system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why increased atmospheric CO2 levels don't just mean healthier more lush plant life which in turn offset that CO2.

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gpt
Photosynthesis = H20 + CO2 + Light (particularly UV/V/B light) made into O2 (oxygen gas) + C6H12O6 (Sugar).... Let's keep that simplification in mind for now.

When we think about plants making O2 to offset the CO2 production, you run into two key limitations: water and rate of photosynthesis. When there is no water (particularly clean, harmless water), it prevents the process of photosynthesis from occurring. That is why a drought can be devastating in that regard. This usually isn't an issue as rain forests contribute much of the plant-based global-O2 generation and these environments are not insanely water deprived (but global warming and deforesting have still affected this cycle in some regards).

The biggest issue is rate of photosynthesis. Just because more CO2 is present, that does not mean the plant will engage in photosynthesis much quicker. Think of it like this: let's say your car tops out at 100mph. Does adding more gas in your tank make your car faster? The answer - no. Your car (plant) is still limited by its internals (plant chlorophylls) regardless of how much gas (CO2) is in the tank (atmosphere).

Also, the Ocean makes about 70% of oxygen. Phytoplankton and the sorts, and they are also in danger due to the dropping quality of ocean water in some regions.

EDIT: Phone auto corrected "in danger" to "endangered."