First, it's all voluntary.
So there's really no incentive to do anything.
Second, the U.S. would share that contribution with other developed countries.
By "sharing" do you mean like the nations that are suppose to be paying NATO but are not?
Third, there's no time frame for the contributions.
So there's really no incentive to do anything.
Fourth, the contributions can be in the form of private loans which are more of a business transaction and cost taxpayers nothing.
The Government is funded by taxpayer money not private loans.
Fifth, the U.S. is the second largest carbon emitter and the largest per capita, so it's fair that they help undeveloped countries use alternative energy sources.
I recently read that the U.S. is the largest carbon emitter. So it makes sense to spend the money (taxpayer) at home cleaning up our own mess and also where it's going to have the biggest impact on carbon emissions.
Now I know what you're thinking. "But Trump won't spend that money on cleaning up the mess"
You don't know that. But you have a voice in that decision.
You also don't know what the $100B is going to be spent on in 3rd world countries and you have absolutely no voice in that decision.