Based on my comments I take it that I am still not making myself clear on Climate Change in America.
A few more direct points
(1) Its not that rebuilding America would be a good thing because it produces jobs. Or that spending money or seawalls or air conditioning contributes to GDP. Its that these are bad, but they are not that bad. They are a cost but not a great cost.
(2) Cranking up really big damages almost always comes from loss of life. What reason do we have to suspect that lots of lives will be lost in the United States from Climate Change?
(3) There is a big difference between whether Climate Change will be bad for the United States and whether it will be bad for the rest of the world. Again big damages come from loss of life.
Yet, if those lives are in other nations they don’t show up as damages for the US. This is a central issue. Many folks may want to inflate the damages for the US so as to motivate Americans to do things that primarily benefit people in other countries. I understand the motivation here but simply see it as my role to say what, as best I can tell, is true.
Additionally, calculating damages outside the US is extremely tricky because its going to depend crucially on how developed the rest of the world is by the time the effects kick in and what international immigration policies are.
If in 100 years we wind up with a mostly developed globe and relatively free immigration the damages to climate change will probably be small for everyone.
If Africa and South Asia stay poor then there is a much different story.
(4) It is of course possible that climate change could be much worse than we imagine but this is true for all sorts of risks. The next flu could be worse than we imagine. The violence from revolutions in the Middle East could be worse than we imagine. Nuclear proliferation could be worse than we imagine. The probability of an asteroid strike could be worse than we imagine. The probability of mega-earthquakes could be worse than we imagine.
All of these things could go really badly and cause lots of death and destruction. We have to try to weigh these things as best we can and balance our efforts. This is the point of trying to get a handle on expected damages.
Moreover, recognizing our general uncertainty about the future makes it less sensible to invest heavily in one particular cause.
All that having been said you can still make the case for a carbon tax. Its just that the case centers around damages that are in the 5% of GDP range for the US.