There is an overwhelming consensus supporting the basic tenets of anthropogenic global warming theory. Those tenets are that CO2 levels are rising, this rise is caused by human activity, this rise is causing a rapid warming trend and this trend will continue unless CO2 levels stabilise. Contrarians still like to deny this, but the existence of this consensus is an indisputable fact.
Additionally, but with much less certainty, the general picture emerging from scientific research is that the kinds of temperature changes that may be in the near future (~50 years) will cause problems ranging from very difficult to catastrophic.
Emerging from the comments recently are two well formulated answers to the fallback argument that consensus is nothing significant anyway.
Consensus, what is it? Well it is a group of people that agree with each other. Nothing more nothing less.
The first answer is here from "ali baba":
You make it sound like a show of hands. Every scientific theory either rises to the level of consensus or else it is abandoned. Every single one. Consensus implicates a consilience of evidence and a preponderance of evidence for the best explanation. Consensus is how science works, and it is the difference between truth as we know it and poorly supported speculation we don't. The difference between science and its denial.
This is really a debate over the nature of truth. It seems that the (post)modern conservative movement has adopted the relativist position that all knowledge is political, and there are no objective truths. They've utterly conflated normative claims ("what should be") with objective/positive claims ("what is"). You certainly have a right to your own opinion as to what we SHOULD do in response to knowable facts, because "should" statements depend on subjective values about the good life, obligations to others, etc. But you don't have the right to interpret what those facts ARE.
Fortunately, science is the one area of life where the truth is objectively knowable and ISN'T political. Unfortunately, when science implies that certain policies may be a good idea, ideologues opposed to those policies are all too willing to "interpret" the known facts to suit their preconceived beliefs.
Nicely stated, both of them.
Of course the only answers to those comments were the purest of denials, statements at absolute odds with objectively discernible reality ("evidence in support of the sceptics' position piling up daily", "sceptics believe that when a conflict arises between evidence and theory, evidence is to be preferred."). No surprises there, except maybe how laughable that really is.
Below is a common post of mine when I troll denialist blogs:
So we are left with three possible conclusions:
1) An overwhelming majority of international climate experts agree about much of the tenants of AGW and are honest.
2) An overwhelming majority of international climate experts are ignorant about their own expertise in a sudden and collective manner.
3) They have all agreed to conspire to delude the billions of folks on the planet and just a very tiny percentage of them (and mostly unpublished) are trying to save us all from this mass hoax.
Common sense and a sense of probability should lead one to the likely correct choice above.
The first person to show proof of what IS causing the modern day global warming and that it is not AGW is likely to be the next Nobel science winner.
The shortest version I can come up with:
Observing a bird in the sky doesn't disprove gravity.
Scott, great description of the possibilities. May I use it? (sourcing you of course?)
Certainly Bart. I use your bird quote (which you posted at RC recently) often. :)
Nobel Prize? The person that offers a superior and supportable alternative to AGW is the Einstein of earth science, and the star of whatever universe they want.
Until then we all need to work with the best tool available.
Haven't you heard? AGW debunkers are to be compared, not to Einstein, but rather to Galileo (you know, persecuted by the "church" of AGW).
And of course Galileo is notiecably absent from the Nobel prize winners' list. I suspect fraud.