Increased in CO2 requires ecological adaptations of organisms living in the oceans, for instance. https://www.noaa.gov/education/resource-collections/ocean-co...

"Researchers have already discovered severe levels of pteropod shell dissolution in the Southern Ocean, which encircles Antarctica."

The optimal concentration for the ecology that we recently had is the CO2 concentration we had a bit before that (as it takes times for effects to be felt). Plus or minus, here or there, depending on other factors such as warming.

Forcing the rest of the ecology to try to adapt to increased CO2 levels starts to become really problematic for them when we are also forcing them to try to adapt to things like human-introduced invasives, human-caused habitat destruction and repurposing, population loss due to over-hunting and over-harvesting. It all adds up.