It's not about living with xxx ppm, it's about sustainability and pleasantness of the ecology.

How is the ecology more sustainable and pleasant with a different amount of CO2 than 450ppm?

What is the optimal concentration of CO2 and why?

If you start tinkering with things you aren't sustaining it, you're tinkering with it.

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.

Good point about OA. I couldn’t find what the control was in the pteropod study. What are they comparing them against?

Also looks like OA isn’t uniformly bad. Some species seem to thrive: https://www.whoi.edu/oceanus/feature/ocean-acidification-a-r...

> Good point about OA. I couldn’t find what the control was in the pteropod study. What are they comparing them against?

No clue. I was just using it as an example.

> Also looks like OA isn’t uniformly bad. Some species seem to thrive

Sure, that's why the harm is measured in terms of entire ecologies, not individual species.

The optimal is whatever it median value was before industrialization. Not that Earth hasn't had life at a wide range of CO2 levels, but current life had a million+ years to get used to mid-200s PPM, whereas we've gone from that to over 400 in a few decades. Ecologically, that's like hitting a brick wall. The rate of change is too fast.

It is not a photo, it is not a closed room. We are in a system, that have inputs and outputs. With more than GHG (not just CO2) than preindustrial times, the planet keeps more energy of what it receives to the sun. That affects the pretty complex system that is the climate one, in several ways, and triggering several positive feedback loops that makes things even worse (i.e. losing ice over ocean means less albedo, and more energy capture, whatever is the concentration of CO2 in the atmosphere). And some of those feedback loops even do their own emissions of GHG, like more methane from wetlands, and the one emitted by permafrost thawing, potentially more water vapor that is a strong GHG, and so on.

There is a point where the concentration of CO2 is directly harmful for human beings, 450ppm is still far from that. But having too much for too much time put the system on a roll into a direction that we don't want. And we are not decreasing it, but still accelerating the rate at which we emit it.