Paul B -

An additional factor in CO₂ cycling is that any significant increase in concentration may push more of it up in the atmosphere. Once caught by the trade winds and/or otherwise trapped in high altitude circulation, the excess CO₂ is inaccessible to bio processes taking place at low altitudes, and could persist for very long times.

Some of it is continually brought down by rainfall, but the condensation that might sweep excess gasses down normally occurs at fairly low altitudes within the vertical circulation systems. Ice crystals formed at higher altitudes have little capacity for absorbing CO₂. High altitude gases may be "immune" to cleansing by the vertical circulation.

At present, this is in the category of "suspected processes" without convincing proofs that it does (now) occur to significant extent; but a few people worry that the stabilizing effect of low altitude biological CO₂ cycling could be "broken" if the effect is found to be significant.

John