City governments can have a great impact on reducing carbon emissions. Such activities as reducing traffic congestion, improving transit efficiency, providing safer walking/biking routes, and more, can help citizens move more efficiently. 

This would not only reduce carbon emissions, but make the city more 'livable' and probably improve citizen satisfaction. Read this article and check out their link to the project six major European cities have undertaken to try to use advanced technology (including Artificial Intelligence) to meet these objectives.