Photosynthesis produces oxygen at a rate faster than the rate of depletion due to oxidation of the Earth's crust (think rust). Without plants there's nothing le

Photosynthesis produces oxygen at a rate faster than the rate of depletion due to oxidation of the Earth's crust (think rust). Without plants there's nothing left to spew oxygen back into the atmosphere. Photosynthesis converts CO2 into carbohydrates. Lowering the concentration of atmospheric CO2 slows this reaction. Plants also metabolize carbohydrates, producing CO2. So photosynthesis requires the right balance of CO2 concentration and light. The estimate for when CO2 levels would fall too low appears to come from the following linked article: The latter authors feared that photosynthetic production would cease ~ 0.1 Gyr from now when just 150 ppm CO2 remained in the atmosphere. The latter figure corresponded to the CO2 compensation point, at which photosynthetic carbon fixation would exactly balance losses through respiration. We stress that _ this must not be confused with the CO2 value at which healthy metabolism ceases; with zero net C fixation available, a plant could not repair itself, produce seeds, or even grow. https://arxiv.org/pdf/0912.2482.pdf https://arxiv.org/pdf/0912.2482.pdf There's more on the compensation point here: https://en.wikipedia.org/wiki/Compensation_point https://en.wikipedia.org/wiki/Compensation_point