One argument for CO2 absorption continuing to increase is that,
each layer of the atmosphere radiates onto the next, while the pressure of the
atmosphere drops, hence one can never get to saturation. This is similar to
Xeno’s paradox of the tortoise. Given that Achiles eventually passes the
tortoise, there must be a fundamental flaw in the initial argument.
In combustion calculations, there are also gradients of CO2
concentration. There are (massive) gradients in temperature. Compared to
climate, the gradients are orders of magnitude higher. At a fundamental level,
there is NO difference with between the atmosphere of combustion engineering
(which includes things that are not burning these days) to the atmosphere of
climate science. The atmosphere is the atmosphere.