This is an excellent video put together by consensus in science expert John Cook. John is the author of the excellent must-read book Cranky Uncle vs. Climate Change: How to Understand and Respond to Climate Science Deniers.

I have always been interested in the concept of consensus, even before that word became centered in the pro vs. anti science debate. In Anthropology, we have huge problems with consensus. In at least one branch of Anthropology, consensus can never be achieved because all good work is defined as breaking consensus. The moment you get close to consensus, you’ve failed. (That’s socio-cultural anthropology, modern style). In another branch of Anthropology, we deal with questions that can’t really be answered at that level, but sort of can be. So there is never consensus in the sense that of the many possible explanations for a thing, there will always be a list of possible, and often very distinctly different, alternative explanations. But, over time, the list changes. One hopes for the list we have now being better than the list we had a decade ago, even if both lists are approximately the same length. (Example: Reasons for the origin of bipedality in the human lineage.)

There is a particular kind of consensus that to my knowledge my friend John Cook does not talk about (yet, he’s got most of this covered very well): Beer pitcher consensus. It goes like this. Suppose there is a range of thought on a particular narrowly defined scientific question. Since this is about climate, let’s do a climate one. The question might be: What is the best value for “climate sensitivity.” This is the number of degrees Celsius that the atmosphere at the Earth’s surface will go up with a doubling of atmospheric CO2 from pre-industrial values (say that was about 280ppm). (I’m oversimplifying the concept and the question slightly and I believe forgivably.) The answers range from a somewhat pedantic and absurd 2.0 to an alarming and probably alarmist 5.6 or so.

Now, get a bunch of experts on this question, say at a conference. Sit them down for a beer. After a couple of beers, tell them, “OK, folks, I’m giving each of you a piece of paper and a pencil. Write down a guess on the climate sensitivity value. Here’s the thing. Don’t show each other what you wrote down, only show it to me. And, if they are all the same value, I’ll buy pitchers of beer for the rest of your time at this conference, starting now.”

Had I simply asked this group of experts to tell me the climate sensitivity value, it would start a conversation that would go on for hours, and there would not be a single number. But if I do it as described here, they would all write down one number, and it would be 3.5 (I’m pretty sure).

That is the beer pitcher consensus.

Anyway, have a look at John Cook’s excellent video. It shows why most of the time you as a science oriented concerned lay citizen usually get this wrong, but in a harmless way. There is not a “97%” consensus. There is a full consensus; the number 97% is kinds of silly, and it is only part of the picture. The idea that global warming is happening and is human caused is, simply put, established scientific fact. There is no valid dissent. But, the number “97%” does have an important meaning and history in the debate. More to the point, not only is there a consensus on climate change and the human cause of it, but there is a consensus on the fact that there is a consensus!