Many Californians hope the state can join the Paris Agreement, but the Constitution doesn't offer much latitude on that front—foreign treaties are handled federally.
There is a chance the Paris Agreement could be amended so that large parts of America could join. But California has been aggressively signing memorandums with other subnational governments, and while its progress may falter without federal subsidies for clean energy, the state cap-and-trade program isn't in doubt.
That means that even if California isn't officially party to Paris, it's effectively doing the same thing.