This is the objection I keep meeting and I cannot answer it properly. Carbon dioxide is a few hundred parts per million, while nitrogen and oxygen make up almost all of the air.
Why does the trace gas control the temperature rather than the bulk of the atmosphere?
Accepted Answer
Because nitrogen and oxygen are nearly transparent to infrared, and abundance is irrelevant if a gas does not interact with the radiation in question.
A molecule absorbs infrared only if the vibration changes its dipole moment. N₂ and O₂ are two identical atoms, symmetric, no dipole change, so they are effectively invisible to outgoing longwave radiation regardless of how much there is.
CO₂ bends and stretches asymmetrically, so it absorbs strongly around 15 micrometres, which is close to the peak of Earth's emission.
So the 99 percent is not doing nothing because it is weak. It is doing nothing because it cannot couple to that energy at all. Concentration only matters among the gases that can.
One thing to have ready, because the objection usually has a second half: "the CO₂ bands are already saturated, so more makes no difference".
That was argued in the early twentieth century and it is wrong for a reason that is not obvious. The centre of the band is indeed saturated near the surface. But adding CO₂ widens the wings of the absorption band, and more importantly it raises the altitude from which radiation finally escapes to space.
Higher means colder, and colder means less emission. The imbalance is made at the top of the atmosphere, not at the surface, which is why surface saturation does not close the argument.
Water vapour absorbs more in total, which is the other half of the standard objection. The difference is that water vapour responds to temperature rather than setting it, so it amplifies a change rather than starting one.
The discussion on each answer is open to members.
Join free to read the rest