That would probably also have the effect of reflecting some of the energy leaving Earth back down onto it, essentially increasing the greenhouse effect.

Also, assuming you need to reflect 2% of the incoming light, and using 8ft diameter balloons, you would need approximately 2,744,628,480,000 balloons.

Why think so small? Do something like this

{1] https://en.wikipedia.org/wiki/Cloud_Nine_(tensegrity_sphere)

combine that with the ideas of

[2] https://en.wikipedia.org/wiki/High-altitude_platform_station

and PROFIT!