[alancalverd (OP)]

There is a useful quantity of carbon buried in the ground in the form of coal, oil, gas and shale, and a whole lot more deposited as limestone and chalk carbonates.

All this carbon must have come out of the atmosphere.

Now the amount contributed by volcanic activity is pretty small: if it were significant, we'd find loads of volcanic dust particles (silicates, aluminates, ferric compounds....) in among the coal and chalk, but we don't. So there is no obvious source of continuous replenishment of carbon dioxide. Therefore all the carbon in fossil fuel, chalk etc must have been in the atmosphere before the plants and animals appeared on the planet.

This makes a carbon-rich atmosphere seem a lot less apocalyptic than the climate change hyperbolists would like you to believe. Plant life and sea life will simply become more abundant.

[chiralSPO]

I agree that most of the carbon was from CO₂ in the atmosphere. I don't think that such high concentrations spell the death of life (obviously there was a thriving planet with enormously high concentrations of CO₂), but life as we know it evolved to work at geologically recent levels, and is having a difficult time adjusting to the rapid (instantaneous on a geological timescale) change in atmospheric composition, and the resulting changes in weather patterns and water chemistry.

It appears that we are at the beginning of a significant extinction event. Organisms that can adapt quickly (bacteria, algae, and other species with short lifetimes and high rates of mutation) will adapt and survive, while organisms that cannot keep up with the changes (polar bears, mangroves, and other species with long lifespans and highly specific niches) will not. Those of us in between will have a chance, but I expect the next few centuries to witness very quick evolutionary changes.

The real question is, can humans survive until equilibrium is re-established?