Will a warm object in a vacuum cool more quickly than a warm object surrounded by an atmosphere containing CO2? I have gotten stuck on an endless thread about Global Warming, in which skeptics sometimes make the claim that an object surrounded by a "heat absorber" like CO2 will cool more quickly than one not surrounded by something that absorbs heat, because the "heat absorber" will absorb heat from the warm object, speeding its cooling.  I think this is likely complete nonsense, but would like to see what people with greater expertise have to say. If it is false, a nice, simple explanation to fire back with and try to set these people straight would be much appreciated!
  Could this be tested by lab experiment?  Has it been? any links?
 A: The object can be the planet Venus. If Venus would be a shiny white rock without an atmosphere, its surface temperature would about -40 C. But the surface temperature is about 460 C.
The CO$_2$ in the atmosphere radiates as much as it absorbs, half of it downward half of it upward.
https://www.acs.org/content/acs/en/climatescience/energybalance/planetarytemperatures.html
A: Depends on the situation!
If you have a thermos flask setup, where your warm object is surrounded by some medium and then by air, then the carbon dioxide cools it faster. This is because there's better thermal contact - the carbon dioxide molecules collide with the warm object and carry away some energy, whilst if there's a vacuum then all cooling has to be done by radiation, which is slow.
If, on the other hand, you have a setup more like Earth's climate, where you have a warm object, a boundary layer, and a vacuum, and a constant inflow of energy (from the Sun), then you find that the warm object stays warm for much longer if there's carbon dioxide. This can be shown with a fairly simple treatment, like the idealised greenhouse model.
