Is it fair to assume the earth is a closed loop self healing system?
That is even if we dump a lot of bad stuff into the atmosphere, the planet will eventually “equalize” to “normal” levels.
I would rather we did not of course. I’m just curious about the planetary atmospheric system.
The surface of the ocean is in equilibrium with the atmosphere, so increasing atmospheric ppm increase the ppm of CO2 in the mix layer (think of it as a gas sponge). The ocean buffers that co2 (carbonate/bicarbonate system), so the amount of carbon there is a lot higher than just 420ppm, and in doing so it becomes slightly more acidic.
As the ocean turns over (every ~5000 years), it brings that acidity down to depth where it can react with various forms of alkalinity that are at down there (mafic rock at the bottom of the ocean, sediment, etc), partially neutralizing it. It takes about 20 cycles (very slow) to do this, so ~100k years to return to a pre-industrial level. The pre-industrial ppm is defined mostly by biology.
A more complicated treatment of the carbon cycle: https://www.ipcc.ch/site/assets/uploads/2018/02/TAR-03.pdf
Life on Earth can adapt to a new normal, certainly, and on a geological timescale normal has shifted many times. Humans will adapt too, but will likely endure catastrophic losses on the way.
You must think of the Earth more as a process than a fixed thing. Thinking about this system in terms of feedback loops may be more helpful here.
Drastically increasing C02 levels affects several feedback loops in the system, essentially transitioning it to a state that is different to the current one.
As noted elsewhere in the comments, the Earth has gone through different eras in which the atmosphere had "high" and "low" levels of C02.