1. Arnold writes:

I have no strong priors about the distribution of possible global temperatures going forward.

How about strong posteriors conditional upon the existence of a scientific consensus? If consensus doesn’t impress you, then you should be willing to bet at even odds that temperature will fall.

2. Arnold adds:

My skepticism is about the effect of man-made CO2 on global warming. If Bryan or anyone else can devise a bet that focuses on that parameter, then that would allow me to put money on my skepticism.

What’s the problem with conditional bets? We could bet on (a) Temperature will rise conditional on CO2 emissions being above a certain level, and (b) Temperature will rise conditional on CO2 emissions being below that level. I don’t know enough climatology to specify reasonable cutpoints. But conceptually, what’s the problem?

3. Arnold observes:

Gregory Clark wants to argue that in the 1800’s British workers were much more productive than Indian workers. He asks that if British and Indian workers were close substitutes, why would you ship cotton from India to Britain, turn it into cloth, and ship the cloth back to India? Instead, if they were close substitutes, you would observe English companies building cloth factories in India in order to serve the Indian market more cheaply. In other words, shipping cotton to England and cloth back to India is something that is unlikely to occur if workers are close substitutes. Instead, it provides evidence, meeting my “strangely strict” standards, that British workers were more productive.

I agree that this is a fine argument. But it only works because we have a large body of diverse pieces of evidence that (a) Businesses want to make money; (b) Businesses are not run by idiots; (c) Cotton made in England is a close substitute for cotton made in India; (d) Economic historians are not making up these stylized facts; etc. Clark’s argument is good, but it’s far from “an experiment or a naturally-occurring event where the results are extremely unlikely to occur unless X is true.” Isn’t it?