If the USA goes to 0 (which is the most hilarious notion I’ve read all day) while other countries continue per the status quo, that still makes a difference. It helps to slow the increase, buying more time to actually solidify the numbers.
The thing is, while the consensus is that we need to do something, and that something needs to be done a lot, and fast, we don’t have any quantifiable numbers, just estimations that may be wildly underexaggerated.