It is possible that Trump knew this info in advance and played the dumb guy at the G20. And today it seems he was in reality the smartest guy in the room ;-)
From Quartz: "Research analysts at Morgan Stanley believe that renewable energy like solar and wind power are hurtling towards a level of ubiquity where not even politics can hinder them. Renewable energy is simply becoming the cheapest option, fast. Basic economics, the analysts say, suggest that the US will exceed its commitments in the Paris agreement regardless of whether or not president Donald Trump withdraws, as he’s stated he will."