alone.