If CO2 is heavier than air and sinks to the bottom, how does it trap heat in the atmosphere?

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Yes, I do believe in global warming.

In: Physics

Unfortunately CO2 is only *slightly* heavier than air, which mean ordinary mixing from wind is enough to keep it up there for years.

If you open a bottle of compressed CO2, it will sink to the bottom of the room. But if you let it sit for a while, it’ll slowly mix with the air and rise up. This is a process called “diffusion”, it’s caused by the movement of atoms. It’s the same process that will spread a drop of dye throughout a glass of water, even if you don’t stir it.

Once mixed, it doesn’t separate – the same way that a scrambled egg won’t unscramble. So that’s what happens with CO2 from car exhausts, industry, airplanes and so on: It just mixes with the air around it and slowly but surely rises up into the atmosphere.

Loaded question. Gases, once mixed, don’t separate into layers just based on density under ordinary earth gravity. If you generate a layer of heavier gas with no turbulence, it will mix only slowly with lighter gases above it. But once mixed it stays mixed.

CO2 traps heat by absorbing it, because it’s opaque to certain frequencies of infrared light. That includes both infrared coming down from the sun and radiated upwards from the earth. When CO2 molecules radiate away the energy absorbed as heat, they do so in all directions, rebounding about half the energy back towards the ground again.

The first page of [this paper](https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/wea.2072) has a nice diagram explaining the whole process of energy being absorbed, reflecting, or passing through the atmosphere.

Since heat from the surface trying to escape the atmosphere has to pass through all of it, it doesn’t particularly matter where in the path from ground to space and back CO2 is, though, as others have noted, it can be found pretty evenly mixed into the air in all layers of our atmosphere.