the only effective way to cool the earth...

[dcrockerjr]

is to reflect light back into space, granted if everyone goes around painting everything white the world will look like some sort of dystopian nightmare, but at least it will be a dystopian nightmare with cool breezes.
the "greenhouse" lets light in, it also lets light out, which is why a strategy of reflecting that light will work.
trying to reduce CO2 is the wrong strategy, plants need CO2, an oxygen rich atmosphere causes fires to start and burn faster and hotter, etc.