The control mechanism of C02 atmospheric concentration is rather abrupt. C02 may accumulate in the atmosphere, increasing temperature and therefore evaporation and later rain. the rain leaches soil, sending large amounts of carbon to the sea. increased C03- ion leads to increased precipitation of calcium carbonate, efectively sequestrating carbon and eventually leading to an ice age. the trick is on the different speeds of the fluxes composing the geochemical cycles.
since we had ice ages no so far [1,2], than the C02 concentration should not be above average.
Maybe Nomad can tell.
however: [1] - the configuration of the continents encircling the arctic must be responsible for switches between normal and ice ages, so these ice ages may unusual [2] - is it possible that climate stability is much smaller than we have assumed. the rapid chances of recent past may be better known just because their clues have not yet been erased.
We do not have to worry that we would emit too much water wapour, say, with hydrogen cell cars - the additional H2O would be a tiny fraction of the atmospheric H2O flow. But higher H2O potential as greenhouse gas means that fluctations of H2O circulation can determine very much of how the greenhouse effect would develop. By standard assumptions, H2O circulation is determined by physical causes and effects - there might be dominant positive or negative feedbacks; or by deterministic or stochastic chaos - if the basic underlying dynamical system is chaotic. But it is also possible that the atmosphere reacts to increasing greenhouse stresses (whether anthropogenic or "natural" or accidental) as a cybernetic system, and that compensating (or possibly "rebooting") event sequences are already in place.
