Or would the heat dissipate back into space? For my world building purposes, I need it to cause global warming, I just don't really know if it would work. Maybe the cities would need to be bigger then they are know?
EDIT: I meant mirrors pointed up like giant reflective parasols. And now I see why that would decrease the temperature, not increase it, so... silly me! I'll fix it somehow, find something more compelling to produce global warming. That story with the town in Norway was pretty cool! Thanks for all the feedback, my question has been answered! :)