Arguing that the US has already used up more than its share of the total allowable greenhouse gas emissions, and therefore has an obligation to decarbonize on the double.

I am not convinced that that is the right way to look at the question, but the conclusion may be valid anyway. If we choose a more pragmatic goal -- to avoid climate disaster that could destroy civilization globally, and most of humanity -- it still follows that the US must decarbonize on the double.