One of the many problems with limiting the amount of sunlight reaching Earth is that it reduces plant growth worldwide. This would not only decrease agricultural output, with the obvious consequences for food production, but also reduce the amount of carbon absorbed and stored by growing vegetation. Which of course wouldn't help.
And reduce evaporation, which reduces moisture in the air, which reduces rainfall, etc. I don't know what I'm talking about and already think messing with the weather in any way is a Bad Idea.