Is greenhouse effect beneficial and/or harmful?

Reverse greenhouse effect?

• Gas and dust nebulae often cool through a reverse greenhouse effect. Let me back up. On Earth, the greenhouse effect is that visible light can come down through the atmosphere to strike the ground and make it hot. The hot ground radiates in the infrared. However, CO2 and other gases reflect infrared back down, making it harder for heat to radiate into space. This heats the ground. In space, the dust is opaque to visible light, but transparent to infrared light. So the internal heat generates infrared, which leaves the nebula, cooling it. Such nebulae can cool to a lower temperature than the cosmic microwave background radiation temperature. The question is this. I don't see how any energy is expended to cool the nebula. Does a reverse greenhouse effect violate the laws of thermodynamics?

• Answer:

  No, not if the dust is transparent to infrared. Energy absorbed by the particles can re-radiate in many different frequencies. If the dust is not only opaque, but *reflective* to certain frequencies, it will cool beyond the CMBR temp. My guess is that it doesn't absorb microwave radiation, but rather reflects it, and since it is an efficient radiator, it has no problem cooling past the CMBR temp. Disclaimer: I am not an astrophysicist, but I play one on the internet.