i think it makes more sense to look at the amount of coal and oil being burned than how tiny cities are in comparison to the globe.

for example there still are at least 800 billion tons of coal that can be mined. the density of coal is a bit higher but lets say 1 ton = 1 m³. so you could make a 1 m layer with an area of 800000 km². that's almost 3 times the size of italy. i could imagine that burning it has some effect on the atmosphere. also imagine all the coal and oil that already has been burned.