Even scientific consensus can be in error -- the worldwide consensus of geologists rejected continental drift until well into the early 1960s, even though the theory had been around for decades.
The convincing thing isn't how long the theory has been around, but rather how much has gone into it, over that time, and the trend of the consensus. I'm not sure any scientific question has ever been subject to the kind of intense scrutiny global warming theory has over the last few decades. We're not talking the equivalent of a few dozen working geologists working with woefully inadequate tools, as during the pre-plate-tectonics era. We're talking about thousands of researchers across dozens of specialties looking into the matter with highly advanced tools, for decades, and all of that forming a remarkably consistent picture. And we're talking about a trend where the consensus grows both stronger and broader over time. To make a valid analogy, you'd need a situation where some new theory got that kind of massive, diverse attention over decades, with gradually and consistently growing strength of consensus, only to turn out to be wrong. Continental drift doesn't cut it. That was a case of the slowness of a single field to adopt a new theory in the face of sparse evidence (and then adopting it once the evidence become overwhelming) -- it's just not terribly relevant.
It is a graver error to make huge policy changes based on an "if" no matter how many people believe in the if, than it is to make minor changes while waiting for a higher level of certainty
If we stuck to only minor changes while waiting for higher level of certainty, that would be a permanent state for issues like this, where there will ALWAYS be some level of doubt. That's a recipe for disaster in cases where dramatic action is needed.
Let's look at it in a smaller context. You go a doctor and he tells you that you have very high blood pressure, and you should quit smoking, because you're at high risk for a heart attack or stroke otherwise. So, you see another doctor. Turns out there's a consensus about this. In fact, you'd have to hunt far and wide to find one of the tiny handful of doctors who think smoking isn't a big risk factor for people in your situation. Still, rather than make the "grave error" of a huge change in your smoking policy, you decide to make minor changes while waiting for a higher level of certainty. You'll cut back from 40 cigarettes per day to 39. Surely, in a decade or two, doctors will have a clearer picture of exactly how important smoking is in blood pressure considerations, right?
Even you have acknowledged that we don't have a clear determination that the climate chane we're experiencing is anthropogenic -- so we don't know if the alleged negative repercussions stem from a human source.
What we know is that the vast majority of experts believe it's anthropogenic and that if we don't take major steps, the results will be terrible. If you choose to gamble the liveablility of our only planet on the hope that they're wrong, that's the very opposite of "conservative." It's reckless to the point of being pathological, like that nicotine addict who will rationalize his choice of continuing to smoke in the face of compelling expert advice.
At this point, the earth isn't as warm as it was during the Medieval Warm Period.
I realize this is a popular talking point in the anti-global-warming industry, but it's simply not true. It's based on localized European measures, not global ones.
http://www.realclimate.org/index.ph...the-medieval-warm-period-than-they-are-today/
http://www.newscientist.com/article...d-with-vineyards-in-england.html#.UdSXEazgfpU
http://en.wikipedia.org/wiki/Medieval_Warm_Period
http://www.skepticalscience.com/medieval-warm-period.htm
http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch6s6-6.html
The thing to remember, whenever one of your right-wing climate "skeptic" sites feeds you that kind of propaganda is this: by the time they heard about some piece of evidence in the climate debate, the actual climate scientists had already been aware of it for a long, long time, and it's been factored into the consensus analysis already. The Medieval Warm Period isn't news to the IPCC, or the many national institutes of science that have echoed the IPCC statement of the consensus, nor to ANY of the thousands of climate experts who make up that consensus. It's common knowledge. Hell, even I'm aware of it, and I'm a rank amateur. When experts say they believe the Earth is warming quickly due to human activities and that we're heading for serious trouble if we don't do something major, they do that in full knowledge of the facts about the Medieval Warm Period.
To go back to the smoking analogy I made earlier, it's as if you tried to argue that smoking isn't a problem for your heart because Japanese people smoke a lot and have long life expectancies. The consensus on the problem smoking poses for the heart formed in the presence of the common knowledge of Japanese smoking and life expectancy, so pointing to that small component of the overall evidence picture as if it somehow overrides the consensus would be an act of hubristic foolishness. If that's a point against the smoking/heart-disease picture, it's a point that's radically outscored by all the other points, or the consensus wouldn't have formed. This isn't some new data-point. In the same way, if the Medieval Warm Period were a point against the global warming consensus, it's a point that's already been tallied and vastly overwhelmed by all the other points in the other direction. It's a data point that's been integrated into the consensus already. You might want to pull it out and give it an unjustified amount of weight to overwhelm all the rest, but that's for emotional, not rational reasons. You want the answer to be a certain thing, and so you weight the evidence in accordance with your desire.
A wise lay person accepts that the best we can hope to do on a complex scientific controversy like this is get a clear picture of where the experts are generally coming out -- a meta-analysis of the direction, strength, and breadth of the consensus. Then we set policy based on that. To think that we can add something to the debate through snippets of propaganda we picked up that cherry pick data is just absurdly egotistical. It's like "Gee, scientists, I know you have doctorates in this topic and have spent 50 hours per week for the past 20 years dedicating your life to studying it, while I've been occupied with other matters, but taking a quick glance at a conservative web-site I've become aware that it was warm for a while back in the Middle Ages, and I'm betting you didn't consider that." It's just hilarious, really.
