That's your son, you jerk. It's his world you've just promised to screw up. Do you really want your own child to curse your name forever?
No, we don't have to be in an accord to do what is right, but it does help when everyone (or almost everyone) is heading in the same direction. Listening to John Kerry last night, he said the U.S. was not paying anyone anything and that each country agreed to do their best to meet the goals set forth. Not sure who to believe here. I'm not sure why the U.S. would have to be paying anyone to do anything, but I haven't read the agreement.
EDIT: I just looked this up on the internet. According to the Q&A in the NYTimes regarding the agreement,
"Under the deal, the Obama administration pledged to cut domestic greenhouse gas emissions 26 to 28 percent below 2005 levels by 2025 as well as to commit up to $3 billion in aid for poorer countries by 2020. (The United States has delivered $1 billion to date.)"
https://www.nytimes.com/2017/05/31/climate/qa-the-paris-climate-accord.html?_r=0
Note that it will commit UP TO - not that it WILL commit $3 billion. So if it's a matter of not wanting to contribute any more funds, Trump could keep us in the accord and just not dole out any of the other $2 billion.