Global warming doesn’t stop on a dime. If people everywhere stopped burning fossil fuels tomorrow, stored heat would still continue to warm the atmosphere.
Picture how a radiator heats a home. Water is heated by a boiler, and the hot water circulates through pipes and radiators in the house. The radiators warm up and heat the air in the room. Even after the boiler is turned off, the already heated water is still circulating through the system, heating the house. The radiators are, in fact, cooling down, but their stored heat is still warming the air in the room.
This is known as committed warming. Earth similarly has ways of storing and releasing heat.