To piggyback onto this comment, it really is a matter of rates, of chemical kinetics. The current increase in CO2 from fossil fuel burning and related human act

To piggyback onto this comment, it really is a matter of rates, of chemical kinetics. The current increase in CO2 from fossil fuel burning and related human activities has occurred at a rate that is geologically instantaneous. There is no real precedent for this. CO2 has indeed been higher in the geologic past (e.g., Cretaceous maximum, early Paleozoic prior to vascular plant systems), but these increases are likely tied to very leisurely evolving cycles of plate tectonics and enhanced volcanic CO2 fluxes. Higher pCO2 during these periods produced warmer, wetter climates that drive enhanced rates of silicate weathering. The result? High pCO2, somewhat lower pH, yes, but also higher carbonate alkalinity in seawater as well, which gave rise to higher, not lower, saturation states for marine carbonate minerals, and are associated with higher rates of biomineralization (e.g. Steven Stanley's periods of 'hypercalcification'). So it's really a matter of the balance of rates. "Natural" rates of C oxidation via the slow uplift of buried organic carbon are far slower than what we have produced via fossil fuel burning of coal, hydrocarbons. In this process, silicate weathering acts as an essential thermostat through the feedback to chemical weathering of crustal silicates. This has been understood for quite awhile.