Very simplified (ocean-centric) homeostat: The surface of the ocean is in equilibrium with the atmosphere, so increasing atmospheric ppm increase the ppm of CO

Very simplified (ocean-centric) homeostat: 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 https://www.ipcc.ch/site/assets/uploads/2018/02/TAR-03.pdf