Interesting idea. Here are some numbers. Every hectacre of a good forest contains 250 tons of biomass. Trees are typically 50% carbon thus a hectacre contains 125 tons of carbon and 1.2 bln hectacres would contain 150 billion tons of carbon. Every ppm of CO2 in the atmosphere represents 2.35 billion tons of carbon, so planting 1.2 billion hectacres would reduce atmospheric CO2 levels by a little more than 60 ppm, which is about 30 years of future atmospheric CO2 growth at the current rate.