From CNN:

A person familiar with Trump’s plans says he will trigger the official withdrawal mechanism that’s written into the Paris accord. It will kick off a process that will take until November 2020 to complete.

This was considered the less-drastic option of withdrawing – the other being removing the US entirely from the UN climate change treaty, which would have taken effect sooner.

The person said Trump will make clear that he hopes to renegotiate better terms but that enacting the withdrawal process was the only way to do that.

So basically he’s gonna try to claim a win for pulling us out and leave the problem for others to fix.