But we know from physics that it's a greenhouse gas. It's not a privileged hypothesis we're pulling out of nowhere. It's not like saying "You can't prove there's no invisible pink unicorn in my garage!" AGW is, ceteris paribus, what we should expect to happen if the other things we believe are true.

I realize that this is not a debate about global warming, but respectfully, you are wrong here. It's just that the privileged hypothesis is hidden from view by means of conjunction.

It may surprise you, but the actual global warming hypothesis as pushed by the likes of the IPCC is NOT simply that increased levels of CO2 will result in an increase in global surface temperatures.

The actual hypothesis is that increased CO2 levels will cause an increase in global surface temperatures, which will cause an increase in levels of water vapor in the atmosphere, which will cause temperatures to rise further, and so on, until there has been a dangerous increase in global surface temperatures.

In other words, global warming is a compound hypothesis. And the second part of the hypothesis -- water vapor feedback -- is very much like the invisible unicorn in your garage. There is simply no a priori reason to believe that the climate operates by positive feedback in this way.

I go into more detail about this on my blog.

http://brazil84.wordpress.com

Anyway, I realize this post is a bit off-topic, but I think the point is important. When discussing a claim, it's helpful to make sure everyone is discussing the same claim.