11. The cause of increasing CO2 may be from industrial use. For example, factories have boilers that emit CO2 into the atmosphere. This also correlates with the fact that industrialization has been increasing over the past several decades. Possible other sources include vehicle fuel emissions.
12. Because trees and other plants take in CO2 for photosynthesis, and release oxygen (O2) instead, planting a large number of these would slowly increase the rate of consumption of the CO2. As long as the rate of CO2 absorption from the plants is faster than the rate of production from other factories, the CO2 concentration will gradually decrease.