The U.S. Constitution, Article 1, Sec. 10 states in part (irrelevant wording removed):
- "No state shall enter into any treaty, alliance, or confederation, grant letters of marque or reprisal, emit bills of credit, ..."
- "No state shall, without consent of Congress, lay any imposts or duties on imports ..."
- "No state shall, without consent of Congress, ... enter into any agreement or compact with another state, or with a foreign power ..."
I cut a lot of the text, but the Constitution makes it clear that an individual state cannot be a party to a treaty or make deals with foreign governments. California can pass its own emissions caps, carbon credits, etc., but they only apply within the state. It cannot enforce those provisions outside its jurisdiction, nor can it set up a system of carbon credit swaps with foreign powers or even other U.S. states. It also can't impose tariffs on imports that don't satisfy environmental limits (although it can tax its own citizens for selling/using them).
There is a lot that California can do on its own to enact some of the provisions of the treaty. But they are strictly limited in their ability to cooperate with international efforts, and to have their efforts recognized internationally.
FYI: Letters of marque and reprisal are old English terms for taking actions to sanction another country or state for violating their laws. A bill of credit refers (in this case) to monetizing carbon credits or some such system of cap and trade that would have the effect of turning them into a tradeable commodity on par with currency.