Unintended Consequences
This is an idea to be tested, but when working with a complex system as the atmosphere there will be unintended consequences. A possible consequence could be more water in the atmosphere contributing to weird weather patterns. The current National Geographics has an article about extreme weather events from heavy rain lasting days that lead to local floods to stronger thunder storms and tornadoes.
Prudent actions should anticipate problems caused by the solution. A good example is importing cane toads to deal with local insects; the cane toads become a bigger problem than the insects. If making white clouds to combat global warming has a fast effect then it is possible to deal with the severe weather if that is only for a short time. If the solution takes more than a decade to show improvement then it may not be worth the risks from severe weather.
