He makes the assumption on the base that we don’t need to know whether it’s true. There’s the fallacy.

He puts the 4 different options at 25% probability each.
He also puts AGW or no AGW at 50% each.

And actually he omits two more possibilities:

5. GW is happening, but despite spending trillions the catastrophic event will happen anyway because it wasn’t man-made
6. In 20 years the Earth will start to cool (there may be events that we cannot foresee) and our actions will actually have contributed to that cooling

I would rather make a different case. At least some actions which are proposed to stop the presumed AGW are actually good for different reasons. Clean air, clean water, who’s against it? Clean water for everyone will actually be the major challenge in the next decades.

How if we spent our trillions to achieve tangible benefits for the 4 bn underprivileged people on Earth instead of wasting them on things that will make life for those 4 billions even harder?