The ceremony is only a first step to seeing the Paris agreement enter into force. Fifty-five countries representing at least 55 percent of global emissions still need to ratify the agreement, which the U.S. and China (the two biggest emitters) plan on doing this year. Even once it’s ratified, there’s a lot of work left to be done. Top U.S. climate negotiator Todd Stern told Grist in an interview, “the most important thing is what countries do nationally” to make their needed emissions cuts.