The single most used piece of evidence in the climate change debate is computerized climate models. This is an unarguable fact. Every statement of catastrophic climate change is based in computer models because there is no observable evidence for catastrophe. There can't be observable evidence because there is no observable climate-related catastrophe currently occurring.
Let's put a few things into perspective before we go any further. No one with even the barest education denies that there is such a thing as climate change. In fact, the cynical observer can note that the reason the environmental lobby switched from using the label "global warming" to "climate change" was because the one was highly debatable while the other was not and using the undebatable term would help to silence the rest of the debate as well (the cynical observer also notes that it is difficult to keep the public's attention on a threat of global warming if the planet refuses to cooperate and show consistent warming trends for you). Climate change happens and always has happened. Earth's climate has never been stable. Dismissing skeptics as "people who don't believe in climate change" is ridiculous and a dishonest refusal to address what is actually being said.
What is actually being debated is 1) whether or not the current round of climate change has a significant human influence and 2) whether or not the current round of climate change has potentially catastrophic consequences for humanity (if you think it's about catastrophic consequences for the planet, you're delusional - the planet will adapt and keep going regardless). Contrary to popular belief, the debate on these two questions has not been settled. It cannot be settled because it has never happened. It cannot happen because, from Day One, believers have accepted the premise as proven fact and have vilified doubters with a fervor usually reserved for religious inquisitions. (Want an example of this fanatic extremism? A prominent New York Times columnist recently argued that those who voted against the Cap and Trade bill in the House should be considered treasonous. How's that for open minded discussion?)
To be fair, the first question cannot be answered with any surety. Not in anything resembling the short term, anyway. The only way to prove this point, one way or the other, is to make dramatic changes and then wait around about a hundred years to measure the results. Not the most efficient means of answering a current events argument but this is, essentially, what the environmental lobby wants. It is not, however, what the general public wants. Such dramatic changes would be expensive beyond comprehension and would produce equally dramatic upheavals in everyday life. The general public does not like that kind of change without a good reason.
Which is where the second question comes into play. If there is an imminent catastrophe and such dramatic changes have a real chance of heading off this catastrophe then there is good reason for the changes.
So, is there an imminent catastrophe? The environmental lobby says there is, but that's not exactly an unbiased source. Is there evidence for an imminent catastrophe? Contrary to the constant headlines, the answer to that question is a resounding no. A catastrophe is certainly within the realm of the possible, but every statement of catastrophe that has been made has been a guess or a computer model prediction.
There is no evidence in the historical record. Warming trends and cooling trends have been well documented through the use of core samples and other such data collection methods. While there is plenty of evidence for cooling trend catastrophes (ice ages do tend to make such survival requirements as food collection/production rather challenging, to say the least) there is nothing in the historical record to indicate the kind of global catastrophe being described as a result of a warming trend. In fact, human culture and civilization has tended to thrive during the warming trends!
The proponents of catastrophic climate change do not challenge this fact (except maybe for those who are so uneducated they believe the catastrophe is due within their own lifetimes, despite the fact that their own models do not predict catastrophe for about a century). Instead they argue that it isn't relevant because this time the world is warming faster than ever before, but what is the evidence for this claim? There isn't any because the scientists these people are quoting don't claim this! These scientists claim that the world will be warming faster than ever before. Notice the difference? The scientists are not stating an observed fact, they are making a prediction. They are then using that prediction to support another prediction. In order to make the first prediction, however, they must assume that human activity has a significant impact on the world's climate, which is exactly what these predictions are supposed to be trying to prove in the first place. Nice circle.
As I said at the beginning, the single most used piece of evidence in the climate change debate is computer models. Melting ice sheets? The major ice sheets have always melted and reformed and we have only been actively observing this process for thirty years. We know nothing of any consequence about the major ice sheets - which are currently melting and freezing, depending on where you look - and only computer models indicate that what is happening now is any different from normal fluctuations. Mass extinctions? If you had any idea how many species are assumed to go extinct every hour now, you would only be able to laugh at this claim. The truth of the matter is that no one has a clue how many species are on this planet and no one knows how many go extinct or what will cause or prevent these extinctions. Claims of mass extinction are nothing more than computer model number crunching based on statistics and probabilities. Severe global drought? This one is another joke. A prolonged or unusual warming trend is as likely to increase atmospheric moisture as to reduce it. This is all based on computer models and many of those models actually predict longer growing seasons and more arable land (which is also what the historical record supports). Quite the opposite of a drought. Rising sea levels? This one is actually smoke and mirrors. Sea levels have actually been rising for centuries (to put it mildly - pretty much since the end of the last Ice Age) and most computer models do not predict any unusual increase in this natural, ongoing process. It just sounds bad because no one mentions that it's been going on forever. The models that do predict catastrophic rises have been used for benchmarks for years and, so far, they have not gotten one single sea level increase prediction right.
Notice a trend here? Every single claim made by proponents of climate change catastrophe is based on computer models - models that either cannot be independantly verified or that get the predictions wrong when attempts to verify are made - and then they claim the debate is closed. Computer models cannot prove anything except the fact that a computer can model something based on input. No other branch of science would be allowed to claim something as proven based only on computer models and I don't think any other branch would try.
Astrophysicists had computer models predicting the existence of black holes for decades before the phenomenon was observed and, until it was observed, it was only a theory. Cosmologists have computer models predicting the existence of dark matter but no other evidence has yet been found, so dark matter is only a theory. Computer models exist that show that a cataclysmic meteor strike of Earth now only has happened but can happen again, yet no one is suggesting a complete upheaval of society to prepare for this possible catastrophe.
:et's be clear: Cleaning up the environment is a good thing and should be done. I always considered myself an environmentalist before that label was stolen by irrational fanatics. Trashing your own home is just stupid! Cleaning the environment, however, can be done without record deficits or social upheaval. If there is no imminent catastrophe then it should be done without record deficits or social upheaval. Until there is more evidence than computer models, the debate is still open and arrogantly refusing to listen helps no one.