Question

During photosynthesis, how is the earth's overall mass being increased?

Answer 1

So the majority of mass of a tree/plant is actually its CO2 levels and water. As the tree gets larger, it is taking in nutrition from the ground/soil, water CO2 and sunlight. All these factors allow the plant or tree to grow. Because two of the factors will never change (CO2 and sunlight) you can say that as the tree takes in more CO2 (a molecule heavier than oxygen) it is gaining mass. Also, remember that plants also create glucose. This can account for the mass as well, but the majority of the dry mass of a tree consists of CO2

Answer 2

So to formally answer your question, it is because of plant's ability to "breath" in carbon dioxide, which allows for their mass to increase. If you think about it, there will never be a deficit of CO2 because the earth is overpopulated with humans.