What is the smallest change that would allow an Earth-like planet to have atmospheric COâ‚‚ levels of several percent?

Start with a planet just like Earth as of today (whatever today means when you are reading this). For simplicity's sake, disregard mankind's continuous spewing of greenhouse gases into the atmosphere; if you like, think of this new planet as not having humans, and maybe not even any fossil-carbon fuels such as coal or oil.

Now, suppose instead of a CO₂ level around 0.04% (400 ppm), which is where we're at, this planet's atmosphere has a stable concentration of gaseous CO₂ of around five percent by volume (50,000 ppmv), along with a significant amount of gaseous oxygen in the atmosphere.

That will of course have huge ramifications throughout the biosphere, but my question here is simple:

What is the smallest change to the planet which will enable it to support such a level of atmospheric CO₂ as a stable level, while still supporting oxygen-breathing life broadly similar to what we might be used to?

Note that "smallest" does not necessarily mean of least magnitude (something like changing the geology of a planet could be far-reaching) but rather more like requires the least amount of handwavium in order to explain in another planet. There does not necessarily need to exist a simple way to get from where Earth is today to where this planet is (or the other way around), but there needs to exist a plausible explanation of how a planet (some planet) otherwise similar to Earth could end up in the situation this planet is in.

posted about 7 years ago

Canina‭

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