Oh, another argument is that there aren't just the two states, one where global warming doesn't exist and another where it does. There are really three states, one where global warming doesn't exist, one where global warming does exist and we can do something about it, and one where global warming does exist and we can't do anything about it. That third state isn't necessarily excluded when you narrow the question (as he did) to human-caused global warming, as it's possible that we've set something in motion that we can't stop.

So that adds a third row where not doing anything results in catastrophe and doing something results in a potentially worse catastrophe by exacerbating a fiscal depression.

Of course, there are potentially more columns, too. Doing something can be split into reversing/preventing/ameliorating global warming (which each could potentially be their own column) and preparing for global warming; that is, maybe there's some way we could allow global warming to occur but figure out a way to make it non-catastrophic.

Then each of the do-something columns can be split in two: one where we do something and succeed and another where we do something and fail.

All in all, I tend to agree with his conclusion, though. If we assume there is potential for catastrophe, we should do everything we can to prevent it.