"we should do something just because of what might happen if we don't" doesn't really make sense. you should turn your computer off right now because it MIGHT blow up!

Here's a scenario. I own a very large weapons manufacturing plant (which I actually don't, by the way). I also point out that you can not know whether aliens will attack tomorrow or in the future. I can make the same chart that he does, showing that you should arm yourself to the teeth just because aliens might attack.

I'm not a global warming nay-sayer. I am saying this because he is doing an "expected value" argument and he is essentially assuming equal probabilities for each of the possible outcomes, thus advancing his argument.

So if aliens have a 50/50 chance of attacking, it makes sense that we should arm ourself against them.

JonV