The U.K. has cleaner beaches and natural protection and shouldn't risk the environment by leaving.
Americans "can be proud because this historic agreement is a tribute to American leadership," president says.
Will the Paris Agreement be enough?
"Our responsibility to history is immense," French Foreign Minister Laurent Fabius said.
The sticking points have been finance, pace and ambition.
Oceans produce most of the oxygen we breathe, and they are threatened by climate change.
We've been waiting years for this moment, says Ken Berlin, president of Al Gore's Climate Reality Project. Let's make it count.
Public protests were banned after the deadly attacks in Paris.
Authorities use emergency laws put in place following the Paris shootings.