I would guess not that much - certainly fractions of a degree - because the proportion of the planet's surface is still relatively low, and as much as 60% of th

I would guess not that much - certainly fractions of a degree - because the proportion of the planet's surface is still relatively low, and as much as 60% of that reflected light would be scattered and absorbed by the atmosphere. It'd have a much bigger impact if all those roofs had solar panels, and the resulting electricity was used to replace carbon-emitting energy sources.