The CO2 is largely consumed by plants which convert it back to O2, more/less the reverse of the burning process that created CO2 and water.

Sadly, that's wishful thinking. Plants photosythesise (produce sugars) for two purposes: to live (respire) and to grow. The respired sugars are returned to the atmosphere as CO2 almost immediately. The growth of the plant (leaves, stems, trunks, roots, tubers, flowers, seeds etc.) is sequestered until it rots, when bacteria break it down and return the CO2 to the atmosphere. So in all but the shortest terms, the biosphere is CO2- neutral- it neither adds to it nor subtracts from it. that's why the oxygen content has remained essentially constant for millions of years.

But if we add fossil carbon to the atmosphere, the only way this can be incorporated is if the biosphere grows proportionally, otherwise it hangs about as loose CO2. Most evidence suggests that we are actually reducing the size of the biosphere, though it's difficult to measure. At least, few suggest that it's growing.

The CO2 content of the atmosphere is incredibly small- its concentration is only about 0.03%, about a tenth of the concentration of of argon for example. But it is a very active component of the atmosphere, as it is one of the main causes of the greenhouse effect.