I watched the announcement w/o knowing much about it. He justified the decision by saying:

- We're paying 100b/year when other developed countries do not have to. - Spending on that is unaccounted for. - The impact will be (only) a 2/10 Celcius reduction in temperature by 2100 - The positive impact by 2030 is the equivalent of what China pollutes in 14 days.

Just stating what was said. I don't know which are facts.