Effecting an actual, official exit is quite a headache with no real upside and it can't be done at all inside of four years without essentially breaking international law. There's no rational reason to do it outside of as a general "fuck you" to the world in general and US liberals in particular. If Trump wants to go the denialist road, it's much easier - and more likely - to simply skimp out on putting up ambitious goals, or insist on delaying real commitment for as long as possible. Not to mention that it gives the US a massive bargaining chip.
So that's my expectation: Trump will stay in for the duration of his term, but will fire on all cylinders about protecting the US from "bad deals", won't put up real commitments for the US and won't officially recognise climate change.
That said, it's Trump. He could still withdraw from the accord out of incompetence and spite, no matter how counterproductive that would be.
That would trigger a global financial crisis the likes of which has never been seen and throw the world into enough turmoil that major wars could break out. Nuclear wars, even. It will never happen.