8 billion people running a 2000 watt AC continuously for 8 hours a day = 5 trillion watts of heat. (Only the electricity consumed by the AC is turned into new heat. The heat from inside the house would have moved outside anyway, at the same rate, since it's an equilibrium)

The sun = 175 quadrillion watts of heat.

So I would say, the heat from running ACs is not significant. It's also additive with all the other existing forms of energy use we have. Unlike greenhouse gases, which are multiplicative.